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Musings of an Energy Nerd

Solar Thermal is Dead

It’s now cheaper to use a photovoltaic system to heat domestic hot water

Image 1 of 2
In much of the U.S., it makes little sense to install solar thermal collectors anymore, because there's now a cheaper way to use the sun to make hot water.
Image Credit:
In much of the U.S., it makes little sense to install solar thermal collectors anymore, because there's now a cheaper way to use the sun to make hot water.
Image Credit:
During the summer, a two-collector solar thermal system can produce more hot water than a typical family can use.
Image Credit: Metro Solar Atlanta

In the northern half of the U.S. — and even much of the South — installing a residential solar hot water system doesn’t make any sense. It’s time to rethink traditional advice about installing a solar hot water system, because it’s now cheaper to heat water with a photovoltaic (PV) array than solar thermal collectors.

In short, unless you’re building a laundromat or college dorm, solar thermal is dead.

The idea has been percolating for six years

In the early days of PV, when PV equipment was much more expensive than it is now, homeowners with PV systems (especially off-grid homeowners) were instructed not to use electricity for heating. After all, since electricity is precious and expensive, and since PV power usually costs even more than grid power, it made sense to save electricity for uses like refrigeration, lighting, and home entertainment.

For decades, we all assumed that the greenest way to heat domestic hot water was to use a solar thermal system. But then two things happened: PV equipment got cheaper, and heat-pump water heaters became widely available.

The logic of using a PV system to heat water was first explained to me in early 2006 by Charlie Stephens, a policy analyst for the Oregon Department of Energy. I reported the details of that conversation in an article, “Heating Water With PV,” published in the May 2006 issue of Energy Design Update.

“If you want to do solar water heating and solar space heating, solar thermal remains too expensive,” Stephens told me. “It’s not as cost-effective as using an air-source heat pump coupled to a PV array. In our climate, a properly sized solar thermal system can provide 100 percent of your hot water in the summertime, but it won’t do diddly in the wintertime. So you paid $4,000 for a system that provides 40 or 50 percent of your hot water needs. If instead, using the same money, you just add an extra kilowatt of PV to the roof, you could heat all of your hot water year round with an air-source heat pump.”

You can quibble with the details used in Stephens’ argument — it may take more than a kilowatt of PV to meet your hot water needs, for example, and his 2006 price estimate for installing a solar hot water system is now much too low — but his conclusion is even more valid now than when it was first made.

Some solar-heated water goes to waste

Solar thermal proponents know how to calculate the number of gallons of hot water produced by a typical 4′ by 8′ solar collector in a variety of climates. After calculating the thermal energy that this represents, they usually concluded (before PV prices dropped, anyway) that solar thermal collectors were a better bargain than a PV array.

But the number of gallons of hot water produced by a solar collector is always less than the number of gallons actually used by the homeowners. After all, if great quantities of hot water are produced on a day when it isn’t needed, you can’t really count the energy production in your annual tally.

Solar thermal energy is inconsistent, and during the long sunny days of summer, most solar thermal systems make more hot water than the typical family can use.

Although Charlie Stephens (pessimistically) estimated that a residential solar thermal system in the Pacific Northwest would only supply about 40% and 50% of a family’s annual hot water needs, the so-called “solar fraction” will be higher in other climates. In a 2006 study, researchers from Steven Winter Associates monitored two residential solar thermal systems for a year, one in Wisconsin and one in Massachusetts. Each house had two solar collectors. The solar fractions of these two systems were 63% and 61%, respectively.

Comparing solar thermal and PV systems

Compared to a PV system, a solar thermal system has several disadvantages:

  • Unlike a PV system, most solar thermal systems have moving parts (pumps and solenoid valves).
  • In freezing climates, solar thermal systems are sometimes subject to freeze damage.
  • Solar thermal systems require regular maintenance, including antifreeze replacement.
  • Unlike owners of a grid-connected PV system, who can be credited for their excess electricity production during the summer, owners of a solar thermal system can’t sell the excess summer production of their hot water systems.
  • While a pole-mounted PV array can include a tracking mechanism to follow the sun’s path across the sky, it’s virtually impossible to install solar thermal collectors on a tracker.
  • On average, PV systems probably last longer than solar thermal systems.

There are far more stories of troublesome solar thermal systems than there are stories of troublesome PV systems. Solar thermal systems sometimes develop air bubbles that interfere with the circulation of fluid, suffer from leaking pipes, or experience problems from summertime overheating. PV systems, which suffer none of these headaches, look attractive in comparison.

Let’s do the math

Since 2006, when Stephens first proposed that it was cheaper to heat water with a PV array than a solar thermal system, two factors have emerged that greatly strengthen his case: more reliable heat-pump water heaters have become widely available, and PV modules have gotten dramatically cheaper. (During the same time period, sales of solar thermal systems have also been hurt by a third factor: dropping natural gas prices. But that’s a topic for another article.)

Although it’s always difficult to predict future price trends, there are reasons to believe that the price of PV modules will continue to drop, while the price of the copper tubing used to make solar collectors will continue to rise.

In northern states, a typical residential solar thermal system includes two 4′ by 8′ collectors and a 120-gallon solar storage tank; the installed cost for such a system is about $8,000 to $10,000. Instead of spending $10,000 on a solar thermal system, what would happen if you invested $3,000 in a heat-pump water heater and $7,000 in a 1.7-kW PV array?

The 1.7-kW PV system would produce 2,114 kWh per year in Boston or 2,093 kWh per year in Madison, Wisconsin. Let’s be conservative and use 2,000 kWh for our example. Assuming that the average [no-glossary]COP[/no-glossary] of the heat-pump water heater is 2.0 — a fairly conservative assumption — it takes 0.0855 kWh to raise the temperature of a gallon of 50°F water to 120°F. So 2,000 kWh can produce 23,392 gallons of hot water a year, or 64 gallons a day — exactly equal to the amount of hot water that the U.S. Department of Energy assumes is used by the average American family. So, once you’ve paid for the system, you get “free” hot water.

That’s a much better deal than a solar thermal system that produces only 63% as much hot water — even if you do have to buy a new heat-pump water heater in 12 or 14 years. (Trust me — if you have a solar hot water system, you’ll have to invest in maintenance and replace a few parts over time, too.)

Of course, if your family uses less than 64 gallons of hot water a day, or your heat-pump water heater has a higher average COP than 2.0, or you live in a state with more sunny days per year than Massachusetts or Wisconsin, or the average temperature of your incoming cold water is higher than 50°F, then your new PV system will be producing extra electricity that you can use for other purposes.

In fact, many families use significantly less than 64 gallons of hot water a day. A Canadian researcher, Martin Thomas, monitored hot water use in 30 Canadian homes in 2008; the average hot water use by the monitored families was only 44 gallons a day. If your family uses 44 gallons of hot water a day, you’ll only need a 1.2-kW photovoltaic array (costing about $5,000) — or, in a sunny climate, an even smaller PV array — rather than the 1.7-kW array proposed for northern families using 64 gallons of hot water a day.

What if you use an electric-resistance water heater?

Let’s do the math for those who prefer to use an electric-resistance water heater. If you invest $10,000 in a PV system, you’ll get a 2.2-kW system (assuming a PV equipment cost of $4.54 per watt). The PV system will produce 2,736 kWh a year in Boston. Using electric resistance heat, it takes 0.171 kWh to raise a gallon of 50 degree water to 120 degrees, so you’ll end up with 16,000 gallons of hot water per year, or about 44 gallons a day — about exactly the average water use by U.S. and Canadian families, according to two recent studies.

If the family with the hypothetical $10,000 solar thermal system uses 44 gallons a day, and the solar fraction is 63%, their solar thermal system heats about 28 gallons a day on average. The PV option produces 37% more hot water, even with an electric resistance heater — and with far less hassle.

To make 28 gallons a day — an amount equal to the average output of the solar thermal system — with an electric-resistance heater, all you would need in Boston is a 1.4-kW PV system costing about $6,300.

But — but — but —

Here’s the part of the blog where I admit that my chosen title — “Solar Thermal Is Dead” — was deliberately provocative and somewhat inaccurate.

So I’ll list a few exceptions to my new rule:

  • Solar thermal systems still make sense for off-grid homes.
  • If you can get a two-collector solar thermal system installed for $5,000 or less — an attainable price in areas of the country where people don’t have to worry about freeze protection — it may make sense to install one.
  • In a sunny, warm climate, where a solar hot water system will have a higher solar fraction than 63%, an investment in a solar thermal system makes more sense than it does in Wisconsin or Massachusetts. (On the other hand, a PV system produces more electricity in a sunny climate, too.)
  • If you are skeptical about the longevity of heat-pump water heaters, you may prefer to wait a few years before buying one, and to stick with a solar thermal system in the meantime.
  • Before taking the advice given in this article, compare the costs and energy production figures of a solar thermal system and a PV system in your area, using location-specific energy production figures and local equipment costs and installation costs.

There are many factors to consider when choosing equipment to heat domestic hot water. One point is clear, however: if you plan to install a heat-pump water heater, you definitely don’t want to also install a solar thermal system. The correct solar complement to a heat-pump water heater is a PV array.

Green builders have an emotional connection to solar hot water systems, because they represent a fairly simple technology that’s been around for over 100 years. But it’s time to admit that a PV array is cheaper and less troublesome than fluid-filled solar collectors on your roof.

Author’s postscript: For an updated (December 2014) analysis that compares the cost of heating domestic hot water with a solar thermal system to the cost of heating domestic hot water with a PV system, see Solar Thermal Is Really, Really Dead.

Last week’s blog: “A Superinsulated House in Rural Minnesota.”


  1. User avater
    Marc Rosenbaum | | #1

    Some thoughts
    - There isn't actually a good location in many houses for a HPWH - they need enough heat source and volume to function properly. I don't really know what the lower limits are, and I think it will vary based on the amount of DHW usage, the degree of insulation in the basement if it's a basement application, and the actual HPWH you select.

    - In some places, they are using waste heat off a poor fossil fuel heater (uninsulated pipes or ducts, for example) in others they are stealing that heat from heat you provided by other means (no heat in a basement that is within the thermal enclosure, so heat comes through the floor over the basement),

    - If a house is in a warm enough climate to locate the HPWH in the garage it's probably a great app, or in really warm climates in the house itself, providing cooling that can be used. This will make the noise level of the unit more critical.

    - As renewable electricity gets widespread there will be more valuing of the clearly on-site nature of solar thermal - no import/export grid issues to think about.

    - Drainback systems, properly done, last 25-30 years with a pump or controller replacement periodically. The replaceable elements of the PV/HPWH system are more costly - inverter and the HPWH itself. It would be great to see high efficiency HPWHs with lifetime warranty tanks and easily replaced system components (blower, compressor, controller).

    - The PV argument presumes you already are installing PV, which means the PV cost is a marginal cost, which is a smaller cost than the overall cost per W.

  2. User avater GBA Editor
    Martin Holladay | | #2

    Response to Marc Rosenbaum
    Thanks very much for your thoughts, all of which are certainly pertinent.

    1. You're right: some cold-climate homes don't have a good place to install a heat-pump water heater. However, I'm beginning to consider the possibility that my earlier skepticism about the suitability of HPWHs in cold climates was hasty. I recently heard Robb Aldrich of Steven Winter Associates give a presentation at the NESEA conference on a HPWH pilot project in New England. I hope to write an article about his presentation in the future, but suffice it to say that most New England homes have basements, and most basements are big enough and warm enough to accommodate a HPWH without any comfort or performance issues. Another finding: when you buy a HPWH, you want one with a big tank. That means it's wise to avoid the G.E. model and it's better to buy the Stiebel Eltron model. The measured COP of the Steibel Eltron HPWHs in the pilot study was 2.35.

    2. Yes, HPWHs rob space heat during the winter to make hot water. However, in many homes, the change in the basement temperature doesn't result in any problems.

    3. Yes, a garage is a great place for a HPWH. In a hot climate, you might want one in your living room -- but only if manufacturers can make them quieter, as you point out.

    4. In the U.S., it will be many years before we have to worry about the problem of too much PV. (The worry is that the grid won't be able to handle all that electricity production on sunny days.) When we have to face that problem, I'll sing "Hallelujah" and change my advice about solar thermal systems.

    5. Drainback solar thermal systems probably can last 30 years -- but so can PV systems.

    6. It's true that to get a PV system installed for $4.10 per watt (a price provided by a contractor for a 9-kW system, reported in Jesse Thomspon's blog, PV Systems Have Gotten Dirt Cheap), you need to install a fairly large system. A 1.2-kW system will have a higher cost per watt. However, prices are continuing to fall, so the economics of smaller PV systems will only get better in the future.

  3. Frank R. | | #3

    too soon

    I hate to disagree with you, but it may be too early to declare that solar thermal is dead. Besides the your own "but-but-but" comments, I would add a few others:

    1. The big pump manufacturers (Bell&Gossett, Taco and Grundfos) are producing very efficient variable speed, intelligent small circulators combined with hot water solar. Competition will drive the cost down.
    2. Many of the problems with solar hot water systems are because "green" contractors working with solar manufacturers are installing these systems. Many don' t have the training to select, install and commission heating systems. As more knowledgeable heating contractors and engineers have gotten involved, the less problems that will occur. My limited experiences dealing with a European solar manufacturer/installer were brutal. They lacked the basic understanding on how to select pumps, exchangers or how to prevent air problems in the system.
    3. In northern climates a combined home heating/domestic heating/solar heating system can greatly improve the efficiency of the solar heating system. That may tip the balance back to solar thermal.
    4. In southern climates a solar heating system for pool heaters should be a great investment.

    The February issue of ASHRAE journal had a great article on combined solar/space/domestic heating system and control. If anyone has access to ASHRAE journal (you have to be a member), I would strongly recommend that they read the article. The article goes into detail on how to install and control a simple elegant cost effective system. It would be a great article for GBA if ASHRAE would let you reprint it.

  4. User avater GBA Editor
    Martin Holladay | | #4

    Response to Francis Robertson
    I'm skeptical of your contention that newer pumps will be less expensive than the current generation of pumps, and even more skeptical that a change in pump prices would ever be enough to significantly affect the economics of solar thermal systems. The cost of the pump is a small percentage of the cost of a solar thermal system -- and in general, more efficient pumps cost more to manufacture than less efficient pumps.

    To increase the size of a solar thermal system so that it can provide a portion of a home's space heating needs only makes the system more expensive. It improves neither the cost-effectiveness nor the payback of the solar thermal system. If anything, the disconnect between the timing of solar thermal production and space heating needs is even worse than the disconnect between the timing of solar thermal production and domestic hot water needs -- so any increased investment in solar space heating equipment generally makes the economics of the system worse rather than better.

  5. Kevin Dickson, MSME | | #5

    Overpriced Solar Thermal

    I definitely agree that a solar hot water (SHW) system on a single family house for $10,000 is a worse investment than $10k of PV.

    NREL agrees and has determined that the cost must be $1000-$3000 for SHW to start getting a foothold in this "$123 billion" market.

    The annoying problem that makes SHW cost too much is freeze protection. Overtemperature issues have been cheaply solved by "steamback".

    NREL thus far has been unable to catch the attention of the major water heater manufacturers to develop low cost systems.

    I've been testing some systems that cost under $2000 installed. The secret to low cost is simplicity. These systems use a concept proven in a 2004 study that heat pipe evacuated tube solar collectors can be freeze protected with a simple recirculation strategy:

    These systems use 2%-5% of the collected energy to freeze protect themselves at night and during cold cloudy weather.

    These systems are installed directly on the homeowner's existing water heater, so there is no tank cost. The solar heats the water above the fossil fuel setpoint, so it operates between 110F and 170F. A mixing valve is installed for safety.

    What's notable is what's not there:
    1. Only one pump
    2. No heat exchanger
    3. No expansion tank unless the home has a backflow preventer
    4. No heat dump
    5. No antifreeze
    6. No 110v wiring
    7. The freeze protection works even during a power outage.
    8. Extremely low wattage pump means 100+ COP
    9. No preheat tank

    So my main point is to keep an open mind and don't kill off SHW just yet.

  6. User avater GBA Editor
    Martin Holladay | | #6

    Response to Kevin Dickson
    The DOE and NREL have a long history of underestimating the prices that solar contractors charge to install a solar thermal system. When NREL says, "It will cost $2,000," I interpret that to mean, "Your local contractor will charge you $6,500."

    The trouble with one-tank systems is that these systems have low efficiency. Efficiency depends on the delta-T -- when the water entering the collectors is low, the efficiency is high; when the water entering the collectors is high, efficiency is low.

    So any system designed to send 110 degree to 170 degree water to the collectors will be much less efficient than one that has a separate solar tank with good thermal stratification.

  7. Jan Juran | | #7

    Drain Water Heat Recovery
    Hi Martin: very interesting comments. In some installations, a drain water heat recovery DWHR could nicely complement your suggested heat pump/PV idea. For ex. a R3-78 Power Pipe costs less than $1000 delivered, rated at 60.4% heat recapture (although to be fair, most studies subtract approx 10% additional heat losses incurred between showerhead and shower drain) Of course, a DWHR requires enough feet of vertical drop between the shower drain S trap and the main sewer exit drain, so some installations (e.g. a single floor ranch house with slab on grade) cannot be fitted. However there are lots of Northern two story homes having upstairs bathrooms and showers and basements which could be appropriate, esp. in new construction. As a passive device, a DWHR typically lasts as long as the plumbing system, with essentially zero maintenance. A DWHR device should allow you to downsize the heat pump and PV sufficiently to save more than the DWHR cost. The DWHR should also significantly reduce the number of BTUs removed by the heat pump from a cold basement during winter, as well.

  8. User avater GBA Editor
    Martin Holladay | | #8

    Response to Jan Juran
    I agree with your advice. I have advocated the use of drainwater heat recovery devices (at least for families who prefer showers to baths) for many years.

    Most recently, I included the suggestion in a list of "seven things you can do to reduce the amount of energy used for domestic hot water" in my last blog about water heaters: All About Water Heaters.

  9. Jason Chenard, PE | | #9

    The Laws of Thermodynamics
    Holladay re: Rosenbaum -- "Yes, HPWHs rob space heat during the winter to make hot water. However, in many homes, the change in the basement temperature doesn't result in any problems."

    The Law of Conservation of Energy tends to refute this premise. If you remove heat from the basement, either the air temperature will fall or the heat will be replaced from some adjacent source. Perhaps in an unfinished dirt cellar, the heat could be considered to come partly/mostly from the earthen walls, and therefore not considered a significant tangible cost, or at least partly buffered by the earth's heat. Even then, the floor above will feel/be cold, which is not desirable.

    But in a modern house, it's likely that the basement walls are insulated, and in many cases the basement is partly or fully finished and/or occupied. Then, the heat absorbed by the HPWH has to be completely replaced, and the source of the replacement is the gas or oil being burned to heat the house. Even worse, it is then necessary to expend electricity for the compressor to transfer said heat to the water.

    As noted, this is not an issue in climates where the HPWH can be in a garage, i.e. south of Maryland, more or less. In fact, for those locations, the PV+HPWH suggestion is a fantastic one that I have never thought of (from way up here in New Hampshire.) But in winter zones, the math you're doing just does not work. The heat has to come from someplace.

    To be fair, and this is a point that your article does not make, the HPWH also effectively provides air conditioning while it does its job. In a predominantly non-winter climate, this arrangement becomes a win-win-win system during the cooling season.

    There are various other items that I also am skeptical about:

    I can't really follow your apparent premise that ST systems provide little to nothing in the winter, but PV systems are still online and working. The sun shines on either of them the same way. And on a related note, the advent of evacuated tubes makes ST reasonably effective in cold climates. My system provides on the order of 50% during the winter, with my oil boiler making up the difference, although my solar system admittedly is somewhat optimized to take advantage of lower solar yield.

    While on the subject of evacuated tubes, they also have the distinct advantage of always being "perpendicular" to the sun, which largely negates the supposition that PV panels can be made to "track" while ST panels can not. Even more significantly, the tubes do this by their very nature, while a tracking system is expensive, complex and maintenance intensive. And if fouled by snow or ice, well, the sound you hear is gears being stripped.

    The idea that ST capacity gets 'wasted' on the longest days of the year is subjective, and greatly dependent on the details of the application. As already noted, excess capacity can be shunted to other uses. More cogently, increasing tank capacity provides a buffer between when the energy is collected and when it's needed. And with hot water stored at temperatures approaching 200 degrees, an 80 gallon tank can hold a great deal of energy, and the cost premium over a 50 gallon tank is nominal.

    HPWH's made in the US will NOT last 12 years, and probably won't last 8. The quality of the compressor is the overriding factor, and US makers are not typically inclined to make equipment that lasts longer than whatever is the industry standard. It runs counter to their sales volume. These things are basically an electric tank with a window air conditioner bolted to the top. Even a basic electric resistance tank typically doesn't last that long, and it's an order of magnitude less complex.

    I like the PV+WSHP idea very much if properly applied, but I think ST is still very viable up here in the frozen north, and would be far more so if the cost of installation (which is currently outrageous in my opinion) were lower.

    As always, your mileage may vary! -Jason

  10. User avater GBA Editor
    Martin Holladay | | #10

    Response to Jason Chenard
    I have written many articles on heat-pump water heaters over the years, and I am well aware of the fact that they have the effect of cooling and dehumidifying the room in which they are located, a useful property in hot climates and a potentially problematic one in cold climates.

    When a HPWH is installed in the typical basement, it tends to cool the basement by a few degrees. If the basement is not finished, but is used to house mechanical equipment and for storage, the lowered temperature does not result in comfort complaints from residents, according to several pilot studies.

    Of course the laws of thermodynamics apply. The heat comes from the air in the basement. However, most homeowners don't really care if their basement temperature is 55 degrees instead of 60 degrees.

    You wrote, "I can't really follow your apparent premise that solar thermal systems provide little to nothing in the winter, but PV systems are still online and working." The difference in annual performance has nothing to do with any difference in sunlight striking the panels during the winter; it's simply a function of comparing the annual performance of the two types of systems. The measured solar fraction of a solar hot water system is on the order of 63%, as I wrote. This can be compared to the annual solar output of a PV array. In both cases -- solar thermal or PV -- most of the energy is gathered during the spring, summer, and fall, with very little during the winter.

    You wrote, "The idea that solar thermal capacity gets 'wasted' on the longest days of the year is subjective." On the contrary, it is a fact, and this is a fundamental difference between a solar thermal system and a grid-connected PV system. That's why many solar thermal systems have special valves that need to be flipped during "vacation mode" during the summer to prevent overheating when the homeowners leave to visit Grandma for a week, and why some systems have a "dump loop." If a homeowner with a grid-connected PV array goes on vacation, there is no risk of overheating and no need to activate a dump loop; the electric meter will simply spin backward while the family is away.

  11. Jason Chenard, PE | | #11

    All true, to an extent
    Being a cheap Yankee, albeit of Franco extraction, I decided to dispense with the thermodynamic esoterica and do economic math instead.

    At 2000 kwh in Mass (just up the rud, as we say here ) and the unconscionable 17 cents we pay for power, the PV gives you back $340 per year, regardless of whether you use it or send it to the grid, and regardless of whether you use if for hot water or your plasma TV, for that matter.

    Accepting your 60% number for ST operation, and at $3.75 for oil and a 78% efficient boiler, the ST saves you about $300 per year. This is less any wasted capacity, which I could quibble about, but it's always going to be some modest fraction, to be sure.

    Your concept also has the free-AC-in-the-summer perk. And, your 2.0 COP is giving the HPWH short shrift, so at least when it's warmer, that's a sizable additional additional chunk of savings in your favor. These two points are sort of the same thing, in a way.

    So if the costs really are equal for the 1.7 kw PV and the typical 2-panel/120 gallon ST thermal, then it's not close: Your suggestion is far better.

    But, and this was the point I was making at the end of my post about ST installed cost, the ST really *should* cost a great deal less. If it were $6K for ST and $10K for PV, the life-cycle cost would swing in favor of ST. (I installed my own ST, 60 tubes, for way under $5K.)

    Big picture, in places where the kWh cost is less than here (which I think is practically everywhere), the case to be made for PV becomes weaker. But that's even more true if the advantage of ST is weighed against the cost of using nat-gas: ST can not come even close to being justifiable. And the latter is much more common than the former. Still advantage PV-HPWH.

    Summary: I bow to your point of view. The PV-HPWH will be $better$ in the vast majority of cases.

    Also, full disclosure: I started research/shopping for a HPWH about two weeks ago.

    It's 5pm and 70 degrees and sunny. I think that does it for me on this. Great article, and point taken.

  12. User avater GBA Editor
    Martin Holladay | | #12

    Response to Jason Chenard
    It looks like we end up agreeing after all. As I noted in my article, "Before taking the advice given in this article, compare the costs and energy production figures of a solar thermal system and a PV system in your area, using location-specific energy production figures and local equipment costs and installation costs."

    I bent over backwards to be fair in my math. But if one wanted to play the game a little closer to the line -- and assume 2,140 kWh per year instead of 2,000, and a COP of 2.35 instead of 2.0, and a daily hot water requirement of 44 gallons instead of 64 gallons... one quickly sees how strongly the math favors PV -- IF you have somewhere in your house to put a HPWH, IF you're not scared of the fact that a HPWH steals heat from your home during the winter, and IF you're not worried about premature failure of HPWH equipment.

    Considering the reduced maintenance headaches of PV versus solar thermal, an argument could even be made that PV + electric resistance water heating makes more sense than a solar thermal system -- especially if you live somewhere where solar contractors charge high prices for installing a solar thermal system.

  13. Peter Hastings 4C | | #13

    Is Resistance Futile ?

    Considering the reduced maintenance headaches of PV versus solar thermal, an argument could even be made that PV + electric resistance water heating makes more sense than a solar thermal system

    Given current trends, how soon before the numbers on PV+ER make real sense ? It has the KISS principle on its side. As well as decibels...

  14. Sean Herbert | | #14

    PV + electrical resistance water heater
    You just stole the question from my mouth Martin. What if you spent the extra $3000 (of your initial $10k) on more PV, and just kept everything else the same? Obviously, you would lose out on the COP, but that meter would spin so much faster backwards while at grandma's.

  15. John Klingel | | #15

    Quit a good article for those
    Quit a good article for those of us trying to decide which way to go. I will have to look into this for those of us who are REALLY up north. thanks.

  16. Stephen Magneron | | #16

    What does Thorsten Chlupp have to say???
    Martin, you certainly make HPWH + PV's very appealing. I'll have to do the calculations for Ottawa!

    I'd like to hear what Thorsten Chlupp has to say on this subject. His own house is built to Passivhaus with 12 solar thermal collectors that feed into a 5000 gallon, two-storey tank that provides 100% of his DHW and almost all of his space heating (a masonry heater is used as a back-up). All this in Alaska where there is no sun for two months!

    Although apart from the $1000 tank he had to customize, there was little information about the total costs of a system like that.

  17. Les Dell | | #17

    Solar Thermal Is(n't) dead
    "In northern states, a typical residential solar thermal system includes two 4' by 8' collectors and a 120-gallon solar storage tank; the installed cost for such a system is about $10,000. "

    You did say Northern states, so I cannot speak to that figure, but I install in the south, Atlanta specifically and the typical install you mention would be in the neighborhood of $5K. After the 30% Federal Tax Credit and the 35% state tax credit you are looking at out-of-pocket costs of less than two-thousand dollars. So I won't argue with what you've quoted in your neck of the woods, but here on the ground where we are the $10K figure is entirely out of proportion.

    I checked the DSIRE website and found that in addition to the 30% federal tax credit both Mass. and Wisconsin have state rebates in addition to numerous utility rebates. (Two states your article mentions)

    Was the $10k figure you gave before or after incentives?

    Georgia isn't an especially renewable friendly state and I know that several Northern states, namely N.J. and New York offer much better incentives on SHW than we have here. So I believe that it is very reasonable to assume that a residential install would come in much lower than ten thousand dollars in some parts of the Northeast.

    You didn't specify what type of maintenance is necessary for SHW. There is really very little. The glycol would need to be changed about every ten years if installed properly. Drainback systems do not have this issue at all. It's not uncommon to see Grundfos pumps functioning perfectly after 20 years of duty. Collectors are typically guaranteed for 12 years and up and should last at least 30. I've taken down more systems because the home needed a new roof than because the system failed.

    You did qualify your statement at the end and say it might make more sense depending on where you are geographically, absolutely. Several comments have made mention of evacuated tube technology which is better suited to cold climates than flat plate. Was this taken into account in terms of the winter efficiency you quoted? In my latitude a SHW can produce about 50% of the hot water demand even during the winter. Providing 100% of the demand whether PV or SHW isn't very realistic for most applications.

    (I appreciate the photo plug)

  18. John Klingel | | #18

    "Of course the laws of
    "Of course the laws of thermodynamics apply. The heat comes from the air in the basement. However, most homeowners don't really care if their basement temperature is 55 degrees instead of 60 degrees." After re-reading, this is not as viable an argument, IMO, as it appeared to be at first. What happens when the room goes down to 55 F? Isn't the next cycle going to take the temp toward 50 F? Sooner or later, your boiler/other heat source is going to kick in, or your basement is going to go down to soil temp, is it not? So isn't the governing factor for free heat your soil temp? It just stands to reason that if this contraption is in conditioned space, it will need help sooner or later. Did I miss something? I do like this concept, however, and will be chatting w/ those involved w/ this up here.

  19. Kevin Dickson, MSME | | #19

    Solar Thermal DHW is Better in Sunny Places
    That's pretty obvious, but seriously I wouldn't open a solar thermal business in Seattle or Maine any time soon.


    1. The $2000 for these simplified systems is my actual retail installed price, not NREL's guesstimate. I've been designing & installing SHW systems since 1979. I agree that NREL has often been way too optimistic in the past with their cost estimates.

    2. The low-loss collectors we are testing suffer by less than 10% at those higher one-tank temperatures:

    3. Bradford White, after 35 years in solar, is finally making a gas-fired single tank solution with a timer. This will greatly reduce that 10% penalty:

    4. My dream pump has recently been introduced:
    It uses one-fifth the energy and costs one-third as much as the usual suspects, Laing, El Cid, Wilo, Grundfos, and Taco. This pump is a heckuva lot quieter than my inverter, so decibels aren't a problem. It's rated for 30,000 hours of service.

    5. At $2000, PV will have to come down in price by about a factor of three to compete with SHW. I expect it may eventually happen, though. That's OK with me because I'd rather have just one solar system on my roof anyway.

    6. HPWHs won't have great market penetration in cold climates until they make a split system. Then the heat will be taken from the outside of the house. This will add $1000 of mostly labor to the already pricey $1400-$1600 for a standalone HPWH.

    7. If anyone is still interested in SHW, and you want to get a better understanding of the costs involved, this is required reading:
    Gary Reysa, the retired Boeing engineer, has taken all the mystery out for you.

  20. User avater GBA Editor
    Martin Holladay | | #20

    Response to Sean Hebert
    Your question about using an electric-resistance water heater is a good one. Here's the math you requested: If you invest $10,000 in a PV system, you'll get a 2.2-kW system if you pay $4.54 per watt. That system will produce 2,736 kWh a year in Boston. Using electric resistance heat, it takes 0.171 kWh to raise a gallon of 50 degree water to 120 degrees, so you'll end up with 16,000 gallons of hot water per year, or about 44 gallons a day. That's just about exactly the average water use by U.S. and Canadian families, according to two recent studies.

    If the family with the hypothectical $10,000 solar thermal system uses 44 gallons a day, and the solar fraction is 63%, that means that their solar thermal system only heated about 28 gallons a day. The PV option produces 37% more hot water, even with an electric resistance heater -- and with far less hassle.

    To make 28 gallons a day -- an amount equal to the output of the solar thermal system -- all you would need is a 1.4-kW PV system (in Boston) costing about $6,300, not $10,000.

    Thanks for requesting these calculations. I have decided to edit my article to include them.

  21. User avater GBA Editor
    Martin Holladay | | #21

    Bids from Les Dell and Kevin Dickson
    So Les Dell offers to install a solar thermal system on an existing house for $5,000, and Kevin Dickson offers to undercut Les Dell and install one for $2,000. All I can say to you guys is -- open a franchise in Massachusetts. It won't be long before one of you has all of the business in the state.

    However, Les, as my latest math shows, a $6,300 PV array in Massachusetts will produce just as much hot water as your $5,000 solar thermal system using an electric resistance water heater, and has the potential to produce twice as much hot water using a heat-pump water heater.

  22. User avater GBA Editor
    Martin Holladay | | #22

    Response to Stephen Magneron
    I don't know what Thorsten Chlupp paid for his solar thermal system, but I'd be happy to do some back-of-the envelope calculations to estimate what it would cost someone to reproduce his system. A 4' by 8' flat-plate solar thermal collector costs about $900 to $1,000 plus shipping; let's call it $1,100 each in Fairbanks, or $13,200 for 12 of them.

    I couldn't find a price for a 5,000-gallon tank, but a company called SWHIFT in Idaho will sell you a 1,570-gallon unpressurized stainless-steel tank for $14,448. Let's call it $16,000 with shipping, and let's buy 3 of them. (It is wildly unrealistic to assume that a contractor could install a 5,000-gallon water storage tank for $1,000, which is Thorsten's reported cost.)

    So, along with the 12 solar collectors, we're up to $61,200.

    After we buy the pipes, pumps, controllers, and tank insulation, I'd say we're up to $70,000 in materials. I'll let you estimate the labor charge.

  23. User avater GBA Editor
    Martin Holladay | | #23

    Response to Les Dell
    You're right, of course, that the federal tax rebates and local incentive programs lower the cost of a solar thermal system to most homeowners. However, the same incentives also apply to PV systems. So if you want to use the net cost after incentives, the math lowers the costs on both sides of the equation, and PV still comes out ahead.

  24. User avater GBA Editor
    Martin Holladay | | #24

    Response to John Klingel
    Concerning your worries about the problem that a HPWH might lower the temperature of a basement to the point where performance or comfort are affected: You're right -- it's possible.

    You only want to install a HPWH if you have a good place to put it. A bad place to put it would be a small, cold room or a small closet. A good place to put it would be a big basement with at least one heating appliance (usually a furnace or a boiler) that shares the same space.

    Here's what researchers are finding: as long as the basement doesn't get below about 55 degrees, these HPWHs work well. Many basements don't get colder than 60 degrees, even with a HPWH running all winter. The calculation as to how much harder the furnace or boiler has to work under these circumstances is complicated, and the answer probably ranges from "a little bit" to "not much."

    Finally, as I pointed out in another comment, the math works out fairly well for PV + an electric resistance water heater. So even if you don't want to buy a HPWH, it's worth considering the advantages of a PV system compared to a solar thermal system.

  25. John Klingel | | #25

    Martin: Got it. Thanks. I am
    Martin: Got it. Thanks. I am going to ask our elect utility today (home show) about the specs for PV, etc, here, as well as a company that quoted me "about $8000" for an installed "state of the art" SHW system last year. And, in no way did Thorsten install a SS tank for a grand, unless someone was drunk when they sold it. A septic tank costs far more than that. I think a zero was left out of the printed price, as that tank is not empty of structure inside, if I recall. I will try to remember to ask Thorsten today, too. Very good blog, and very timely for me. Thanks.

  26. User avater
    John Semmelhack | | #26

    Cheers, Martin
    Cheers, Martin, for a good article. I've been telling this to my clients for the past couple of years...with mixed results. On a recent project, against my advice, the client opted for an $8,000 evacuated tube system (30 tubes, 80 gallons storage) that will provide in the neighborhood of 50-60% of their annual hot water needs and leave them with about 2,000kWh in electric resistance heat to cover the remainder. For less than $3,000 they could have installed the Steibel Eltron HPWH (EF=2.5) to get about the same energy use.

    I'm also looking forward to the not too distant future when split system heat pumps for water heating are commonplace. In my climate (central VA), I expect we'll get annual COP between 3.0 and 4.0, have better hot water recovery and less noise (than HPWH's)...for just a little bit more money.

  27. User avater GBA Editor
    Martin Holladay | | #27

    Response to John Klingel and John Semmelhack
    John and John,
    It's interesting to hear both of you mention that, in your areas, a typical bid for a solar hot water system is $8,000.

    At a social event last night, a friend told me that he was at a home show / energy fair and talked to a solar thermal contractor who quoted the same ballpark price -- $8,000 -- for a solar hot water system. Of course I don't know if that is a one-collector or two-collector system -- whether or not it is for a one-tank or two-tank system -- and whether the tank would be small or large.

    In most cases, these "home show" bids tend to be lower than the invoice received at the end of the job.

  28. John Klingel | | #28

    don't recall
    Martin: My bid was a year ago and from ABS Alaskan, 907-452-2002, who does a lot of installations here. I don't recall the details, as I was more interested in knowing if our site was good for solar thermal and knew prices would change in 2 yrs. I just told the tech "Here is the spot, and there is South. 3 bdroom, 2 bath, 4 or 5 people. Estimate w/in a few hundred bucks is fine." From this link, I'd have to guess at which product, if any, was in the tech's head. Their big focus is on domestic hot water, as we don't get a lot of sun when we need the heat the most. So, I have no idea what system we talked about back then, but whatever it was, it was $8K installed. As we get closer to build time, I will have to re-check all this. In the meantime, if anyone has any comments on what they feel I need, quality of this system, etc, I am all ears. ABS has a good reputation of being knowledgeable and truthful; that is all I know about them. Thanks. BTW: At the home show, the opinion from ABS was that PV was getting very competitive, and panels may even get cheaper. Solar thermal prices are about where they will be for the near future. The electrical company said "Run your numbers, but you will likely pay for PV in several years." (Define "several"??) That is all I know so far. And, yes, home show prices from at least some places may have a little bit of "angle" in them.....

  29. John Klingel | | #29

    semmelhack and i did some price fixing before posting

  30. Kevin O'meara | | #30

    large tank costs
    Agree with the article entirely, although large tank costs are much cheaper than you might have guessed. I installed two 5,000 gallon tanks at $1/gallon including delivery. These were concrete not stainless steel since the water inside them is used to store the heat and never used for drinking, showers, etc. It made sense for us to go this route as these were required by the fire marshall in our county for fire suppression purposes anyway. We just retasked them to store hot water for use in the winter (it is underground and wrapped in 12" of EPS, and has custom bent heat exchangers inside)

  31. Jonathan Teller-Elsberg | | #31

    Stuck on thermo theory
    As a northerner without a garage option to house the HPWH (not just a bad idea, but I don't have a garage!), I feel like the question "where does the heat come from that the HPWH relies upon?" hasn't been sufficiently answered. Mr. Holladay, you write:

    Here's what researchers are finding: as long as the basement doesn't get below about 55 degrees, these HPWHs work well. Many basements don't get colder than 60 degrees, even with a HPWH running all winter. The calculation as to how much harder the furnace or boiler has to work under these circumstances is complicated, and the answer probably ranges from "a little bit" to "not much."

    You seem to have numbers for almost everything at your fingertips, but no luck on the calculation regarding how much harder the furnace has to run? It sure does seem like there would be a lot of systemic inefficiency if the HPWH is sucking up space heat generated by the furnace. Even if that space heat is in the basement, it is buffering the living space from the ground temps--a 5'F drop in air temp in the basement might not cause people in the home to complain of noticeably colder floors, but that doesn't mean the furnace isn't burning enough excess oil in response to cancel out the efficiency of the PV-powered hot water system.

  32. Rich Cowen | | #32

    What about the Secusol appliance
    Here in Massachusetts I got a quote of $12K+ for solar hot water and assumed it was not economical, but then I talked to a company that only does installs of the German-made Secusol Appliance, a preconfigured system that was originally designed for cold climates. This can be installed for around $7000 and the state and federal rebates bring the cost to $3500.

    links: -- the site says that the efficiency of the Secusol system has been verified by the Massachusetts Clean Energy Center. The president of the company did come out to look at our mounting location so this all seems legit.

    Anyone have experience with Secusol?

  33. John Klingel | | #33

    me, too, but dropping it
    Johnathan TE: I see what Martin is saying exactly, and I am still in your boat on this. I just don't want to drag the conversation on this point on too long, and maybe I have more physics to brush up on. I have been reading about HP's and things are starting to make sense now. Heat pumps do not create energy, they just move it. Therefore, whatever heat they move from conditioned air to the water needs to be replaced, and that will require your other heat source (boiler, for ex). I do not think that fact, as I understand it to be, is arguable. Where these HPWH shine is in their efficiency compared to a standard electric water heater. From "Heat pump water heaters use electricity to move heat from one place to another instead of generating heat directly. Therefore, they can be two to three times more energy efficient than conventional electric resistance water heaters." All that said, I still do not know if heating the air w/ a boiler, then transferring it to a water tank, is more efficient than heating water directly w/ the boiler. I can't see how it could be more efficient. My gut says the boiler-heating-water-directly is far more efficient. Where I can see the HP being super is if it uses toasty warm outside air to heat water. Also, if the heating element in the tank is hooked to the PV system, then the sun is heating water virtually directly (minus losses in the inverter, which, if in a conditioned space, may be adding to the btu's the house needs anyway, so no loss at least part of the time). So, until I get differently educated, it looks like for my situation that using PV to generate electricity is going to be a very useful function of it. The elect can be used for lights or heating water. I have more leg work to do, but that is where my brain is at this point. BTW: AK does not yet give gov't rebates on this stuff, but if the Feds still do it when I need a system, great.

  34. User avater GBA Editor
    Martin Holladay | | #34

    Response to Jonathan Teller-Elsberg
    You wrote, "I feel like the question 'where does the heat come from that the HPWH relies upon?' hasn't been sufficiently answered." The short answer is clear -- "ambient air" -- but the long answer is, "it's complicated." I agree with you completely that the current lack of data and clarity is unfortunate.

    I could have posted some calculations as if the question were simple and settled -- but it isn't. I'd love to see some data from researchers who have performed monitoring studies, and I feel that eventually we will have more data -- but even so, such monitoring studies will be complicated and hard to set up. This isn't a simple question.

    Pretending the question is simple and settled would be tempting but inaccurate. I invite anyone with good data to share it here.

    Marc Rosenbaum made an excellent stab at answering the question in an answer published in the Q&A column of the March 2012 JLC, and even Marc (a numbers guy if there ever was one) answered the question in general terms: "This is probably a good [HPWP] application ... [While] this would not be a good choice."

    Clearly, a HPWH transfers heat from the ambient air and uses the heat to raise the temperature of the water in the tank. In some houses, and some installation locations, this means that a HPWH doesn't make a lot of sense. In other houses, and other installation locations, a HPWH can be an excellent choice.

    The factors to consider include: the size of the room in which the water heater will be located, the temperature of the room, whether or not the room is in conditioned space, whether or not a drop in temperature in the room will cause any comfort problems, and the type of fuel and the efficiency of the equipment used to supply space heat.

    Note that my article also includes calculations that indicate that even homeowners who use electric resistance elements to produce domestic hot water might consider the use of a PV system more sensible than the use of a solar thermal system.

  35. User avater GBA Editor
    Martin Holladay | | #35

    Response to Rich Cowen
    I don't have any direct experience with Secusol equipment, but Alex Wilson recently wrote a blog review of the equipment for GBA. You can read his review here: German Innovation in Solar Water Heating.

  36. User avater
    Dana Dorsett | | #36

    Lots of fuzzy math regarding heat pump water heaters
    The efficiency of the HPWH as specified by an EF test is at a ~65-70F near-tank ambient. In the case where you're talking about running it in a 50-55F basement the EF won't be nearly as optimistic for two reasons.

    A: (and primary), the delta-T between tank water & ambient air is now 10-15F higher than at test condition, which reduces the operating efficiency of the heat pump.

    B: The standby loss from the tank (and near-tank plumbing) is now at least 20-25% greater (assuming 120F setpoint

    Operating in non-conditioned space @ 50-55F would result in a considerably more severe hit on efficiency than the conditioned space room volume problem. See figure 7 in this document:

    I'd be shocked if it actually breaks 1.5 for in-situ EF in a 50-55F basement, which is dramatically less than labeled.

    The other fuzz-factor is the expressed notion that having no impact on comfort is the same as having no impact on space heating load when reducing the basement temp from 55F to 50F. In the typical New England basement there is very little effective insulation or air sealing between the basement and first floor, and a 5F reduction in average basement temp would represent a significant increase in average heat load, even if it didn't change the comfort level anywhere in the house. Even if the whole-assembly R of the separating floor were R10 (U0.1) and the upper room was only 65F at floor level, in 1000' of first floor bumping the delta-T from 10F (55F basement) to 15F (50F basement) you're going from 1000BTU/hr to 1500BTU/hr heat loss through the floor to the basement, a 500BTU/hr increase. In 2000 hours of heating season that's still 1 MMBTU or 10 therms. In a more typical ~R1 uninsulated floor that's on the order of 100 therms.

    Clearly better models are called for, but free lunch is almost never truly free. When mini-split type hot water heaters arrive that take the heat solely from outdoor air the modeling becomes much simpler, but the delta-Ts can also be quite high in US climate zones 5 & up, taking a big bite out of the average efficiency compared to zones 3 or lower.

  37. User avater GBA Editor
    Martin Holladay | | #37

    Response to Dana Dorsett
    It's not fuzzy math; it's monitoring data.

    Robb Aldrich's NESEA presentation discussed an evaluation of 14 HPWHs installations for National Grid, NSTAR, & Cape Light Compact. They were installed in basements.

    The G.E. water heaters had an average COP of 1.82, mostly because they had small tanks. The A.O. Smith water heaters had an average COP of 2.13. The Stiebel Eltron water heaters had an average COP of 2.35.

    Cold basements had lower COPs than warm basements. No basement stayed cold all year long, however. Summer basement temperatures were warmer than winter basement temperatures.

  38. User avater
    Dana Dorsett | | #38

    So I guess I AM surprised...(response to mholladay)
    ...that the monitored in-situ COPs were actually that high when operated in cool/cold basements!?! And was that strictly COP, or was it effective EF (including standby &/or distribution losses)?

    Did (or how did) they measure the space heating load effects related to basement operation of said water heaters? Without the true net-energy use to the house any simple (or even monitored) COP is still in the fuzzy zone, since it's not an isolated system unto itself whenever heat is drawn from anywhere inside the thermal envelope of the house. Measuring this can be difficult or awkward but it doesn't mean it can be discounted or ignored simply because it didn't result in a comfort issue. (I'm pretty much in sync with Marc Rosenbaum's take on it as written up in that JLConline piece.)

    Is any of any of Robb Aldrich's NESEA presentation (or the data behind it) available on the web?

  39. Aj Builder, Upstate NY Zone 6a | | #39


    No soup for youze guys and your slanted studies.

  40. User avater GBA Editor
    Martin Holladay | | #40

    Response to Dana Dorsett
    As far as I understand it, Robb Aldrich measured the COP of the HPWHs in his study by monitoring hot water draws at the water heater and electricity use by the water heater, as well as incoming water temperature and outgoing water temperature. Therefore the calculation included standby losses but not distribution system losses.

    Q. "Did (or how did) they measure the space heating load effects related to basement operation of said water heaters?"

    A. As far as I know, they didn't. One of Robb's slides pointed out, "Hard to predict specific impacts on space heating."

    Q. "Without the true net-energy use to the house any simple (or even monitored) COP is still in the fuzzy zone, since it's not an isolated system unto itself whenever heat is drawn from anywhere inside the thermal envelope of the house."

    A. I agree! Let's hope more data come down the pike in the future. In the meantime, HPWH skeptics in northern states can still adopt the PV + electric resistance water heater path.

  41. User avater GBA Editor
    Martin Holladay | | #41

    Response to AJ Builder
    One more time... you don't have to install a HPWH if you don't want to. I nevertheless urge you to consider PV + electric resistance water heater before you decide to spend thousands of dollars on a solar thermal system.

  42. Keith Gustafson | | #42

    I dunno if it is any crazier to replace the basement heat loss with fossil fuel use than heating the basement with resistance electricity, which is what happens when you use an electric water heater in a basement normally. Where does one think the heat escaping the tank and pipes goes anyway?

    Since at the current state of electronics I cannot get an object as simple as a coffee maker toaster or digital camera to last more than a year,I have doubts that current pv [or HPWH] customers, will have the astounding good fortune that Martin has had.

    I saw these guys selling systems that look somewhat reasonable:

  43. User avater GBA Editor
    Martin Holladay | | #43

    Response to Keith Gustafson
    I wasn't the only person buying PV modules in the early 1980s. Trust me -- my own experience is not unusual. I have several friends who all bought PV modules soon after I did (a bunch of us live off-grid in northern Vermont), and not a single one of my friends has had any PV module failures. My first inverter (a Trace model) lasted 20 years.

    On the page you linked to, the inexpensive solar thermal systems all have very small tanks. The systems with a reasonable tank size start at $3,350 (for a system with a 105-gallon tank) or $3,875 (for a system with a 132-gallon tank).

    Add a few hundred dollars for shipping. Then double the materials cost to cover labor and installation charges; add profit and overhead. That sounds about right -- $8,000 or $9,000 installed.

  44. Keith Gustafson | | #44

    Wasn't trying to imply your experience was unusual, I have equipment from the 70's and 80's that still works. I just am wondering aloud as it were if that experience is going to be repeatable going forward

  45. Robert Del Mar | | #45

    Solar Thermal still has a pulse
    I have to agree with much of your logic but - but... I thought I would lend some perspective from the Oregon market.
    The Oregon market bears witness. In 2011 there were less than 100 solar water heaters installed in the state compared to more than 1200 PV systems (avg size ~3.6kW avg cost ~$6/watt). But the “non energy” economics are stacked heavily in PV’s favor. In Oregon the state tax credit and utility programs provide about four times more financial incentives for PV than a for a solar thermal system with comparable energy production. The result has been many homeowners paving their roofs in PV.
    I was one of them …but I have some buyers remorse. I was able to fit 2.8kW of PV on my modest roof generating about 3100 kWh annually. Had I saved 64 square feet for thermal I could have increased my total collection by more than 60%. I would have had a 2.3kW PV system and a solar water heating system that together would generate about 5000 total kWh annually. Like many I have a gas water heater and a new baby (ie plenty of load…).
    Moving parts / reliability: I would put my money on a good bronze circulator over most inverters and all heat pump water heaters.
    Cost: I wont belabor but 4.10 a watt may be a "China-is-dumping" rate and we may not continue to see such steady declines in PV cost.
    The PV / HPWH is an exciting prospect with potentially compelling economics but may not be the best societal solution in the long run. Solar thermal is too expensive right now but in a world of limited roof space we need to apply the defibrillator to the solar thermal market to drive costs down and save some roof space for the technology with twice the energy density of PV. After all, in much of the world it is possible to install solarwater heating for $1,000 or less.

  46. User avater GBA Editor
    Martin Holladay | | #46

    Response to Robert Del Mar
    I like bronze circulators too, but they aren't cheap, and they do eventually fail. It's amazing how tough and long-lived PV modules are.

    You seem concerned about the area of your roof required for PV modules compared to solar thermal collectors, so I did the math. In fact, the required area is exactly the same. In Boston, a solar thermal system with two 4'x8' collectors (64 sf) produces 63% of a family's hot water use (an average of 28 gallons daily out of the family's daily use of 44 gallons). To make that much hot water with a HPWH with a COP of 2, you would need a PV system rated at 0.7 kW. Since a PV array in Boston produces 0.037 kWh/sf/day, such a PV array will measure 64.5 square feet -- almost exactly the same size as the solar collectors.

    If you use an electric resistance water heater instead, you will, of course, need a larger PV array -- one measuring 129 square feet.

    Concerning your observation that "in much of the world it is possible to install solar water heating for $1,000 or less," I might answer, "So what?" If you're talking about Africa and India, where temperatures never drop below freezing and labor is cheap, you may well be right.

    But your statement is similar to the observation that gasoline costs 12 cents a gallon in Venezuela. That's true enough, but it doesn't help me in Vermont.

  47. Chris Herman | | #47

    PV cost
    I would love to know how to get a good PV system for $4.54/watt. Maybe if you are a dealer/installer you can get it that cheap. The proposals I have seen are no quite that inexpensive. Maybe if you buy chinese modules. I prefer not to.

  48. Tom Gocze | | #48

    When you're hot....
    Hi Martin,
    You do know how to get comments.
    I have to say that I agree with you to some degree.
    The fact that PV's are basically an electronics technology make them somewhat susceptible to Moore's Law although it has taken many years and the Chinese to drive prices down to this threshold.
    A dollar a watt for PV's is a game changer. No great insight there!

    I suspect that we will see heat pumps integrate with PV systems in all climates over time. The simplicity of grid-tied systems is hard to ignore.
    As one who has dealt with his share of leaks and failures, it is appealing to think about having simple systems.
    I think that it still is premature to call solar thermal dead, though. There will be someone who sells a low cost consumer system that will show up in mass marketer's stores. It might only be a Fafco system but there is nothing wrong with that. Of course, the cost of the Fafco system is a little stiff for what it is, but this will change.

    We have messed a lot with low cost systems over the years. My sense is that we will have a very low cost polymer system that will function inexpensively in cold climates.

    We have been working with Gary at on low cost thermal systems. Solar thermal does not require China Inc to make the collectors. If it is black and in the sun, it usually works. And making a black thing to put in the sun can be really inexpensive. We all know that.

    Any solar thermal heating system will (as would any PV/HP system) have to be working with a low energy building.

    I can say in my own case, living in an 1100 square foot antique Cape Cod home on the coast of Maine that is basically R-65 thermal shell this is do-able. Our usage of oil was 250g for heat and hot water when we used oil.
    We now use 1-2 cords of wood in a wood boiler with a storage system and a HP for DHW in our brief summer.
    My guess is that 200 sq. ft. of solar thermal collectors would cover us annually for heat and hot water, if it was sunny every other day. That can be an inexpensive system.
    My sense is that at this point, if I was doing another house, it would be similar thermally with 3-5kw grid tied, along with a 1 ton split system for backup to the wood/solar system. (I have been using less than an 1/8 of a tank of oil for backup for the past three years.)

    I suspect (and hope and pray since we manufacture thermal storage systems!) that a thermal storage system tied to a cold climate air source heat pump "boiler"
    is certainly do-able and something that a utility's demand side management scheme would love.
    Nyle Systems up here in Maine has several air source cold climate units waiting in the wings.

    At least I would still be making tanks!
    Tom Gocze
    American Solartechnics

  49. User avater GBA Editor
    Martin Holladay | | #49

    One more point about solar thermals systems
    I can't help adding another point about solar hot water systems: many of them are performing much worse than the homeowners think (whereas PV systems usually provide about as much energy as predicted).

    I'm reading a book called "Trail Magic" by Carl McDaniel. He tells the story of the design and construction of a net-zero energy house in Ohio. He had an energy consultant and a LEED consultant -- everybody he should have needed to make good decisions. The house included an evacuated tube solar thermal system.

    To his credit, McDaniel monitored the performance of the system for six months. This is what he discovered: "A mere 7 percent of the energy used to provide hot water comes from sunshine (28 kWh or $2.80). ... The pumping of fluids in the solar hot water system uses a substantial amount of electricity, 200 kWh annually. ... After two years we had the evacuated tube system removed."

  50. Frank Flynn | | #50

    3 thoughts
    - I have a 1970's vintage drain back Solar Hot Water system; it is still working well (although it has had maintenance over the years). I live in the SF Bay area and it produces essentially all of our needs (for 3 teenagers, my wife and I) taking perhaps 100 Kwh a year for the back up heater which I do meter.

    - I was thinking of using an absorption chiller in the summer with the excess heat - the problem being they are not really available for single family homes and are expensive. Perhaps the answer is not to worry just put up enough PV to run an efficient conventional AC.

    - Even if your number are off a bit the fact the the number are even close is remarkable. PV prices are going down while Solar Thermal prices are somewhat stagnant. Now I have the dilemma that if I install a PV system today a newer better and cheaper version will be available before I get pay the bill.

    What will really get my attention is an article arguing that the "clothesline is dead" because not spending the $20 on clothesline, clothespins and 2 hooks and spending that $20 on PV will produce enough electricity to run my dryer all year.

  51. Frank Flynn | | #51

    1 last thought
    These number work because you can use the grid as storage for your surplus electricity. This is true today most places here in the US anyway. But this will most likely change at some point in the future as the Utilities point out that they are providing you a service that you are not paying for. So far the number of PV installations is small and it's not a problem but as this number rises it is likely the "Net Metering" will have to go away in favor of an energy tariff.

    Today Net Metering makes it easy to store surplus electric for 6 months or so but if there's a fee for that and who knows how that fee would be structured it might change the equation to give Solar Hot Water benefit because you can store hot water for a day or two fairly easily.

  52. Ice Rabbit | | #52

    HPHW in unheated basement in ME does fine
    First. Thanks for the article.

    We have pondered about going solar and/or wind, but our site and location are not optimal. The terrain and house don't make for an easy installation and it did not seem like solar hot water was ever going to pay off. >$10k is a lot of money for a system that in the winter probably can't produce enough heat to thaw out, so to speak.

    Second, I wanted to chime in with a personal experience, after one reader posted a HPHW should never be used in a norther climate, etc.

    We replaced our conventional hot water heater last spring with a heat pump hybrid model and have seen lower electric bills, generally speaking. It is located in the utility basement that only gets radiated heat from hot air duct work and the boiler. Insulation to the outside is minimal, to the rooms and spaces above is none and under the concrete is granite ledge. The HPHW has not had any noticeable effect on our heating situation. The basement has been 55. 50 and recently 47.

    I did obviously notice it took longer to recover hot water during the winter then the summer, but as far as we are concerned it should still be more efficient then a conventional heater. Even if it is only partially more efficient (1.25?? vs 2.0) in the winter, so be it.

    I can't quantify any savings this winter, because
    a) our meter has a factor 80 multiplier, so we don't get accurate readings month to month. Our meter needs to turn 80 times to count as one or runs at one 80th speed (however they do it) and so we do get sometimes where the use won't add to 1 or then the next time jump over 2.
    b) Hot water use and other big draw units (oven, hot tub, ...) depends on occupancy and has varied month to month, year to year.
    c) we had this significantly warmer winter, which meant we were able to use more wood heat and used less oil ... about half of last years amount ... which meant a lot less heat going into the basement etc.

    Overall, I think a hybrid water heater is a good thing ... and I will agree that it may be a gamble as far as longevity / durability.

    It does have a good side effect of de-humidification. Something that basements typically benefit from. So you don't have to use a dehumidifier as much.

    I would advise against installing it in a living space, because it is quite audible when the heat pump is running. It also needs a minimum number of square / cubic feet to breathe, so you can just lock it in a closet. At a minimum it would have to have a pair of louvered doors and I think the sound would just be amplified in a small space. And, I doubt you could insulate well enough against it. Sound control isn't as simple as installing insulation.

    I think a HPHW might be ideal in a garage in a southern climate, where it doesn't need to heat the water that much, has unlimited heat to draw from and can combat humidity a little.

  53. User avater
    John Semmelhack | | #53

    PV prices, Chinese modules, and the Grid
    Yesterday, my utility company installed the "smart net-meter" (as opposed to the previous "smart" meter) at my house so I could turn on the 6.2kW PV system that will bring my household to slightly "plus energy" (site annual). The price for the system was $4.32/Watt. The panels are made by Q-Cells, which has manufacturing facilities in Germany and Malaysia...not sure where mine came from. In addition, my church is currently installing a 14.6kW system with panels made by Sharp in the good ol’ U-S-A. The price was $4.60/Watt. PV does not require China Inc. to make the panels.

    Frank Flynn wrote regarding the grid: “These number work because you can use the grid as storage for your surplus electricity. This is true today most places here in the US anyway. But this will most likely change at some point in the future as the Utilities point out that they are providing you a service that you are not paying for.” Yes, I do not pay the utility company for using the grid as storage. Also, my utility company will not pay me a premium for the valuable energy I will send to the grid when they need it the most. That the utility company thinks the current setup is a fair deal is a testament to how much it is not.

  54. RICHARD SCHMIDT | | #54

    Hooray for Lo Tech Solar
    I always get a kick out of setting up straw man to make a point -- like comparing expensive PV-heated water to expensive solar heated water. There's another way -- a classic tank-type (breadbox) solar water heater. I put one on my house in 1983. Building it myself would have cost about $350, not the $10,000 quoted in Martin's article for a "current" tube-type collector system. (It consists of a tank like one from inside a water heater, an insulated glazed box propped to an appropriate solar angle, and piping to connect with my gas water heater.) The heater is really a pre-heater, but it heats water to scalding in summer, and can be disconnected and drained in freezing weather. Even if it only warms 50 degree water to 80 degrees, that's a lot of Btus captured from the sun, which is free, and a lot fewer that need to come from natural gas or electricity. In the 29 years since installing the thing, I've had one repair -- a faulty PTR valve that dripped a bit of water, which I replaced myself for about $5. There are no moving parts, no energy consumed beyond its embodied energy. These heaters were common as flies in the early 20th century -- a big improvement over boiling water on the stove for bathing or having a dreadful "boiler" baking everyone in the kitchen. Then cheap natural gas caused the market for them to collapse. This totally passive solar water heating technology is still great -- but I guess it's not high-tech enough to attract much attention from today's techie greens.

  55. Kevin Dickson, MSME | | #55

    Installation Logistics Favor PV, WREN is Coming
    Almost 3 years ago I wrote on this same subject:

    The highlights:

    1. Running the pipe for SHW is usually more difficult than the wiring for PV, and finding an appropriate place for the panels is easier with the smaller and lighter PV panels.
    2. As Martin just mentioned, actual field SHW performance is often disappointing.
    3. An insulated two-car garage with a perimeter insulated slab is a well-matched geothermal energy source for an HPWH

    All this is, however, is before the potential SHW cost breakthroughs I mentioned in comment 19.

    The "smartest guys in the room" are convening soon at the World Renewable Energy Forum here in Denver:

    Advancements in RE Technology, 5/14/2012 4:15pm - 5:30pm

    FORUM - Radically Reducing the Cost of Solar Water Heaters
    Jay Burch

  56. User avater GBA Editor
    Martin Holladay | | #56

    Response to Richard Schmidt
    I'm not a "techie greenie." Before my house had a water heater, I used to take outdoor showers during the summer with water from a garden hose that heated up in the sun. I love low-tech solutions, and I'm delighted to hear about the success you've had with a breadbox solar water heater.

    In New England, contractors charge $8,000 to $12,000 to install a solar hot water system in an existing house. Although you assumed I was talking about an evacuated-tube system, contractors are charging about the same price for a system with two flat-plate collectors.

    Breadbox heaters won't work in cold climates. They're a great solution in Florida, but most homeowners don't have the time, discipline, or knowledge to keep an eye on the thermometer and drain the system seasonally before it freezes. I have friends who installed a flat-plate collector on their roof, hoping to use the "remember to drain it before it freezes" method of freeze protection. Needless to say, they ruined the collector.

    Low-tech solutions work fine for the right homeowners. But it is unrealistic as a matter of policy to advocate solutions that require seasonal draining and keeping an eye on the thermometer.

    The fact is, PV systems have fewer problems and provide more dependable energy performance than solar thermal systems. I wish it weren't true, because I'm an old hippie who remembers the "get a 55-gallon drum and paint it black" days -- but it is.

  57. User avater GBA Editor
    Martin Holladay | | #57

    Response to John Semmelhack
    Thanks for sharing the price of your PV system ($4.32 / watt) -- one more job to add to the growing list of PV systems installed for under $4.50 / watt.

  58. User avater GBA Editor
    Martin Holladay | | #58

    Response to Frank Flynn
    You propose a new worry: namely, that today's net-metering arrangements may change in the future. Of course, you're right -- they may. But all kinds of economic factors may change; that doesn't prevent us from making informed decisions today based on current conditions.

    In the future, we may see global water shortages, steeply increasing oil prices, low natural gas prices, a copper shortage -- or the opposite of all of these predictions.

    For the time being, I expect net metering contracts to be honored for at least 10 and probably 20 years -- the type of type horizon used for most water-heater decisions. Of course, I could be wrong. But I don't recommend that any of us make economic decisions that assume factors that are contrary to the current situation -- life would just get too complicated if we did. Moreover, our predictions could easily be wrong.

  59. Kevin Dickson, MSME | | #59

    Fun Factor
    I'm not quite sure how much value can be attributed to it, but I can tell you that a solar thermal system is several times more fun than a PV system. PV is DEAD boring.

    HPWHs could be made more fun for mechanical engineers if there were more sensors onboard and more data shown.

  60. Don Mallinson | | #60

    Solar Thermal IS dead
    My approach to hot water is to use a Stiebel Eltron Tempra Plus 24 which is a nearly 100% efficient, on-demand electric water heater. No venting required in my air tight envelope. Besides, I produce my own electricity via a 5.4kW PV system. There was no room for a proposed 1,500 gal tank in my 1,300sf slab-on-grade house. A SHWS seemed un-necessarily complex and pricey by comparison.

  61. User avater GBA Editor
    Martin Holladay | | #61

    Response to Kevin Dickson
    "PV is dead boring"? Clearly, you need to buy more meters for the PV system. Put several digital and analog meters -- both ammeters and voltmeters -- on your living room wall. They are fun to watch.

    If you need more fun, hook up some buzzers or bells that go off at the high end of your array's output, so that the bells only go off on an unusually productive day -- a cool March day with a few cumulus clouds, for example. More fun! Check the meters! Oh boy!

    Or maybe you should install a remote video camera on your utility meter outdoors -- so you can watch the meter spin backwards on your living room TV.

  62. Leslie Baer | | #62

    Whoa... Look at where you are before you leap to this conclusion
    ST certainly faces some challenges -- not the least of which has been inequitable subsidy structures for renewables. However, NREL itself has substantiated that certain parts of the country are ideally located to improve the economics of SHW systems. Colorado is one of those locations, where lots of sunshine and cold ground water conspire to make ST one of the most economical heating propositions in the state. That is especially true for certain kinds of commercial applications (as pointed out in the article) and for residences and businesses currently heating with electricity or propane. In these cases, the systems can be looking at a ~6 year payback and a respectable ROI, not to mention independence (or reduced dependence) from volatile fuels costs.

    Further, that target of $2,000-3,000 for systems costs is actively being pursued by NREL researchers, with marketable systems in this price range expected in the next five years. Finally, we all know that natural gas prices will rise -- eventually. Even if the big plays in the center of the country pan out, the infrastructure will be built to transport it to the high-priced coastal markets, evening out costs and, as a result, raising them in states like Colorado. As those prices rise, ST system prices come down, it will make even more sense for folks like me with hot water heat and domestic hot water needs to pursue ST.

    So, ST may be certainly be in stasis in some parts of the country, but in Colorado it makes good economic sense for 25-30% of our residents. Check out the Colorado Solar Thermal Roadmap for more details on this perspective:

  63. Graham Irwin | | #63

    I don't get it
    Perhaps I'm missing something, but I can't see how the physics behind this works out:

    1) PV vs Solar Thermal Output

    PV (optimistic estimate for Northern California, which is pretty sunny): 1500 kWh/kW/yr / 100 ft2/kW = 15 kWh/ft2/yr
    Solar Thermal: 1000 BTU/ft2/day / 3413 BTU/kWh x 365 days/yr = 107 kWh/ft2/yr

    Even with the COP of the heat pump and the fact that the solar thermal output can't be utilized at 100%, etc, this is a big efficiency gap. Further, solar thermal is less affected by clouds, sub-optimal orientation, etc. In the quote above about a "properly sized" solar system, the description of "properly sized" is debatable - I think this is referring to a pressurized glycol system that is sized for 100% summer input deliberately to avoid overheating of the fluid. With a standard drainback system (simpler, cheaper, lower maintenance, freeze and overheating protected, more efficient) we are seeing 70% of the year's DHW load and with another panel (+$1500) we can get 90% of DHW and space heating in our Passive House projects. This is northern California, granted, but it would take a lot more PV to do this. Europeans I've spoken with feel that solar thermal is overpriced in the US, mainly due to low market penetration and the fact that each system is largely custom built, rather than "installed" on site. As you lower the overall heating demand, the size of the required solar thermal system decreases and the usable fraction goes up at the same time. Efficiency is great for renewable energy systems!

    Heat Pump Water Heater

    Volumetric Heat Capacity of Air (25ºC/77ºF): 0.001297 J/cm3/K
    Volumetric Heat Capacity of Water (100ºC/212ºF): 4.216 J/cm3/K
    4.216/0.001297 = 3251, so you have to cool 3251 x the volume of air to raise an equivalent volume of water by the same amount

    There are 7.48 gallons/ft3 and a 2000 ft2 house would have about 2000 x 0.75 x 8 = 12,000 ft3 of air inside, so I can heat 12,0000/3251 x 7.48 = 28 gal of water 1º by cooling the entire house by 1º.

    If we assume this 2000 ft2 household uses 28 gallons of 120ºF water each day (conservative) and the water comes in at 60ºF (optimistic), we need to cool the house by 60ºF every day to supply the DHW. This could be great in summer, but pretty rough in winter time.

    By most accounts, our society must move toward an energy infrastructure that is more based on renewables and less on fossil fuels. As such, we ought to work toward strategies that are compatible with this, and emphasize load reduction WITH generation over load shifting alone. In European countries where there is a high degree of renewable electricity generation, they are already experiencing problems with this. If there are economic subsidies in place that steer people in illogical directions, it's money unwisely wasted, IM(H)O.

  64. User avater GBA Editor
    Martin Holladay | | #64

    Response to Leslie Baer
    Sorry, I don't buy it. Sunny regions of the country like Colorado also benefit from a higher PV output compared to Vermont -- not just a higher solar thermal output. And I don't care how often NREL mentions its target of finding mythical contractors willing to charge $2,000 to $3,000 for an installed solar thermal system -- repeating a target over and over again doesn't make it a reality.

  65. User avater GBA Editor
    Martin Holladay | | #65

    Response to Graham Irwin
    Your largest error is your assumption that all of the thermal energy collected by solar thermal collectors is usable. It isn't. This stands in stark contract to a PV system, since all of the electrical output of a PV system is usable.

    Very few homeowners or solar thermal installers have actually monitored a solar thermal system for a full year, and even fewer have done so accurately. To do so requires a water meter on the hot water tank; sensors to record the temperature of the incoming water and the outgoing hot water; and a gas meter or an electrical meter on the backup water heater. Those who have done this exercise find that most solar thermal equipment installers exaggerate the solar fraction provided by the equipment and underestimate the fuel used by the backup water heater.

    Moreover, many homeowners forget to include the parasitic energy use required to run solar thermal pumps when making these calculations.

  66. Graham Irwin | | #66

    What Error?
    I said "Even with the COP of the heat pump and the fact that the solar thermal output can't be utilized at 100%, etc, this is a big efficiency gap."

    The solar fraction I reported was based on monitoring by LBNL.

    You seem to be ignoring the parasitic heating load for running the HPWH in winter in your analysis. Thoughts on this?

  67. Katy Hollbacher | | #67

    Load reduction vs. load shifting
    I've been a big fan of PV w/ heat pump water heaters but was taken aback at the idea that PV with electric RESISTANCE might now make economic sense. What a notion; has definitely thrown me for a loop. I buy it, but the two potential issues I see are:

    1. for the paranoid bomb-shelter types out there, they'd rather have a solar thermal system that can still (potentially) deliver hot water when the grid's out (pumps can be run off batteries)

    2. as Graham alluded to, Germany's got a problem w/ TOO much PV, eg:

    "... (solar) capacity generates nothing at night, while still putting 1 MW into the grid at noon on a bright summer day... The difference affects how much backup capacity must be available to the grid and likewise how much other capacity must be taken offline as solar output ramps up daily and seasonally"

    We're sadly a long ways off from this issue in the US, but on an infrastructure level it needs to be considered. Maybe the next things to subsidize are batteries?

    Thanks for the new topic--it sure has inspired great discussions!

  68. User avater GBA Editor
    Martin Holladay | | #68

    To Graham Irwin: about your error
    Your error is that you are greatly overestimating the useful solar thermal output of a solar hot water system, because you are basing your thermal calculations on the theoretical maximum output of the collectors instead of monitoring data.

    You estimated that a solar thermal system in northern California will produce 107 kWh/ft2/yr of thermal energy (the area refers to the area of the solar collectors). That may be true, but no monitoring study that I know of has come up with a number like that for actual hot water used by a family in a residential installation.

    Here are the numbers for two monitoring studies I know of:

    1. Robb Aldrich and Gayathri Vijayakumar (of Steven Winter Associates) analyzed data from two residential solar thermal systems: one in Hadley, Massachusetts, and one in Madison, Wisconsin. Aldrich and Vijayakumar reported their findings in a paper, “Cost, Design and Performance of Solar Hot Water In Cold-Climate Homes,” presented on July 12, 2006 at Solar 2006, the American Solar Energy Society conference in Denver, Colorado.

    Their results: the average output of the two systems (solar energy used by the families) was 36 kwh/ft2/year.

    2. The second monitoring study I'm aware of is “Performance Results from a Cold Climate Case Study for Affordable Zero Energy Homes,” by Paul Norton and Craig Christensen of NREL. The paper was presented at the ASHRAE Winter Conference in New York City in January 2008. Norton and Christensen report one year of performance data for the zero-energy Habitat for Humanity house in Wheat Ridge, Colorado.

    Their results: the output of the system (solar energy used by the family) was 6.7 kwh/ft2/year. If you subtract the parasitic energy used by the pump from the solar thermal output, the output drops to only 5.7 kwh/ft2/year.

    [For more information on this topic, see the table in my comment below, after my response to Katy Hollbacher.]

  69. User avater GBA Editor
    Martin Holladay | | #69

    Response to Katy Hollbacher
    1. You advised "paranoid bomb-shelter types" to stick with a solar thermal system. I wrote something similar: "Solar thermal systems still make sense for off-grid homes."

    2. You wrote that "We're sadly a long ways off from this issue [having so much installed PV that it's hard for the grid to handle] in the US." I agree -- we are a long ways off from having that problem here.

  70. User avater GBA Editor
    Martin Holladay | | #70

    Energy output per square foot
    Both Robert Del Mar and Graham Irwin have raised questions concerning the energy output per square foot of collector, the main point being (they claim) that if you have a limited area on your roof, you can get more BTUs per year from a given area of solar thermal collectors than PV modules.

    It's an interesting question, but frankly not that important for most Americans. However, if the area of your south roof is small, it's worth doing the calculations.

    In general, you get more BTUs per square foot with a solar thermal collector -- usually, but not always. There are a couple of important factors to remember:

    1. If you are using your electricity production to operate a HPWH with a COP of 2.0, you can double the kWh figure for PV when comparing these two technologies.

    2. The useful energy produced by a solar thermal system varies widely. The two most important factors are climate (sunnier climates produce more than cloudy ones) and the number of gallons of hot water used by the family. High-use households obtain more usable energy from their solar thermal systems than households that conserve hot water.

    The table below summarizes the data under discussion. The referenced research reports are the following:

    1. Robb Aldrich and Gayathri Vijayakumar, “Cost, Design and Performance of Solar Hot Water In Cold-Climate Homes,” presented on July 12, 2006 at Solar 2006, the American Solar Energy Society conference in Denver, Colorado.

    2. Paul Norton and Craig Christensen, “Performance Results from a Cold Climate Case Study for Affordable Zero Energy Homes,” presented at the ASHRAE Winter Conference in New York City in January 2008.

    Click on the image below to enlarge the table.

  71. Robert Del Mar | | #71

    Response to Response to Robert Del Mar
    Hi Martin,
    Agreed. Most PV modules last a lifelltime. I was puting my bronze circualator against the HPWH not the PV module. I know a HPWH fan who is on his third unit in 2 years. But I don't wish to use specific examples of failures to bring down HPWH's which I believe are a promising viable technology for many homes. As are Solar water heating systems.

    The reduced space requirement for PV is valid for homes that have limited sunlit roof space. If a homeowner only had 70 square feet of sunlit roof I doubt they could install a 0.7kW system for less than about $5,000. This solution with a HPWH would be about the same cost as a solar water heater with about the same number of moving parts.

    I believe lessons from low cost systems in other parts of the world will deliver freeze tolerant SWH systems for less than $5,000 for the US market.

  72. Graham Irwin | | #72

    No Error!

    There are, as I see it, (at least) four aspects to this discussion: 1) a physics aspect, 2) an engineering aspect 3) an economics aspect and 4) common sense. My own tendency is to look at the physics first, as a guide to what's possible from an engineering standpoint and to what's sensible from an economics standpoint. When I evaluate a proposal, I look for all four aspects to align. If not, I need to go back and examine where the various aspects may diverge or conflict.

    The physics says that much more energy is available per square foot of collector for solar thermal than PV, but also that the output of either is much higher in summer than winter. The economics says that solar thermal collectors are much cheaper per square foot than PV as well.

    The engineering issue is how to utilize the energy available. With PV, the best option is to grid-tie, so that summer surplus can be used elsewhere, and one can offset winter fossil fuel costs. For solar thermal, if one doesn't have large seasonal storage (an expensive solution for many situations outside of extreme climates like Fairbanks, AK) there is no option but to accept less than 100% utilization of available energy. In a solar thermal system that has glycol in it for freeze protection, the typical approach is to size the collectors for 100% summer solar fraction so that the glycol never overheats. The physics tells us that this means that the winter output is much lower than that, as was alluded to at the beginning of your article. A system sized for 50% summer solar fraction would have an even lower yearly output, and so on. As I described in my original "erroneous" posting, we wouldn't describe this type of systems as "well designed," which is why we engineer drainback systems, which do not have this overheating issue. This allows us to cost optimize the solar thermal collector array for yearly solar fraction not glycol overheating. When this is coupled to a high efficiency building like a Passive House, the results are significantly better than the reports you allude to. Since our goal is high performing, low energy use projects, we find that solar thermal for our DHW is a good idea, and that amortizing the cost of the tanks, pumps, etc. over a larger collector array that also delivers a good deal of space heating makes economic sense, even when PV is also included in the project. Thermodynamics tells us that the least complicated system is best, meaning that the energy is transformed the fewest times, ie sun to water directly.

    Anyhow, back to your proposal. Even if I accept the premise that it makes economic sense for people in northern climates to run air conditioners in their basements all winter to heat their hot water and then net meter away the cost of the electricity to run the heat pump and the extra heating fuel with summer PV production, it makes no sense to me from a physics standpoint, nor, for that matter from a common sense standpoint.

    Your point may not be that this is a good idea, perhaps it is that current incentives for PV are producing logically perverse results. This was not clear from your article, which is why I was asked by someone to respond. As I said at the end of my first posting, if there are subsidies available to make such an approach economically feasible, it is, in my opinion, money wasted, and it is doing society harm.

  73. User avater GBA Editor
    Martin Holladay | | #73

    Response to Graham Irwin
    You haven't presented any information that contradicts anything I have said. Regardless of the theoretical calculations made on paper, in the real world, without subsidies of any kind, it costs less to heat domestic hot water with PV than with a solar thermal system. That's because PV equipment is cheaper for an equivalent output of energy. PV systems also have fewer maintenance issues, by far. Moreover, poorly designed solar thermal systems are far more common than poorly designed PV systems (although I've seen both).

    Paper calculations about the summer energy production of solar thermal collectors aren't relevant if you can't use the energy and if you can't afford an insulated tank to save it for winter. To determine how much hot water from a solar thermal system is actually usable, the only relevant data are monitoring data from installed systems.

  74. Michael Horowitz | | #74

    What a great discussion!
    This article has really made me think. It has made want to pay out of my own pocket to monitor a bunch of solar hot water systems for actual solar fraction, and also to monitor a few water heater heat pumps to ascertain any increased fuel use for the household heating system due to their use! Martin, you really picked a winner.

    I install the occasional solar thermal system in Vermont at what I know are very fair prices and agree that it does, generally, cost about $8000 to install a standard two-tank pre-heat, closed loop SHW system with the collectors on a roof, and the existing water heating system in the basement. The bulk of this cost is equipment, not labor. I hear tell of other "turn-key" systems in our state priced at near the $10K level. Thus, I am intrigued by your assessment. I intend to run some numbers in a spreadsheet myself, as time allows. Who would've thunk?

    There is one troubling aspect to this that I feel is still being swept under the rug. I live in Vermont, not far from you. If I were to place an air-to-water heat pump in my basement with a COP of 2.0, I know where the heat would come from. I heat my home from October 1st until May 1st - seven months of the year. During those seven months I replace BTU's that are, say, conducted through my walls, lost to an open window, sucked out of my exhaust only ventilation system, or "moved" into my hot water system by an air-to-water heat pump. Its all the same. Its thermodynamics.

    The economical implications of the COP of an air-to water heat pump inside the conditioned envelope is not the same as the economics of the COP of a heat pump outside of the envelope. Thermodynamically, or economically, COP is not a total energy equation. It is a ratio of the benefit in BTU (or kWh) divided by the "cost" in electricity to run the heat pump. If I am, as I would be in my home, moving heat that I already made (read: paid for) with my heating system, I need to include the fuel used to make (or replace) the heat in my equations. In this case the heat pump is actually inefficient, as I could have moved that heat into the hot water system directly without first heating the air, and then using an electrical heat pump to "recover" it from my heated space.

    The difference between the solar thermal and the heat pump is that the solar thermal is gathering it's heat from the sun. If we quantify the electrical requirements of the solar thermal circulator relative to the heat we collect, we can also assign a COP to the solar. The PV-HP combination is getting its pump electricity from the sun, but its heat is coming from inside the envelope where the heat will be replaced during the heating season. It's not a comfort question. Not if we are comparing apples to apples for economics. Thermodynamics still requires 25,656 BTU's to be "moved" into any water heater for 44 gallons of hot water to be heated from 50 to 120 degrees, which would cool, (at .028 BTU/ft3/degree F) my entire home, basement to second floor, of 21000 ft3 the equivalent of 44 degrees. For seven months I am producing 25,656 more BTU's from my heating system to "move" it through the air into the hot water system. That is 7.5kWh extra per day, on top of the pump energy. That seems to have slipped out of the economic equation. I cannot find either the fuel cost or, since we are comparing equipment, the capital cost of the equipment that is providing that 7.5 kWh of water heating..

    I agree the future split system makes all the difference. If the heat was actually collected by the heat pump outside the envelope it would be different, . Otherwise someone has to account for that other 7.5kWh/day. It doesn't just magically materialize. I would add the 210 days worth of 7.5kWh to the PV system, to be really fair.

  75. User avater GBA Editor
    Martin Holladay | | #75

    Response to Michael Horowitz
    Thanks for verifying that solar hot water systems cost $8,000 to $10,000 in Vermont.

    I'm glad you are intrigued enough to want to monitor the performance of a solar thermal system in Vermont. I will be astonished if you discover that a two-collector system has a higher solar fraction than 63%. Here is a back-of-the-envelope calculation of the simple payback period for one of your $8,000 solar thermal systems, assuming that we are comparing it to an electric resistance water heater using electricity that costs 14 cents per kWh:

    - The family uses 43.8 gallons of hot water per day, or 16,000 gallons per year.

    - It takes 0.171 kWh to heat each gallon of water, or 2,736 kWh per year if the hot water is all heated with an electric resistance water heater. The cost of that electricity is $383.

    - If you can displace 63% of that expense with a solar hot water system operating at a 63% solar fraction, you will save $241 per year. The simple payback period for this $8,000 system is 33 years. (Of course, if the family used natural gas instead of electric resistance elements to heat their domestic hot water, the payback period would be much, much longer than 33 years.)

    As I have already noted several times, HPWHs clearly steal space heat from a home in winter when operating, unless they are placed in a garage. If that bothers you, use an electric resistance water heater -- the numbers are still pretty good.

    Moreover, if the HPWH is put in a large basement that is not used for living space, it won't necessarily steal enough space heat to make much of a difference in your space heating costs -- it will just lower the temperature of your basement, where you probably don't hang out anyway.

    If you include the cost or the energy value of the additional space heating fuel (for some homes) attributable to the operation of the HPWH, the COP of the water heater drops (on an annual basis) from 2.0 to a different number. But it doesn't drop to 1.0.

  76. Michael Horowitz | | #76

    Issue with the details, not the conclusion

    Not arguing your main point. I did and do believe you that in some, or even most instances PV, at $4.50/watt installed can deliver hot water less expensively than SHW systems that cost $8000, with a 2/3 solar fraction (which I believe most installers estimate more like 60 gallons for the average family of 4, and the basis of the 63%). I will, as I have said, do some spreadsheet work to get to the finer mathematical details, like stand-by losses, cost per kWh, solar fraction vs. actual volume, where PV/ SHW costs reach parity, etc. Instinctively, I do have a red flag waving that says there is more to this, as I see others have, but I believe you that it is at least a major consideration and sheds doubt on the medium hanging fruit status of SHW. As a matter of a fact, because of your thought-provoking article I am now thinking of converting to all PV a combined PV-SHW system, that I am currently designing.

    What I am commenting on is analyzing HPWH as a mechanism to heat water using the COP as an economic consideration. It is also being done by the marketing departments of the HPHW's. There is a bit of smoke and mirrors here in trying to overstate the benefits. These machines are costly, and complex, and they MOVE heat. COP has its origins in external sources of free heat, like wells and ponds and ground temperatures, not internal sources of high value heat.

    I mean no disrespect, but you flip flop a bit from hard numbers about the amount of kWh it takes to heat hot water, to brushing off the heat removed from the basement or utility space, as a comfort issue "it will just lower the temperature of your basement, where you probably don't hang out anyway." Yes, I realize you have admitted that if it is living space, it will affect you, but please address my numbers with the HPHW:

    It takes ~8.33 BTU to heat each gallon of water 1 degree. 42 gallons (is there a teenage daughter in that house?) heated 70 degrees takes 24,500 BTU's or 7.2kWh. This is not insignificant and that heat in $ value needs to be added to the HPHW economics. Whether one can feel it is separate from whether one is paying for it. Using only COP is externalizing that cost. Thermodynamic calculations require that we see the transfer of heat from the adjacent floor, and the conservation of energy says that 24,500 BTU's cannot just appear, but will have to be added inside the envelope to make up for it during the heating season. That this only lowers the temperature of the basement a few degrees is because the basement is adjacent to a warmer space. As has been stated, 24,500 BTU's would lower the basement air temperature well over 100 degrees if it was removed in the course of one hour with no heat added. Practically, that does not happen as increasing the delta T increases the transfer rate from any warmer adjacent surfaces, like the upstairs floor. Whether it happens so no one can sense it is not the issue. If the heat pump moves heat that you have made from another source, it cannot be externalized. 24,500 BTU x 7 months = 5.1MMBTU or 1508kWh. When added to the economics this simply reduces the shiny patina of the HPHW, which I believe was overly optimistic. That $240 worth of electricity or $200 of oil in a year changes the ROI of the HPHW.

    In areas with a cooling load, or where dehumidification is warranted, then those need to be taken into consideration. And where heat is a nuisance my argument is baseless.

    Also, not stated by anyone, if I was to add PV to my roof to compensate for the hot water element or the HPHW, I could also "overproduce". Though it would not lead to overheating, it might lead to changes in the economics. I would have to guess how much PV would cover 100% of my load. Hot water demand changes as children grow and then leave. In many states and situations we only get paid net to parity on grid fed PV, and we cannot recover the value of overproduction. In that case the cost of the PV electricity over parity is not recovered so ROI is decreased. It is the efficiency conundrum. The less you use, the less you save. Obviously the same issue applies to SHW. Again not against the main argument, but pointing out the limitations of PV.

  77. User avater GBA Editor
    Martin Holladay | | #77

    Response to Michael Horowitz
    No argument from me on the fact that when a HPWH is located within the conditioned envelope of the house, it steals space heat from the house. That lowers the annual average COP from 2.0 to a lower number, but a number that is always higher than 1.0. If you don't like the math, buy an electric resistance water heater.

    You are overthinking the problem of "too much PV." For heaven's sake, the grid is the cheapest way to get electricity almost everywhere; it's usually cheaper than PV. And, if you don't have access to natural gas, grid power is a cheaper way to make hot water than a solar thermal system. You don't need any panels on your roof! Just buy electricity from your local utility. I'm pointing out that PV is cheaper than solar thermal, not because we should all run out an buy PV modules, but to show the folly of investing in a solar thermal system when even a PV system (a fairly expensive thing to buy) is cheaper than a solar thermal system.

  78. Mary Florence Brink | | #78

    GSHP and conditioned space
    Very interesting discussion. I, for one, would be delighted not to have to install 2 different types of solar systems on our roof. I wonder, however, why everyone assumes people are not using the basement as living space? Also, why no discussion of ground/water source heat pumps as an option? The question of extracting heat from the indoor environment then becomes a moot point. We are building in Minnesota. Mary Florence Brink

  79. User avater GBA Editor
    Martin Holladay | | #79

    Response to Mary Florence Brink
    Q. "I wonder, however, why everyone assumes people are not using the basement as living space?"

    A. I made no such assumption. All I said was, if you have a big basement that you aren't using for living space, it would probably make a good place to put a HPWH.

    Q. "Why no discussion of ground/water source heat pumps as an option?"

    A. The reason I didn't mention ground-source heat pumps (GSHPs) is that they ave very expensive to install. The point of the article is to look for ways to heat domestic hot water with a smaller investment than required than for a solar thermal system; in most cases, a GSHP costs even more than a solar thermal system.

  80. User avater
    James Morgan | | #80

    HPWH discussion a distraction?
    Though reluctant to add to what is already a very long comment list where nearly everything has already been said, I feel moved to advance the notion that the HPWH debate is not critically relevant to the core insights which Martin offers with this article. They are clearly not appropriate to all possible installations, and even here in the south there's very often not a good place to put one. The unheated northern-climate basement discussion is perhaps an interesting unresolved issue which perhaps deserves its own thread sometime in the near future?

    As an ancillary thought aimed at site administrators, why do I have to scroll to the bottom of two pages of comments (with a repeat of the main article at the top, no less) to read the latest contributions to what has been a very informative ongoing discussion? Is there a technical reason we can't have a 'latest first' option like on the Q&A discussions?

  81. User avater GBA Editor
    Martin Holladay | | #81

    Response to James Morgan
    1. Yes, I am now working on an article about heat-pump water heaters and where to put them. Look for it soon!

    2. I agree -- the long payback period for solar thermal systems, coupled with dropping PV prices, makes the topic of this blog relevant, even if we disregard controversies swirling around HPWHs.

    3. I don't know if your request for a "most recent comment first" option on the blogs is technically difficult, but we'll add it to our growing list of desired website improvements. Unfortunately, our team of programmers has faced daunting hurdles recently, with many website improvements proving to be hard to implement. Each improvement appears to introduce a new site-crashing glitch -- but we're working on overcoming all these hurdles. Thanks for your suggestions and patience.

  82. Bob Z Rational Energy Solutions | | #82

    Excellent article and comments
    Martin, you have really hit on a great topic that everyone commenting here appreciates, and this with only a small portion or readers actually adding comments.

    I can say that I've been pleased with my GE HPWH. I fully instrumented it and in the past 14 months my water heating electric usage has been reduced to exactly 1/3 (800kwh/yr.) using 45gal/da on average, in an uninsulated basement (60-62F), with incoming water at 60F and output set at 120F.

    I had the same conclusion against the SHW system when deciding between it and the HPHW, even if I need to replace the heat pump in 9-10 years vs. the SHW system in 20+ years. A big deciding factor was that I would "lose" the excess hot water produced by the SHW system in the summer months, and still need to use resistance heating for a majority of my hot water in the winter months.

    Also, the noise from the HPWH is about the same as my few year old basement dehumidifier which I now only run ocasionally due to the same function being performed by the heat pump in the water heater.

    p.s. one more web site desire is to change the comments from grey to black lettering to make it higher contrast and more readable.

  83. User avater GBA Editor
    Martin Holladay | | #83

    Response to Bob Z
    Bob Z,
    Thanks for sharing your data. It's great that all you need is 800 kWh per year to make domestic hot water.

    I don't know where you live, but if you lived in Boston, all you would need would be a 650-watt PV system to supply that much electricity. If you could get such a system for $4.50 a watt, the PV system would only cost about $3,000. That's cheap!

  84. Bob Z Rational Energy Solutions | | #84

    Air source HP and PV

    I do live in the Boston suburbs, but since I installed 2 air-to-air heat pumps last year I want to add PV to minimize my electric purchased. I just need to get rid of those pesky trees that are are in the way of my south facing roof. I'll want to install the maximum PV I can fit since my total electric budget in winter is about 50KWH/day.

    I did the same type of tradeoff analysis with a ground source heat pump vs. the air source heat pump and oil backup for low temp days. The long..... payback on the very expensive ground source system couldn't compete with the installed air source systems.

    If you are saddled with oil (i.e. nat gas is not available or you're adverse to carbon fuels) the air source solution plus PV is a good answer.

  85. Steve Dyck | | #85

    Ontario says Solar Thermal is King
    I think a typical price for a domestic solar thermal system is $7,500 - and this system will operate for 10 years with almost no maintenance, much like a PV system. And $7,500 happens to be right in the middle of the lowest dshw price of $4000 (expects some maintenance) mentioned in the blog and highest price of $10,000 (probably oversized ).

    You might be able to self-install a large residential PV system (>5 kW) for $4.54 per watt in some places, but $5 per watt is more realistic for Ontario where I am. For a smaller system (<2 kw) the price will be as high $8 per watt, unless you do it yourself, and avoid structural engineering review. so is either a $25,000 investment for 5 kw (plus $3,000 heat pump water heater), or $16,000 2 plus pump.

    In a home with lower hot water usage (28 gallons per day) it is possible to meet more than 80% of domestic hot water load with a solar water heater.

    Also, heat pump water heaters (hpwh) do not work well in Canadian winters. Basements are typically cool and dry in the winter, so the back up resistance heater in the heat pump water heater will be needed to meet the water heating load. The hpwh will also make the basement cooler, cooler floors are not comfortable.

  86. Peter Hastings 4C | | #86

    By Definition

    inequitable subsidy structures for renewables

    Please describe an equitable subsidy structure.

  87. User avater GBA Editor
    Martin Holladay | | #87

    Response to Peter Hastings
    I'm assuming that your question is directed to Leslie Baer, the reader who posted the comment with the phrase in question.

    My guess is that Leslie was referring to a long-standing complaint by the solar thermal equipment industry that in some areas of North America, financial incentives and rebates for PV systems are more generous than incentives and rebates for solar thermal systems.

    It should be pointed out that some areas of the country do, indeed, have solar thermal system incentives.

  88. User avater
    Marc Rosenbaum | | #88

    Response to Graham Irwin
    Let’s re-do the numbers provided by Graham Irwin (comparing the output of a solar hot water collector with a PV module, per square foot) for the Northeast.

    Solar hot water: take Aldrich’s number of 36 kWh/sf/yr. I think a well-designed system would hit 50 kWh/sf/yr. No way 107. A Heliodyne Gobi, in an SRCC test, makes 42,800 BTU/day in clear conditions and 36 F° ∆T. That's close to Irwin’s number, in best case conditions.

    PV - Sunpower at 17 W/sf in the Northeast makes 1,250 kWh/kW/yr, so 21 kWh/sf/yr. With a HPWH at 2.35 COP, that’s 49 kWh/sf/yr.

    Hmmm - 49 vs 50?

  89. Keith Davis | | #89

    Does this assume net metering
    Does this assume net metering throughout the year such that the banked PV power can be used when extra power is needed from the grid with no premium (when the DHW demand is greater than the HP and PV capacity)?

  90. User avater GBA Editor
    Martin Holladay | | #90

    Response to Keith Davis
    Yes. The vast majority of grid-connected PV systems in the U.S. are installed in locations where homeowners have some type of net-metering arrangement with the local utility. Details vary, but in general excess PV production is credited to the customer.

    For more information on net metering, see An Introduction to Photovoltaic Systems.

  91. Talon Swanson | | #91

    Big picture time..
    Let's face it: pretty much ANY of these ideas is better than your standard gas or electric water heaters. One can quibble about the nickels and dimes all we want with all the variables that entails (e.g.- electric rates, room temps, rebates, tax credits, etc), but the fact is that, until these units are seen as affordable and idiot-proof, they aren't going to become commonplace and save this country the billions of BTUs/kWhs they are capable of. Here in WA, things are getting close for HPWHs as the GE units themselves can be had for around $500 ($1,000 - $500 utility rebate) + installation.

  92. Mike S. | | #92

    New Technology in ST
    Dear Martin,

    Very well written article. Did you know there is already a company out of Florida who patented a PV Water Heating product? You can google the company. Name is Premium Solar. Location Tallahassee Florida. Their website is Name of the product is “Liberty Box”. If you go on their website look under products you will see a“Liberty DC Powered Solar Box”. They are using a special patented inverter that gets hooked up to 1.2KW of PV panels and the lower heating element of an existing electrical water heater. There are no moving part, no expensive copper and no freeze issues. Actually as opposed to solar thermal, this products provides hotter water in colder climates since PV panels work more efficiently in cold. They have been testing this product on certain low income homes in North Florida since 2009 and claim the efficiency is same as a regular ST system with added benefits. The cost of the system is significantly low not to mention it’s almost a DIY since it’s an off grid application. I’m told by the owner of the company (Victor) they will showcase the product for the first time in one of the upcoming national solar shows.

  93. User avater GBA Editor
    Martin Holladay | | #93

    Response to Mike S.
    Thanks for the information; it's interesting.

    For interested GBA readers, here is a better link:

  94. Colin Dumais | | #94

    Someone just sent me this article because I was suggesting to them today that solar thermal is a dead man walking. PV at 85 cents/watt has done solar thermal in for most climates. I just sold the 20 collectors I was going to use for space heating and will replace them with a 12kW PV system. Either system will cost me about 20k wholesale and the advantages (and lack of disadvantages) of PV tip the balance in it's favour. As the price goes down it will be more obvious to the others - but you are spot on.

  95. User avater GBA Editor
    Martin Holladay | | #95

    Reply to Colin Dumais
    Thanks for the support. I agree that with every passing month, the arguments in favor of PV (over solar thermal) just get stronger. Solar thermal equipment isn't getting any cheaper -- that's for sure.

  96. Roger Davenport | | #96

    Late to the Party but Still Intrigued
    Very interesting discussions. Here's a thought: For those in Northern climates, maybe they could duct the HPWH and their refrigerator or freezer together and just pump the heat from one to the other.... (if you need more hot water, leave the refrigerator door open?!). Also, I'm surprised that no one made the point that most of the heat from hot water ends up going down the drains, which probably run through the basement. So maybe the HPWH system just pumps the heat into the water tank, and then the heat leaks back from the drain pipes into the basement. I did see one comment about drain heat recovery, which seems like an idea that should have come decades ago. My two cents worth: that the case studies did not show a huge effect probably shows that even with insulated basements, there is enough heat transfer from the ground into the basement to help supply the energy without needing to replace all of it with a heater (think about bottom of the basement slab, for instance -- lots of area in intimate contact with the ground and not often insulated in older houses). Of course that might not work as well if people are heating the basement, but even then there is likely some seasonal thermal storage from summer going on down there that would be tapped by such a system.

    I just did an analysis comparing a HPWH to a solar thermal system in several modes (solar alone, preheating the water to the HPWH, supplying the heat source for the heat pump, etc.). For my climate (southern CA), the heat pump by itself gave about 70% savings, about the same as a 3 sq.m. solar system, but paid back 10X faster (I was looking particularly at the AirTap A7 unit ($699 retail, $300 on EBay)). All of the "hybrid" configurations were worse than the heat pump alone, but you could get to 90% savings and 3X the heat pump alone payback with a 1 sq.m. solar thermal system preheating water to the heat pump. Have you heard any feedback on the AirTap system (reliability, etc.)?

    Finally, for what its worth we never recommend a domestic solar hot water system that is not PV powered. 20W of PV will run the pump we need for circulation, and is cheaper upfront and in O&M than plugging the system into the wall. Maybe we'll have to re-purpose our Solar Wands(TM) to interface to heat pumps instead of solar water systems....

  97. User avater GBA Editor
    Martin Holladay | | #97

    Response to Roger Davenport
    You wrote that "drain heat recovery ... seems like an idea that should have come decades ago."

    Drain water heat recovery devices are at least two decades old. Check out Renewability and GFX.

  98. Bob Lemaire | | #98

    Real world data
    I know this thread is stale, but I just got onto it from a link in Marc Rosenbaum's new ZNE course syllabus. As a longtime solar thermal DIY guy, I followed Marc's HPWH experience in his blog. I know he expressed some reservations about his choice at one point but I never saw a follow-up. This blog pretty much runs the gambit from both sides.

    Anyway, I have a year's worth of real world solar thermal data for what it's worth:

    10,398 gallons of 120 degF hot water delivered
    51 degF weighted average inlet temperature
    1259 Kwh electricity.
    Average 28 gallons/day usage.

    Dedicated hot water meter reads cold into the system, including the cold that gets mixed. The dedicated meter reads power to the tank as well as the pumps and controls, and additional resistance heating in the distribution system (more about that later). The net result is .121 Kwh/gal as delivered to the tap.

    The heat energy embodied in the delivered water is .172 Kwh/gal (avg 69 degF rise). So that's a COP of just about 1.42.

    My data takes into account standby heat loss, pumping costs, seasonal variation in inlet temperature, and distribution costs. But some of the data presented here, and I think the initial post as well, seems to only talk about the energy calculated in the delta T between the inlet and outlet temperature. Almost all are talking about tanks with standby loss and depending on your daily usage, that becomes pretty significant as the usage decreases. Mine is 28 gallons/day and I think Marc's is less than 20.

    I've found that at some point, paying attention to storage efficiency (extra insulation), and distribution issues, can make a bigger dent in annual costs that the method of heating. For reasons beyond my control, my kitchen is over 60 feet from the tank, and even with a 1/2" line, we would waste 1/2 gal each time we purged the line. Wasted water aside, each new purge represents 2% of our daily usage. Doing that 10 times or more a day adds up fast. So the solution for us was to put a small super insulated electric booster tank in-line under the kitchen sink. It uses about one kwh/day between standby loss and re-heating the cooled water in the lines after inactivity. My point is that if we simply let the water run until hot, our kwh cost/gal would be much better, at least in the summer, but our overall HW cost would be greater because we would use a lot more. It;s a tradeoff that I rarely see factored into the kwh/gal comparisons.

    I have to question some of the other data that was presented. I have found that all attempts to calculate cost of indirect (or ugh tankless) boiler produced HW are almost always grossly underestimated because boiler efficiency is a steady state measurement. Unless your boiler runs straight out, the boiler standby losses, especially in off season, will dominate with low usage.

    The guy who says his HPHW heater used 800KWH for 16,425 gallons (45/day average) works out to .049 KWH/gal or a COP of 3.54 without even considering the standby loss of his tank, which should be baked in as the data is presented. I don't believe it.

    I'm not going to get into the economics of my particular system, but as a DIY install of some recycled components with new collectors at trade prices and tax incentives it works for me. It's in New Hampshire, and the system faces ESE with shading after 1pm from the fall equinox to the spring equinox. It's a summer hummer and a winter bummer, and that magnifies the major issue with ST in northern latitudes: In the winter you need significantly better performance because you get less sunshine, the inlet temps go down, and the standby loss increases and in the summer you throw it away.

    Starting with a clean sheet of paper, I would probably not be looking at ST.

    I think the key to the PV/HPHW (or resistance as you point out) is that there is free seasonal storage available.

  99. User avater GBA Editor
    Martin Holladay | | #99

    Response to Bob Lemaire
    Thanks for sharing your data. So, to get 0.172 Kwh of "free" solar thermal heating, you had to input 0.121 kWh of electricity.

    I agree with your conclusions: "Starting with a clean sheet of paper, I would probably not be looking at solar thermal. I think the key to the PV/HPHW (or resistance as you point out) is that there is free seasonal storage available."

  100. Bob Lemaire | | #100

    Hindsight is 20/20

    Yea, that's about right. Seems silly the way you put it. Collecting the real data was brutally enlightening. It cost me $2,000 to convert from indirect oil fired and between the fed and town tax credits it will pay off in less than ten years. Not sure how I feel about that. At the time, I had looked into the Nyle HPWH but was still smarting from the spectacular failure of Hallowell, another Maine air source HP firm and as I understood it, a scion of the original Nylotherm.

    Anyway, the unique problem of my kitchen distribution adds .036kwh/gal to the cost. That wouldn't change with a HPHW or any other central heater. So by way of comparison, the numbers are .086 vs .172. But the seasonal variation was my main point. Without the distribution fixed cost it's .058 in the Spring and Summer but double at ,114 in the Fall and Winter. Monthly variation is .023 in July and .152 in Jan. The farther north you go with solar thermal systems, the more significant storage becomes in efficient utilization of the investment. I think the numbers would work a lot better in Arizona.

    To me it's not about the HPHW technology so much as PV/net metering being the best use of capital to harness the sun because of the free storage. The only reason that this thread isn't about ST space heating vs. air-source HP space heating is that the scale and seasonal demand of ST space heating amplifies it's deficiencies and takes it off the table. (Apologies to passivehaus but my impression is that's really about good envelopes).

    Thanks for the reply.

  101. Jeffrey Morrow | | #101

    Solar Hot Water Springs Back To Life
    Solar thermal's value is a function of both cost and benefit. Heating domestic hot water with natural gas rather than solar may be cost effective. But if you're heating more than domestic hot water, the math changes quickly. Attached is an image of a Simple Drainback hot water tank plumbed in thermosiphon with a gas backup tank, with combi-system capability. This low cost system can heat floors, do snow melt, heat hot tubs, even be hooked to a wood stove. Solar thermal is alive, and healthy in circumstances where electricity or propane is displaced, and where a single solar hot water system serves several functions.

  102. Sherwood Botsford | | #102

    Your Mileage May Vary
    Here in Canada EnMaxx will lease you a 3 kW system for $60/month for 15 years. At the end of that time you can buy out at $350. About $11,000.

    In our climate one kW installalation will generate about 1100 kWh/year.

    Our local power rate is about 15c/kWh

    So the system will save about $500/year. Thus, the simple payback time is 22 years -- 7 years longer than the lease. Looking at it another way, it costs you 720/year to save 500/year.

    This assumes that the system stays in perfect working order for the entire time. In practice PV systems degrade at about 10% per decade.

  103. User avater GBA Editor
    Martin Holladay | | #103

    Response to Sherwood Botsford
    As far as I can tell, your comments refer to the payback period for a PV system, not a solar thermal system.

    While PV payback is not the topic of this blog, your comments are interesting and welcome. PV payback periods vary widely from one region to another. The important variables are the number of hours of sunshine each year, the local cost of grid-supplied electricity, and the local availability of subsidies, incentives, and tax breaks.

    For more information on PV payback, see PV Systems Have Gotten Dirt Cheap.

  104. Jin Kazama | | #104

    Sherwood and MArtin :
    i was

    Sherwood and MArtin :

    i was just about to post a similar comment !!
    ( after reading through "most" of the comments in 1 shot ... )

    I don't believe that PV systems are appropriate for us here in Quebec either for now .
    Their installed price would have to cut down almost in half , which will probably not happen anytime soon.

    We are still at ~ 0.8$/KW/h of pretty "clean" electricity,
    and production of PV are usually in the 1 for 1 region from what i heard around
    ( as in 1KW array = 1000 KW/h produced yearly )

    Since ~60% of our electricity bills go toward heating
    i'm considering researching advanced solar thermal gathering, but not for hot water ..
    Storage during daytime and release during nighttime is what i believe to be the key here.

    Though, we don't see any/much thermal solar systems here, it never really went ON so it cannot really die i guess :p

  105. Peter Crisp | | #105

    PV Panels to Preheat Water
    I'm late to the table joining this conversation. I have an idea, which might be dumb or useful and I can't find any examples where it's been done. Could a small PV system and storage tank be installed to pre-heat water for either conventional hot water tanks or tankless units using heat trace and a gravity loop for warming? If it works, it should be small, easy to retrofit (no house circuit re-wiring and no double-wall heat exchangers to start with) with a fast payback if it works.. I'm an engineer but I won't profess to being too sharp on thermodynamics. I live up in Canada where the incoming water temperature can get pretty cold. I'd really like some feedback on if this could work, or why it wouldn't. If it could work, we have a couple of rental houses to test the idea out on. Thanks!

  106. User avater GBA Editor
    Martin Holladay | | #106

    Response to Peter Crisp
    If your house is grid-connected, and you want to install a small PV system, go ahead. All of the electricity produced by the PV system will contribute to lowering your electric bill.

    If you want to install an electric-resistance water heater to pre-heat your domestic hot water -- or, for that matter, to bring your domestic hot water up to any temperature you want -- go ahead. You are free to do so.

    The two systems -- the PV system and the electric-resistance water heater -- are independent. The decision to install either system does not depend on the decision to install the other system.

  107. David Jones | | #107

    vacuum tubes, other uses for hot water
    "Green builders have an emotional connection to solar hot water systems, because they represent a fairly simple technology that's been around for over 100 years. But it's time to admit that a PV array is cheaper and less troublesome than fluid-filled solar collectors on your roof."

    Well said. I admit I am reluctant to let go of solar thermal as the best value per unit of energy collected. However any zero energy home needs some electricity production and in most cases that is PV. Regardless of the exact economics having 2 systems (PV and solar thermal) is more complicated than having a single larger PV system.

    I have some concerns about the how the deck was stacked here, but that probably doesn’t change the conclusion.

    "Some solar-heated water goes to waste
    "After all, if great quantities of hot water are produced on a day when it isn’t needed, you can’t really count the energy production in your annual tally. Solar thermal energy is inconsistent, and during the long sunny days of summer, most solar thermal systems make more hot water than the typical family can use."

    A pool or hot tub can use all that extra thermal energy. The extra energy happens to come at the perfect time of year. Granted most green homes won’t have a pool/hot tub.

    Does anyone ever have too much hot water? Wash your car or your dog with warm water. Heat the soil under your vegetable garden to extend the growing season.

    Low temperature water gathered in colder months that is not usable for hot water or space heating could go to snow melting.

    "In a 2006 study, researchers from Steven Winter Associates monitored two residential solar thermal systems for a year, one in Wisconsin and one in Massachusetts. Each house had two solar collectors. The solar fractions of these two systems were 63% and 61%, respectively."

    Were the solar installations SWA monitored flat plate or vacuum tube?

    I am not trying to start the flat plate vs. vacuum tube debate. I know there are many who prefer the flat plate collector for good reasons. However; a vacuum tube system will produce higher temperature water, more days per year, and more hours per day. That can have a significant effect on the solar fraction, (as can adding a 2nd storage tank).

    "Compared to a PV system, a solar thermal system has several disadvantages:
    "Unlike a PV system, most solar thermal systems have moving parts (pumps and solenoid valves). "

    True but circulators and solenoid valves are cheap compared to replacing an inverter.

    "While a pole-mounted PV array can include a tracking mechanism to follow the sun's path across the sky, it's virtually impossible to install solar thermal collectors on a tracker."

    A tracker is a lot of complicated and expensive moving parts. If Honda can’t build a minivan with a dependable automatically closing side door, I can’t imagine that a site assembled low production solar tracking assembly could be considered a maintenance free item.

    A vacuum tube collector is always perpendicular to the sun with no moving parts!

    "On average, PV systems probably last longer than solar thermal systems."

    Inverters are reputed to have short life cycles. These are big ticket items compared with circulators and solenoid valves.

    The PV panel seems to last a very long time, (but possibly suffer a minor loss of efficiency over time). Solar thermal panels should last a long time as well. No moving parts in the panels. In both PV and solar thermal the maintenance is likely to be in the mechanical room.

    "(During the same time period, sales of solar thermal systems have also been hurt by a third factor: dropping natural gas prices. But that's a topic for another article.)"

    I my experience, solar thermal sales are hurt far more by subsidies that are radically tilted toward solar PV rather than solar thermal. (I suspect the PV industry has more money to lobby for subsidies because PVs are bigger ticket items.) In many cases it less expensive for a homeowner to acquire a $30,000 PV system than a $6000 solar thermal system.

    "Of course, if your family uses less than 64 gallons of hot water a day, or your heat-pump water heater has a higher average COP than 2.0, or you live in a state with more sunny days per year than Massachusetts or Wisconsin, or the average temperature of your incoming cold water is higher than 50°F, then your new PV system will be producing extra electricity that you can use for other purposes."

    Of course if you live in a state with more sunny days than Massachusetts, your solar thermal system will produce more hot water too.

    "If the family with the hypothetical $10,000 solar thermal system uses 44 gallons a day, and the solar fraction is 63%, their solar thermal system heats about 28 gallons a day on average. The PV option produces 37% more hot water, even with an electric resistance heater — and with far less hassle."

    I suspect the solar fraction from the SWA study of 63% is for a flat plate collector system and that it would be significantly better for a vacuum tube collector system. However it will never be 100% without massive storage. The PV system does appear to come out on top.

    The economics of this debate can be affected by:

    1. New Construction vs. Existing
    2. Available roof space. (I believe solar thermal can collect a great deal more energy in a given amount of space).
    3. Size of system. A large portion of the solar thermal pricing in your comparison is installation. That figure is non-linear. If a house can utilize a larger solar thermal system for space heating/pool heating then the cost for the domestic hot water production comes down.
    4. Heating system in the house. A hydronic radiant system is a perfect match to low temperature solar thermal. A radiant floor system can potentially utilize very low water temps which makes it possible to get more energy out of a given thermal collector by directing the energy toward space heating or domestic hot water production.

    It’s generally accepted that people should focus on improving (or building) the best envelope possible before they add PV or solar thermal. If we assume a very well insulated building (near zero energy), then it seems reasonable to assume that essentially all of the energy being “pumped” by the indoor air source heat pump water heater is actually coming from the building heating system. In that case I don’t think it makes sense to use the heat pump water heater at all. There is probably a significant efficiency penalty in heating the house with a mini-split and then moving that energy again with an indoor heat pump water heater.

    I do like the idea of the air source heat pump water heater during the cooling season however. I think the unit could be located in a closet, and the air from the closet exchanged with a small fan to provide some cooling and dehumidification to the house.

  108. JP Jon Pierce | | #108

    "But it's time to admit that a PV array is cheaper and less troublesome than fluid-filled solar collectors on your roof."

    IF the water is on the roof: AGREED.
    But why would a geothermal nut
    (both direct fluid tubes without a heat pump / as well as with...)
    look in 1980 at the Solstar(tm) system to heat Hot Water?
    Perhaps it is STILL under 5000 bucks today for the nominal-normal-to-Sun 2 collector system to do 6800+ btuh (2kwh) water-heating WITH SPACE HEATING.

    Air-Solar/ filtered closed-loop air circulation; simple 3way combo-blower-to-air-diverter ducted; all flexduct (that has withstood 30+years of non-brittle-izing); fin-tube for water-heated-from-air exchanger; 80w magnetic coupled impeller-pump; 1/2" short tubing to HW tank; simple snap disc and paralleled redundant safeties; one Temp-differential controller; one 4 pole double-throw relay (small ice cube style); small induction relay to fan motor; a "relay in box addition if needed; two great roofing-flashing-mounting labor-techs; plumbing is too simple.

    one or two duct discharge to warm air space heating changeover when winter HW is 'FULL' !

    Air collectors of 3-layer window screen 3/4 inch separations of an OEM baked 500f charcoal gray. box like that of the Gutter-Pipe SUCCESSFUL air solar guy uses (see youtube) of 1.1/2" poly-foam or equal to keep from roof surfaces overheating air solar collectors.
    Clear PTFE layer above screens ~ 3/4"; and solar glazing (Filon (sp?)) was replaced in 12 to 14 years although yellowed early on. Professionally installed under 5000 and provides SPACE COMFORT.

  109. User avater GBA Editor
    Martin Holladay | | #109

    Response to Jon Pierce
    Your writing style is hard to decipher, but it sounds as if you are a fan of using solar air collectors for space heating.

    This is an old topic. Go ahead and build a few such collectors if you want. They are harmless. But the energy they provide isn't worth the cost of the equipment used to collect the energy.

    The basic problem with solar air systems is that storing the energy (usually in a bin full of rocks) uses a lot of fan energy, and the rocks only stay warm for a few days. The other problem with any solar space heating system: when you really need the heat, the sun rarely shines; and when the sun shines, you don't need any space heat (because all you need are a few south-facing windows to keep your house warm if your house has a decent thermal envelope).

  110. Jeff Auxier | | #110

    Limited space
    I believe a thermal collector is far more efficient use of available space than a number of PV panels. This is an important consideration in most areas.

  111. User avater GBA Editor
    Martin Holladay | | #111

    Response to Jeff Auxier
    As my article points out, the average solar thermal system with two 4'x8' collectors produces about 28 gallons of hot water a day in a northern climate (annual average production). To produce that much hot water with a PV system and a heat-pump water heater with a COP of 2.0 would require (in a northern climate) a PV array rated at 763 watts. Since typical PV modules produce 12 watts peak per square foot, that means that you would need a PV array measuring 64 square feet -- exactly the same size as two solar thermal collectors.

    So the area of the rooftop solar panels is exactly the same, whether you prefer a solar thermal system or a PV system.

  112. Jonathan Mitchell | | #112

    Solar Thermal vs PV maintenance costs?
    Having read back through previous comments I have seen attention drawn to the fact that solar thermal DHW systems are costly, especially so becasue of future maintenance issues. Unfortunately I havent seen anyone mention the fact that Inverters for PV systems don't last forever and will typically need replacing every 10 years or so! These a lot more costly than perhaps a small Grundfos pump that may need replacing or refilling the system with Glycol once every 5 years.

    The fact is, Drainback solar thermal systems are easier to install, quicker to install and require almost zero maintenance and are lower cost than pressursied systems. The do not suffer from Stagnation issues and neither do they suffer from Freezing conditions.

    As a UK Manufacturer of both PV & Solar thermal flat plate collectors we see benefits with both technologies. And by far the most suitable and reliable system for residential properties is flat plate collectors taking into account normal house roof space availability.

    Another thing you fail to mention with PV panels is that for them to work effectively at maximum efficiency you have to ensure there is no shading. If shading encroches on one panel it can knock out an entire string rendering it completely powerless. How many residential roof tops can gurantee no shading at all - either from trees, chimneys, other roofs, other buildings, etc.... Solar thermal is not affected by shading in the same way.

    Anyway, the fact is you can have both technologies side by side roof integrated if you want! Solar Thermal for your DHW needs and PV for your electricity needs or selling back to the grid if you benefit from a Feed InTariff.

    Great to see this article get so much attention - it clearly demonstrates there is a lot of interest in the US for residential solar - whichever technology it may be.

  113. User avater GBA Editor
    Martin Holladay | | #113

    Response to Jonathan Mitchell
    It's very hard to gather objective, quantified data on maintenance costs, so most of us have to rely on anecdotes. Here are two anecdoctes from my family:

    1. My first inverter (manufactured by Trace) lasted 18 years before it needed to be replaced.

    2. My brother's solar thermal system had expensive maintenance problems with its pump, and the tank began to leak after 14 years.

  114. Tom Scheel | | #114

    solar thermal professional weighs in
    Excellent article - it is a conclusion I have been forming myself, even though greatly reduces the value of my hard-won solar thermal expertise.

    A few quibbles
    * you seem to have moved from 63% SF of a 64 GPD government/SRCC figures to 63% of other studies that show the actual figure is 44GPD - but the SF would go up with lower usage - not by 50%, (the difference in water usage) but by much more than zero. I apologize if I have missed where you accounted for that.
    (note you check my figures by going to SRCC and looking at a 64 square foot drainback system annual SF for New York, NY (I thought a fair compromise given all the locations discussed) - and further note that SRCC assumes 64 GPD)

    That is a quibble for DHW - maybe PV would need to be 150% of solar thermal (worst case for your argument). But where solar thermal really shines (pun 100% intended) is in space heating - and there the math means the south facing roof becomes the limiting factor - as I have often told my customers - PV is great but you would need two rooftops to hold enough PV to heat your house (using electric resistance).

    * solar thermal has a steeper curve related to low water temps (PV gets marginally better in colder weather; solar thermal gets markedly better as the delta T between ambient and inlet approaches zero). This means that as we leave the design conditions you specify (ie colder water temperatures in the north or warmer air temps in the south) - all of this is captured in SRCC data (the gold standard of objective 3rd party evaluation of solar thermal - but still a model) - and not captured in the studies based on northern climates. {this is a much smaller issue that the first one I raise}.

    I whole heartedly agree in the need for solar thermal monitoring - I do it on all our major systems and we inevitably find ways to tweak better performance because we have the real time and aggregate data - and we can verify for our customers that we are hitting the savings upon which we sold the system. I think the solar thermal industry is really behind the curve in not monitoring every installation.

    As for reliability - I find HUGE variability - I have serviced systems with 20 year old pumps, and I've replaced pumps on systems within a year (with both Grundfos and Taco pumps). I strongly suspect the inverter reliability will be resolved, and or dealt with via insurance (share the risk).

  115. Seth Maciejowski | | #115

    Nice Article!
    Interesting article. I ran the same calculation recently when deciding on an approach for my renewable energy system. I am running a 4.6KW Allsun dual axis solar tracking system making around 7600Kwh/year and a GE Geospring Heatpump hot water heater for my main hot water loop. My house is in vermont and the temps hit -22F a few days last winter. The heat pump is in my basement and I must say that one of the unexpected benefits of the heat pump is the dehumidfying effect when it runs in heat pump mode. The temps in my basement are mid to high 50s in the summer with very low humidity which is very nice when its' 90 outside and 90% humidity. GE says that the heat pump can operate with a decent COP well into the low 40s. The Geospring can be run in straight heat pump, resistive mode or hybrid mode where it decides what to do in high demand situations. So far I am pretty pleased with this arrangement which has displaced my oil fired tankless hot water system (so direct negation of fossil fuel usage). The fan is MUCH quieter than the old oil fired system and is pretty much not noticeable upstairs. So far, I am very pleased with this approach and am considering an air source heat pump for shoulder season heating when the wood stove is hard to start as I am generating excess kwh off my solar system right now (no reimbursement under net metering rules).

    Thanks for the informative article!

  116. Samuel Hay | | #116

    Inadequate description. Worthless is more like it. This truth found in this article is about as rare as a rifle rack in a Volvo. So many less than accurate statements. I could blast them all but in the interest of time, I will be brief. "Heat all your hot water year round from a heat pump". Like if you take the heat to heat your water in winter you will freeze your ass off. Reliability is, well, non existent even now with solar PV ancillary equipment. For what Ive paid to replace inverters I could have bought 3 thermal systems. And then there are the batteries. Storage systems for thermal systems dont eat expensive batteries. I have both systems. Wanna know what works best for me? My passive solar 'thermal' greenhouse. It grows citrus year round in North Georgia. Oranges, date palms, lemons, limes, kiwi, avocado, olives, pineapple(My personal favorite). And doubles as a sprouting base for my spring gardens. It has one small fan. Open windows in spring, close them in winter. Air is drawn into the greenhouse (connected to the house) after raising two operating windows on each side of the entry door about 2". Then heated air is brought back into the house via an open transom window above the previously described structures. The material is twinwall tuffex and in the middle of the swamp, it has never been damaged by flying debris or falling limbs and it is insular. I have over 3000 watts of PV. I have four thermal collectors. But the secret to it all is design and insulation. As for the article, forget what you read, chose what will work for you. I have seen my thermal collectors (Dry) in mid winter at 425 degrees F. Diffuse radiation from clouds and snow works wonders on the thermal panels. Making the most heat when you need it most! I dont have a sophisticated delivery system, only copper finned baseboards where the excess heat flows when the set point for the hot bathing water is met. It is totally bulletproof and WILL NOT BE DESTROYED by EMP from the sun or a nuclear blast. I can stay warm and take warm baths while you are sitting there looking at your fried panels and all the control systems that go with it.

  117. User avater GBA Editor
    Martin Holladay | | #117

    Response to Seth Maciejowski
    I will address your points in the order you raised them.

    1. You question the reliability of heat-pump water heaters. I addressed your concern in my article; in fact, I wrote, "If you are skeptical about the longevity of heat-pump water heaters, you may prefer to wait a few years before buying one, and to stick with a solar thermal system in the meantime." Moreover, I also performed calculations that show that in many areas of the country, an electric-resistance water heater (with or without a PV system) makes more sense than a solar thermal system.

    2. You question the reliability of PV equipment, citing the need to replace inverters and batteries. First of all, very few PV systems have batteries; the vast majority of PV systems are grid-connected. The only homeowners with batteries are off-grid homeowners. It sounds like you live off-grid. If you do, you apparently missed an important point I made in my article: "Solar thermal systems still make sense for off-grid homes." I'm sorry to hear about your inverter problems. My first inverter lasted 18 years; I'm now on my second.

    3. I'm glad you have a greenhouse attached to your home. I do, too. They're great -- especially if you like to grow things. No argument there.

    4. If you are worried that electro-magnetic pulses will hurt your electronic equipment in some future solar-flare catastrophe, then it makes sense to move to an off-grid location and grow your own food. Good luck.

  118. User avater GBA Editor
    Martin Holladay | | #118

    Response to Andrew Gray
    Parabolic troughs, 12-volt motors, gear drives, vacuum tubing, stainless-steel rods, computer-controlled angle adjustments, counterweights, hydraulic hose -- all cobbled together by a backyard tinkerer, in order to make sure his water heater reaches 150 degrees.

    Not exactly simple or cheap -- and the net effect is the same as my simple 2-collector system (which has only 2% of his system's complexity).

    I'm guessing that the payback period for this Rube Goldberg device is -- oh, I don't know -- about 200 years.

  119. Andrew Gray | | #119

    Parabolic Troughs
    OK, Martin. Got it. Your perspective is indeed helpful, believe it or not. One more thing. Tell me how your simple 2-collector system works in the wintertime, for my info. Do you get 120F temperatures in your tank when it is say sunny but 35F outside? Thanks 1,000,000.
    Andrew Ancel Gray

  120. User avater GBA Editor
    Martin Holladay | | #120

    Response to Andrew Gray
    Clearly, I was being flippant, and was exaggerating to make a point. Parabolic trough collectors are ingenious and useful, especially for utility-scale solar thermal plants. I don't doubt that your backyard invention makes a lot more hot water in the winter than my two solar collectors. And I love backyard tinkerers -- they contribute to the strength of the U.S. economy.

    I just don't think that this is a cost-effective way to make hot water.

  121. Andrew Gray | | #121

    Parabolic Troughs
    Yeah, I hear you about cost effectiveness. I did manage to put a self-designed heat exchanger into an existing electric water heater (saves about $1200 for heat exchanger WH), and I have an EcoSmart Tankless backup so there is no 2nd-tank-wasted-energy-loss for a single person household (the first two showers are free instead of the 2nd and 3rd). Also, this parabolic system will mount on the North or East side of my home, LEAVING MORE ROOM FOR PV PANELS(!) (You will like that). So I am not displacing PV space. Finally, the materials are cheap for this system. It is the labor required that adds up. It would need an economy-of-scale to get it priced to your liking I am betting. Thanks again for your input.
    Andrew Ancel Gray

  122. Andrew Gray | | #122

    Parabolic Trough Vacuum Chamber Water Heater
    Martin, I wonder if I could get your comments on the idea of a parabolic trough water heater like George Plhak has started:
    What say you about this system? I works in the winter because of the vacuum chamber, and it has heat dump capability in the summer because the whole array can be rotated towards the north (to "turn it off").
    Andrew Ancel Gray

  123. Michael Clarke | | #123

    Nice post
    You gave very useful information. I will keep follow your post. Keep it up.

  124. Kelly Livingston | | #124

    Doing Solar Thermal incorrectly SHOULD be dead

    I think you are misleading people about ST and what it is capable of. What this article should be telling people is that the ST industry is falling very short of what it could do rather than hyping up PV.

    1) First, I want to comment just on conversion efficiencies, that is what is the most efficient way to make hot water from the sun. Like most things, the physics can go either way depending on conditions. ST can be anywhere from 80% to 0% thermally efficient at collecting solar energy depending on flat plate vs. evacuated tube and on conditions partial vs. full sun. PV maxs out at 20% for commercial nonconcentrating cells and can go as low as 5% for thin film. PV efficiency doesn't change with light intensity like in ST but shading can really hurt PV without proper inverters/bipass diodes. An important thing to note is that PV performs worse in the heat whereas ST performs better on a hot day (with a low delta T to the hot water especially with flate plate collectors). So yes, if you slap a flat plate collector in Massachusetts or Wisonsin where it will only be 15% efficient for much of the year and compare it to a PV array that is near its max efficiency of 20% due to the cold sunny days, then PV is better. However, if you look at testing of a heliodyne gobi vs a fairly efficient panel that Marc Rosenbaum cited, it's clear to see that ST is roughly 2.35 more efficient than PV which is why you need the HPWH to make up that difference. It's already been challenged that the HPWH wouldn't even come close to that efficiency if it was in a cold basement or sucking the heat out of an already cold house just as you've challenged the gobi wouldn't be able to perform to its tested outputs. I think both claims are absolutely true and there is no real data to say which one would perform better given real world conditions. My vote is on properly designing a ST system.

    2) As people have already mentioned, PV grid-tied has free storage so that problem is solved (until utilities start charging storage fees which I'm sure they will at some point). With ST, storage is a critical factor that must properly be designed. This is what people get wrong. There are 2 ways and thus 2 design parameters for thermal storage: temperature and volume. The common 64 sqft collection with 80 gallon storage is a primary reason ST isn't efficient and gets a bad reputation, because this ratio favors temperature storage more than it should which renders collectors more inefficient when solar resources are available and doesn't have enough volume/total btu storage given the load to last more than 1 day. So why do people choose these ratios? Because they are mass produced for other purposes and therefore cheap to contractors. 2 40 gallon hot water heaters and 2 4x8 (common dimension of plywood and other pallet transported items) are typically used in ST applications. This gives a 1.25 gallons/sqft ratio which isn't nearly enough. A ratio of 2:1 would be best and since storage isnt the dominate cost for many of these installations, I'd go for 3-4:1. There is a reason dedicated solar tank companies like have a 119 and 211 gallon model. Just look at it from a common sense point of view, you want 64 gallons of hot water a day from a 80 gallon tank (75% of the volume and at minimum 46% of the btu storage assuming 160 degree max tank temp,120 degree outlet temp, 50 degree inlet temp = 37184 btu/79680btu) Unless the sun shines consistently everyday in your area, of course you'll only get 63% fraction. Its obvious to me why the post from California said he didn't have a problem with his system. The sun shines there more consistently. The BEST thing you can do for a ST system like this is spend the extra $450 on a 3rd hot water tank and you'll get 20-30% increase in system efficiency for 5% more capital cost. Better yet, get a tank that's actually designed for ST and you might get good performance that can easily beat any PV+DHW of your choice.

    3) People usually get the tilt wrong. The best way to destroy the performance of a ST system is to lay it flat on your roof, and that's what so many people do. Why is the tilt so critical? Because you need hot water all year round! During the summer when it's hot outside and the days are long, a ST collector doesn't even break a sweat and usually the pumps shut down, the system stagnates, steam is generated, and it lowers the longevity of the panels. In the winter, when its freezing cold and there are a shorter number of sun hours available to capture energy, the collector struggles to get water even luke warm. So what can we do as designers to help our collectors more reliably heat water *every day of the year*? The answer is tilt it with a bias towards winter production! shows my point exactly if you look at "Tilt Fixed at Winter Angle" table. Notice that even tilted at 60 degrees, a collector in Vermont would receive 30% more insolation in the summer than in the winter. Although the intensity of the insolation will be much less which doesn't favor ST production, the summer heat usually makes up for it. You make an incredibly valid point that a SRCC year long production prediciton is almost meaningless. The goal isn't to produce the most hot water in June or through the year. It's to produce enough hot water when you need it all year round.

    4) I'm absolutely in support of data collection and system monitoring. I'd go even further and say that a good ST system should be simulated with TMY data if the installer is to do the job right. There is no other way for anyone to learn what you're doing right or wrong unless you get your report card back. Unfortunately, the studies you cite really only proves my point that people don't do ST correctly, and it doesn't show that ST can't collect all the btus that a SRCC test claims a collector should because they were both poorly designed studies. This is evident from the hard data and the sparse experimental descriptions given from your studies. In the Wisconsin/Massachusetts study they got the storage wrong. As you can see from my 2nd point, they underprovisioned storage using the typical 80/64 ratio. They appear to actually attempt to adjust the tilt to the proper amount although it doesn't appear to be at the proper tilt in the ballpark of 60-65 degree, but they used flat plate collectors (I assume single pane but no description is given) in a northern climate. This is a mistake! Twinwall polycrabonate/double paned flat plate, or evacuated tube collectors would have been the proper choice for that climate. Not surprisingly, they had 87% and 93% fraction in the highest month (bet that was summer) and 28% and 19% in the lowest month (bet that was winter where poor tilt and cold temperature got them). The data is vague on energy accounting for pumping as I don't believe the PV pump was included, but the COP for the Wisconsin study is 390 (390 kwh for ever 1 kwh of electricity used). This is not unreasonable for an indirect loop system. The Colorado study made 2 other very important mistakes. While they had the proper storage ratio of almost exactly 2:1, they 1) oversized the system for the load and 2) poorly tilted the collector. For the area of Colorado they experimented, the ballpark year round optimal tilt would be 33.5 degrees. The optimal summer tilt would have been 16 and the optimal winter tilt would be 54 based on the website I shared. They laid the collector flat on the roof at 23 degrees! They didn't even tilt it high enough for year round maximum insolation! So during the hot long sunny hour of summer the panels sat there mostly unused (2.8% energy collected) and again in the winter they did poorly because they were off tilt flat plates either covered in snow or too cold to even circulate. Yet they saw better utilization (7.8%) because they didn't even meet with demand. You can see (not easily) in the energy consumption data that a huge amount of natural gas was burnt in December when the lowest sun angle would have rendered the collector nearly useless. And lastly, they oversized the system 3 fold (20 gallons per day consumption vs 64 gallons per day design parameter) so when you cite 5.7 kwh/sqft/day it is meaningless to the larger point. It should be made clear the system provided 64% of the hot water needs including getting rid of parasitic loses and that's with an incorrect collection angle. Why do you even bring up that study? NREL should be embarrased to even publish it.

    5) Your claim about parasitic pump losses is going to be a thing of the past. Pumps for ST systems, like many industries, are either sized for worst case scenario or even oversized. This is horrible for efficiency in most applications but especially so in ST. Since pumps weren't variable speed, you consumed the same power no matter what level of insolation you are receiving. This is even worse for drainback systems since pumps must have a high head requirement (even though it's only needed for 5 minutes until the drainback syphon kicks in) There are single speed pump solutions such as installing a booster pump with check valve in parallel to a low power circulator but I speculate most ST contractors don't do that due to the increased points of failure or ignorance of its importance. So yes, if you use a bad single speed pump, you can expect a thermal COP of 1-20. But the entire HVAC and ST industry is about to change due to variable speed pumps using VFD and ECM motors. These pumps are going to be great not just for ST but all water pumping with variable loads. Condensing boilers that don't operate at their peak efficiency (one commenter mentioned this) will now have circulators that can slow down flow to ensure a lower temperature return and ensure condensing actually happens. Zone valves will last longer since the pressure of the system can be lowered dynamically. Here is a great explanation of that.

    So ST has a promising future of very low parasitic pump losses and thermal COP from 20-400. This is the future.

    6) I think the economic analysis you have about ST vs PV that is implied by your title and the article itself is the opposite of what is true. First of all, there is NO cheaper way to generate a BTU at less than 200 degrees F than a piece of polycarbonate. We both agree the attached greenhouses are amazing and it's a no brainer! Of course, there is some cost in collecting and storing that BTU in your hot water tank, but the industry has made this stuff overly expensive and for no reason. I don't doubt that it cost 8k for a ST system to be installed professionally, but does it really need to be that expensive? Gary from builditsolar ( built a system for $2k using plywood, pond liner, polycarbonate, water pipe, aluminum soffits, and insulation. That's it. Oh wait I forgot the silicone caulk and the pump. He has a 99% solar fraction and on a good day the COP of his SINGLE SPEED pump is 63 in February in Montana (these are all data driven numbers). His labor is free so I'm not expecting systems to cost $2k but given that he basically put together a system with popcicle sticks, I'm sure an industry with variable speed pumps and years of experience could provide a reliable system for $4k that provides 99% fraction and used minimal electricity. That is if the costumer knew it was possible and demanded this kind of product and performance.

    7) PV, on the otherhand, doesn't have the promise of %50 reduction in cost. Thin film relies on many rare earth metals that are hard to mine in massive quantities and are highly toxic. Silicon-based solar cells have dropped in price mostly to speculation ( and there is a limit to the price decrease due purely to physics. The base material of silicon like many metals requires trememndous amounts of energy to produce. Nearly 1/3 of the cost of a module is silicon and almost all of the cost of silicon is energy (sand is pretty bountiful). Just as you'll see copper and aluminum prices track with energy, so will solar cells.

    Now clearly since you wrote this article silicon cell prices have gone down even further to .73$/watt yet the installed costs are still in the $4.5 range ($3-$7 depending on region). I think I'd still rather do ST right instead of buy some Chinese made PV.

    I look forward to your response, and I've really enjoyed the article and comments even though I disagree with you.

  125. User avater GBA Editor
    Martin Holladay | | #125

    Response to Kelly Livingston
    I agree with you that some solar thermal system designers and installers do a better job than others. But even when we look at systems designed by top-notch designers and installers, my conclusions hold.

    After all, PV systems are governed by the same constraints: some are well designed, and others are not. Yet, on average, PV systems are outperforming solar thermal systems, in terms of dollars saved per dollar invested.

    Efficiency alone (BTUs collected per square foot of collector) doesn't tell the whole story. As I noted in my article, not all of the heat that is collected by a solar thermal system can be used.

    Changes in net-metering contracts have the potential to change payback periods for PV systems, of course. But PV system costs are continuing to plummet ... unlike solar thermal system costs.

    Coincidentally, last week my brother's solar thermal system developed a leak in the expansion tank -- the solar thermal contractor quoted him $900 for repairs -- while my solar thermal system required all of the circulating antifreeze solution to be replaced (due to the fact that the solar panels that provide power to the DC pump were covered with snow on a day that the solar thermal collectors were snow-free -- this situation leading to the fluid in the collectors to overheat). My system is now repaired -- I did the work myself -- but my brother is still mulling whether the $900 investment is worth it.

  126. Gary Steps | | #126

    Solar Thermal is Dead article
    I haven't seen any responses lately. A lot of things have changed in two years. PV costs continue to drop. Grid electrical costs have continued to rise. But from my standpoint as an energy consultant and PH consultant, it is more important that buildings continue to get a lot better.

    A client moved into the first Passive House in Missouri last summer. The house is extremely tight and with super insulation compared to modern building codes. For a number of reasons, all of them good, it has no basement, is all electric, and has a maximum heat load of ~9000Btu/h. We laugh about heating with a hair dryer, but we could do that.

    Because of the above, the portion of the total heating load associated with domestic hot water is a much bigger fraction of the total than in a "normal" home. If we were to install a HPWH, it would chill the air in the conditioned space. Of course, with a tiny ground source heat pump with a very high COP under normal conditions, it would cost very little energy to fill in for the heat lost to the HPWH. However, a desuperheater in the GSHP would deliver the DHW at a fraction of the energy cost of the HPWH, without either the cost of the HPWH hardware or ant comfort issues with stealing heat from the air in winter.

    So - with the addition of a small PV array and a smaller ST array, the building produces 100+% of the total electrical power on an annual basis, and ~80% of the DHW. If there is a need to fill in hot water, an electric coil delivers it at a very low cost of electricity. And the ST unit delivers high quality heat in the middle of winter, when the PV array is sucking wind when it is not covered by snow. .

    So - I believe that there are two answers to this discussion - "That Depends", and as we say a lot about the weather here in St Louis, "Just Wait, things will change fast"

  127. User avater GBA Editor
    Martin Holladay | | #127

    Response to Gary Steps
    As PV prices continue to fall, the cost of solar thermal systems is, as far as I know, remaining constant or rising. So in the months since this article was written, the argument in favor of PV over solar thermal continues to strengthen.

    Of course it's possible to meet the domestic hot water needs of a client with a solar thermal system, or even with a combination of a solar thermal system plus PV. But the PV-only route will be cheaper.

    I agree that in some climates, for some homes, a heat-pump water heater doesn't make much sense. So in those cases, install an electric-resistance water heater. Once you do that, it's still cheaper to meet your domestic hot water needs with PV than with solar thermal.

  128. Gabriel Stinson | | #128

    All the information on Solar Thermal this article left out
    I know this article was written a couple of years ago, but I still could not disagree with it more. Where to start?
    Under Comparing solar thermal and PV systems, lets go point by point:
    -Yes solar thermal has moving parts, no argument there. They consist of one inexpensive circulator for the glycol loop, and a standard 3-way mixing valve.
    -Freezing climates is a non issue when you have a pressurized propylene glycol system which prevents any freezing. Drain backs are rarely used in modern systems.
    -Solar thermal will require maintenance and so will PV. But propylene glycol for a reputable system (think German, like Viessmann) is rated to last 8-10 years. So over the course of a systems 25-30 lifespan, we're talking 2 single day service calls including cost of glycol, maybe a few hundred dollars each. Versus replacing your inverter after 10 years for easily $1000-1500+
    -This article seems to ignore the heightened storage capability of solar thermal tanks. ST tanks must be rated to hold water up to just below boiling, meaning, we can keep pumping solar BTUs into the tank generally loading up to 180°. Despite the tanks super insulation, there are standby losses, but you're still storing and saving potential hot water, even if efficiency drops the hotter you heat the tank. PV may have net metering back to the grid, but no one ever tells you that you're selling back kWh for half the price you buy, because your utility owns the delivery lines, so you won't get paid the delivery portion.
    -Pole mounted tracking systems are nearly always prohibitively expensive. I've yet to see the numbers justifying the added expense once.
    -This last one is great, "On average, PV systems probably last longer than solar thermal systems." While PV may not have moving parts, you're grossly washing over two pertinent points. Your inverter will fail after 10-12 years, resulting in a costly replacement every time. Most importantly, the silicon wafers used in PV panels degrade over time. Industry standard warranty is that they'll be at 80% in 25 years. Of course, this is only based on lab projections, because none of these systems have been in operation more than 10 years, so hopefully it will be 80%. Conversely, solar thermal collectors have zero degradation, and have been around since the 70s. I've personally seen still functioning 30 year old collectors.

    A couple more thoughts:
    -The efficiency of PV panels is grossly inferior to solar thermal. We're talking the best that money can currently buy (and most expensive) SunPower E19 modules are 19% efficient. Usually affordable Chinese collectors are more 12-14%. Compare that to high quality ST collectors at 70-80%. Even if you're running the PV in connection with an efficient heat pump at a COP 3.0, which isn't the worst idea, you're netting only 42-57% overall efficiency.
    -Not sure why statistics here keep quoting at 61% solar fraction for ST? A residential boiler backup Viessmann system has an 80% solar fraction as rated by the SRCC.

    Lastly Cost:
    This is the end all point, if you don't have a reasonable ROI then all the efficiency in the world is irrelevant. Depending on your state, there may be incentive money up front in addition to tax credits. At least at the federal level you'll get a flat 30% no cap credit on any solar install, PV or ST. Speaking from experience specifically in New York State, even with NYSERDA incentive money, the 30% federal credit, and our 25% state solar tax credit, the PV payback remains at ~10 years, and that's before spending another $3000 on a heat pump. Solar thermal also has incentive money available from NYSERDA, plus the two credits, which brings the net cost of a system down to $2700 with an ROI of 4-5 years if you're on anything other than natural gas. Even without the $4000 from NYSERDA, with just tax credits that's a net cost of $4500, which still presents a reasonable ROI.
    -Lastly I will admit that neither PV or ST makes much sense at current pricing without some combination of tax credit and/or state incentive program, so take advantage of them while they're available.

    I invite anyone to dispute my points.

  129. User avater GBA Editor
    Martin Holladay | | #129

    Response to Gabriel Stinson
    Some of the information you provide is accurate, but some of it is flat-out wrong.

    You wrote, "PV may have net metering back to the grid, but no one ever tells you that you're selling back kWh for half the price you buy." Net metering contracts vary from utility to utility, but the vast majority of net metering contracts are exactly as described -- the customer pays for the net electricity used, after PV production is subtracted from total electricity used. In other words, the customer is credited for the full retail price of the electricity. Some utilities don't like net metering contracts, of course, but such contracts are common.

    I'm not a fan of tracking mounts for PV arrays, and I didn't advocate their use in this article.

    You wrote, "None of these [PV] systems have been in operation more than 10 years." My system has been in continuous use for 34 years, and the PV modules are working fine. For more information, see Testing a Thirty-Year-Old Photovoltaic Module.

    You wrote, "The silicon wafers used in PV panels degrade over time." This is not true in the case of my own PV module which I tested at age 30; see the link above.

    You wrote, "The efficiency of PV panels is grossly inferior to solar thermal." But efficiency (output per square foot) is irrelevant; what matters is cost-effectiveness, and that's where solar thermal falls down. Moreover, not all of the energy collected by a solar thermal system is usable, while 100% of the energy collected by a PV system is usable.

    You wrote, "Not sure why statistics here keep quoting at 61% solar fraction for ST." Here is the source: “Cost, Design and Performance of Solar Hot Water In Cold-Climate Homes” (a paper by researchers from Steven Winters Associates).

    Concerning the question of whether investments in PV are a good investment, your information is out of date, and the situation is fast-changing in favor of PV. In many areas of the country, PV investments are cash-flow positive from Day One, and the payback situation is only improving.

  130. Gabriel Stinson | | #130

    Response to Martin Holladay
    Let me state before my responses as someone that previously worked in the PV solar industry, I 100% support PV, I just feel its grossly inferior specifically for the purpose of heating water. Solar thermal is great, but as super efficient at heating water as it is, its mostly a use it or lose it scenario, and obviously its not making you any electricity if that’s the use you need.

    You are correct on the net metering part, I guess I misunderstood your statement in the article. Net metering essentially uses the grid as we'll say your battery storage. As you produce more than currently using, your bi-directional meter spins backward as its pushed into the grid. However, if you somehow manage to make more than you use at the end of the year, I am correct (at least in NYS) that your end of the year payout from the utility supplier for excess kWh production will only pay you for supply and Not delivery, so effectively half the cost. That is a unique situation, when PV system design is limited to 110% your annual historical use (again in NYS) that almost never occurs. I guess my response stems from frustration of uniformed people thinking they will be selling their kWh back to grid and making money from it.

    Glad to hear you're not a fan of tracking systems; only brought it up because you did in the article.

    I honestly wasn't familiar with any residential applications that are 30+ years old, and that’s my oversight. That is fantastic that your panels are still operating 34 years later, but I would say you are one of the few and far between with such a unique system. To that extent, there are also few properly installed ST systems over 30 years old, I just think there are more examples to draw from in the later. Since you've volunteered this info about your 34 year system, I'm genuinely curious at how many times you've replaced your inverter up to now? Approximately how much was a new inverter, and for what size kW system?

    Hate to burst your bubble Martin, but PV collector degradation is very real. Check out this study ( conducted over 40 years across 2000 different rates, which clearly shows 0.5%/year. At that rate your collectors should have likely decreased about 17% since their installation, now operating at 83% of their likely 10-12% maximum potential if they're that old.

    On efficiency or output per square foot, it most certainly is relevant when you have limited real estate on your southern facing roof. When 2-3 solar thermal panels can match the same kWh output as 7-8 PV modules, there's the best proof of efficiency right there. Your vague statement about how "...not all of the energy collected by a solar thermal system is usable, while 100% of the energy collected by a PV system is usable," I'm guessing is referring to the tank storage capacity. Lets be clear, all the potential energy is being collected and utilized, but efficiency drops as the tank gets hotter. Generally a ST systems is set to a delta of 12°, so the collectors need to be 12° hotter than the tank to get the circulator to turn on. Its relatively easy to heat to 120°, but getting to 150°+ is much harder. All the potential energy is being collected, and you're losing >1° an hour due to normal standby loses. Guess that will drop the efficiency slightly. I'll still take 70+% efficient over 14% any day for heating water. It certainly isn't cheaper buying 3 times more modules to produce the same kWh output.

    I'm going to point out that that article about 61% solar fraction is now 13 years. So lets move on. In the meantime, I've attached current SRCC system ratings for boiler backed up Viessmann systems with solar fraction ratings of 96 and 86. Granted that will drop if you're say backing up with an electric element, but if you're going for efficiency, you can make it happen.

    I don't think my information is out of date, at most by maybe two years since I left the PV industry, but it certainly is more relevant than that 13 year old article you linked to. I had mentioned the Sunpower E19 module that was 19% efficient, but yes in the last two years they've pumped it up to 21.5%. The added expense of those modules should set you up for a 15+ year payback. Yes PV systems are cash positive from day one, and so are solar thermal systems. The difference is most people don't want to wait 8-10 years for a PV system payback. Once again I'll add that I fully support PV (although the prices still need to come down for greater adoption), but I think its simply a terrible idea to use it for water heating.

  131. User avater GBA Editor
    Martin Holladay | | #131

    Response to Gabriel Stinson
    Q. "I'm genuinely curious at how many times you've replaced your inverter up to now?"

    A. I'm on my second inverter. My most recent inverter cost $1,100; I installed it myself. Total inverter cost over 34 years has been about $50/year.

    Solar thermal systems work fine; I have a solar thermal system at my house. If you want such a system, feel free to install one.

    My main warning to inexperienced homeowners -- the one on which I base the arguments in my article -- is that the same investment in PV equipment will yield more useful energy than a similar investment in solar thermal. I present the math in my article.

    Your costs may vary, but all signs are pointing in the opposite direction than the one you claim: PV is getting cheaper, while solar thermal equipment is not.

    So do your own math. You have been warned. If you still want a solar thermal system, it's your choice.

  132. Jason D | | #132

    Looks more and more like PV is the way to go
    2 years later and your analysis still holds. PV is even cheaper now. Only way ST makes sense is if you build it yourself or can't get net metering.

    One thing I noticed reading the comments is that people think a heat pump only moves heat. This isn't true - the electricity used to run the HP is (mostly) converted to heat as well. So a HP both generates heat like resistance heating and "moves" heat. Several calculations throughout the comments don't account for this. As an example, a HPWH with a COP of 2 that heats water 10000 BTU's is only "moving" roughly 5000 BTU's from the air with the other 5000 BTU's coming from the electricity running the HPWH.

  133. Doug Nichols | | #133

    I know this may be a no-no as I haven't read every comment and may be repeating information. I agree with Martin on this and would also add these benefits:
    1. In new super insulated construction you can forego the gas service all together. Where I live it's 10$ a month just to be connected to gas service. Not to mention the initial savings of not installing gas lines. I know many people prefer to cook on gas but new electric stoves are quite nice and indoor air quality improves when we don't burn fuels in our living space.
    2. PV can do everything solar thermal can do (efficiency aside) but not vise versa. PV also does so much more-- it can even charge your car at this point. Anyone looking to off set energy use will almost certainly want PV and it's uses are much more versatile and adaptable to meet their needs. Solar thermal panels do one thing, provide hot water. And like noted, especially in hot Utah where I live, they sit idle because they heat the storage tank water in the first few hours of the day in summer.
    3. As we hopefully move to a more renewable energy future solar PV dovetails while gas distribution does not. Meaning that there are lots of ways to produce and deliver renewable power over the the electrical grid; I know of no utility scale projects to bring renewable gas to your home. Therefore, it makes more long term sense to invest into the electrical grid and its sequential parts.
    4. One professional is better than two. One post commented he only had to replace one 5$ part on his SHWS over the years most people will have to pay a professional to do this-- $60 minimum to have someone come and fix stuff. If you have to have two professionals fix two $5 parts... you get the idea. I think Martin and I are both talking about the mass market here-- not us nerds who would fix it ourselves.
    5. The collateral damage potential of a liquid filled system is much higher than PV system.

  134. LUIGI LOVAT | | #134

    ST is dead? Yes, here the proof
    2 Years ago I built a electronic PCB that control PV production and home consumption in real time mode. This PCB unit, drives three power relays that activate three 1200W resistors inserted into 200L iron tank ( I studied all details: Hydraulic , mechanical, electric power and electronic, safety certification etc. ) the system is completed with a feedback temperature and command for a recirculation pump connected with boiler.
    The first my installation was in May 2013 on customer with 3 trackers ( 5980 Wp) and so far all work very good.
    I confirm what Martin says:
    a) the trackers works very good
    b) Insertion and deactivation of 3 resistors depend by sun irradiation and home loads. All is working in real time and when water temperature is up ( 60 - 65°C) and recirculation pump have worked to equalize temperature of boiler tank and resistors tank, the system enter in standby mode.
    c) All energy for heating is produced from PV and the resistors working are modulated in stepping mode.
    d) The customer say ( he use GPL for boiler) that more 40% cut the cost of GPL in 1 year!
    e) All surplus energy is sended to distributor after water is hot. Therefore the owner
    can sell the residual electricity.
    f) All domestic machines (wash machine dishwasher etc..) do not suffer of presence of the pv-tank. They have the priority.
    g) The European Union and Fraunhofer Insitute are undertake to study a smart system to use energy for heating in more houses with a nearest homes equiped with PV plant.

    h) Very easy to install

    i) I am grateful for this discussion because 2 years ago I was worried about the project and I did without feedbak on results of this new technology

    All my customer are satisfied and the ROI is planned in 3-4 years
    thanks a lot

  135. J Moore | | #135

    SHW costs
    This article should be rewritten or at least revised to reflect recent incentives which bring down net costs for solar hot water systems. Like PV, tax incentives and rebates are critical components in cost calculations. In MA, the net cost of a SHW system for an average home (3 collectors; 80 gallon storage tank) is now below $2,500! The state of MA is encouraging SHW technology for good reasons- it is an accessible, inexpensive, efficient and proven renewable technology. Check out:

  136. Robert Berkman | | #136

    Using solar to heat hot water in radiators?
    My wife and I just moved to Rochester NY in a 1929 home with an old oil tank, burner and circulating hot water (not steam) radiators. We are exploring all sorts of options for our upgrade, ranging from just putting in a new gas boiler, mitsubishi hyper heat pumps and/or solar.

    One option--would it ever be possible to generate enough heat from photoelectric for circulating hot water in the radiators? (house is 1400 sq feet; lower floor is 1000 sq ft. and we could keep the upstairs turned way down). We also are going to be making our house much tighter after getting an energy audit--thanks! Bob

  137. User avater GBA Editor
    Martin Holladay | | #137

    Response to Robert Berkman
    Q. "Would it ever be possible to generate enough heat from photoelectric for circulating hot water in the radiators?"

    A. The electricity needed to operate the circulator(s) that are part of your hydonic heating system isn't much. But you are probably also thinking of finding a way to use electricity to heat the water used for your hydonic heating system.

    The most efficient way to do that with electricity would be with an air-to-water heat pump like the Daikin Altherma. That would be possible, but a Daikin Altherma system is expensive -- usually in the $20,000 to $30,000 range.

    Here's the bottom line: if you decide to install a photovoltaic (PV) system, the limiting factors are usually your budget or the area of your roof. If you can afford an 8-kWh (for example) PV system, and you have a good place to put it, it's possible that you can make enough electricity to meet all your needs. If you install a smaller system, you will probably make just some of your electricity. But there is no way to determine what the PV power is used for. It just reduces the amount of electricity you need to buy. A PV system lowers your electric bill, but the PV system doesn't care what the electricity is used for.

    If you want to use electricity for space heating, the most economical way to proceed is to buy one or more ductless (or ducted) minisplit heat pumps.

  138. Eric McKinney | | #138

    Evacuated tube solar collectors
    [Editor's note: Eric McKinney is employed by a company that sells evacuated tube solar collectors.]

    Given that solar PV and Thermal provide different types of energies the only way to compare these two technologies is by comparing applications that each technology can provide and it’s correlating ROI. While Thermal collectors do not produce electricity, the heat generated by evacuated thermal tube collectors when used in a properly design system far and away beats the return on investment into any PV panel/system on the market. Thermal energy also reduces the reliance on electricity by not requiring the need of electricity to fire up a system.

    One solar evacuated thermal tube collector capable of producing 300,000+ BTU's per day (or 1,095,000,000 BTU's per year) would suffice quite nicely for a properly designed residential system for the application of both space heat and domestic hot water and would work as such: First priority: space heat via forced air (electric, gas, geo-thermal) with dissipation heat (left over heat from the system) used for domestic hot water.

    This system would be geared toward both applications to maximize the thermal heat generated by this collector. The ROI for this system would be less than 3 years, while a PV installation targeting only hot water would be, at best, 6-8 years and perhaps closer to 10 years.

    Other advantages of Evacuated Thermal (Tube) Collectors over PV technology:
    1) Solar thermal energy will extend the life of mechanical equipment, which PV solar technology absolutely can not do. This is something proponents of PV conveniently overlook when comparing PV technology to thermal.
    2) Evacuated Thermal (Tube) Collectors work from the sun's UV rays not direct sunlight, as do thermal flat panels and PV technology. An evacuated thermal collector capable of producing 300,000+ a day during peak seasons will produce 200,000 BTU's per day, even during the cloudy winter months.
    3) Thermal heat applications provide for industrial/commercial and residential space heat, space cooling and domestic hot water with the possibilities limited only by the imagination and creativeness of engineers.
    4) Ease of repairs: ETC consist of tubes and there are no moving parts to the collector other than the transfer fluid running through the manifold. If a tube becomes damaged, the collector will still perform at a very high level while the damaged tube is easily replaced (within 5-10 minutes). In the heat of the day an actively UV collecting tube from an ETC would feel, to the touch of a bare hand, like an empty Coke bottle sitting out in the sun (a staple of heat collecting efficiency). PV panels, on the other hand, require substantially more time to disassemble/repair with concern for “hot to the touch” surfaces (a staple of heat loss inefficiencies for all solar panels).
    5) Dissipating heat in a solar thermal collecting system is required. Dissipation protects and enhances the performance of the system regardless of how the dissipation occurs – either redirected to another application, via the dissipater of the system or both.

    Here is a nice comparison of PV and Thermal: The Indianapolis International Airport has an installation of 44,128 solar PV panels with an “expectation” of producing 16,500,00 kWh of electrical energy annually. See the link for a few more bits of info. The figures for the cost for this installation “range” from $30 - $40 million dollars. While BTU’s and watts are different types of energy, when converting these 16,500,000 Kwh to BTU’s it would require only 685 evacuated thermal collectors capable of producing 300,000+ BTU’s per collector/per day at peak (with sizing for this raw comparison at 225,000 BTU’s per collector/per day) to produce this same amount of energy at a cost of less than 1/12 of the 44/128 PV panels. How about that for an ROI comparison?

    My suggestion to all – understand the expectations for your targeted applications to truly weigh the ROI when comparing Thermal to PV. And most importantly, do your research. An evacuated thermal collector capable of producing 300,000+ BTU’s per day does exist and is available.

    Yours in sustainable energy ~ EM

  139. Mike Mazzeo | | #139

    Solar Thermal isn't dead
    You stated "installing a solar hot water system doesn't make any sense" in the beginning of your blog, I wish you had an asterisk that linked to the end of your article where all of your "buts" were listed.

    PV may be a cheaper install, but a limiting factor for PV systems on many homes is the lack of roof space. Most homes in my area of NJ do not have enough roof space to accommodate a PV system that can produce 100% of their existing electric usage let alone switch from gas/oil/propane to an electric HPWH.

    If an OG-300 STHW system is used, with an COP of up to .90 versus .63, the economics for STWH become much better and require roughly 80 square feet of roof space versus the up to 170-200 square feet needed in your 1.7-2.2KW PV system size requirements. When roof space is limited, a properly designed and installed STWH will help save a homeowner more on their energy bills. Your math even makes this case but you didn't broach that. And only PV systems installed with ideal tilt and orientation produce at the 1.2-1.3 level you use in your examples. In NJ, few homes have roofs with ideal tilt and orientation. Many contractors new to the industry will install panels on any roof that sees the sun part of the year, further reducing the output per installed kilowatt annually. But most STHW systems, because of the small footprint required, can be installed to ideal tilt and orientation on most homes with full sun exposure.

    I question the two cases systems with 63% and 61% solar fractions. What system designs were used? And how can you tout an argument on such a low quantity of samples? We use a separate pre-heat tank for the our STWH designs to provide more solar energy and not compete with the supplemental fuel heat. Most homes have an existing, and functional water heater, so adding the separate solar water storage tank makes the system more efficient. And the cost for a second, or pre-heat tank system is within your estimated install costs in our area. I have average winter tank temps of 80 degrees F plus, reaching 120 degrees with an outside high temp of 20 in February. And I have 2 x 32 sf collectors on my roof that are 30+ years old. PV is guaranteed to produce 80% of what it did when new at that age, and it's starting at less than 16% sun to electric conversion when new.

    Summer over-performance and winter lack of performance can be adjusted with panel tilt, favoring the time of year the occupants of the site use the most hot water. Drain back and glycol based systems help negate overheating and freeze damage.

    STHW system maintenance is not much when the system is properly designed and installed. Most of the systems that we service having the greatest service problems are those improperly designed and attempting to heat everything (pool, water, space heat). The most reliable are those simply heating the domestic water with anti-freeze and overheat protection planned in the design. We service systems with original collectors, pumps, control often 25+ years old. When we need to replace components, we use only those proven to last 25-30 years in the field, not from a marketing brochure or lab tests.

    PV systems have a myriad of problems in their early existence. Inverter failures, rodents nesting under systems and chewing on wires causing shorts and possible fire hazards, first responder and local politicians fear of roofs covered with panels and electric hazards. Also, depending on what percent of the roof is covered with panels and where the damage exists, repair labor costs can become astronomical. Central inverters versus Micro or central with Power Optimizers will have an impact on troubleshooting and loss of savings during equipment failure/maintenance. I believe you are premature comparing 30 year old solar thermal system maintenance histories to several year old PV system maintenance. Repair costs will always be affected by the quality of the initial design and installation and how much the homeowner/system owner pays attention to the system like any appliance.

    When do these HPWH achieve COP of 2 or greater? My experience is, like gas mileage on car stickers, they never live up to the hype in the field. The quality of the design and installation contractor will have a major impact on the performance of the equipment. Like any government based program, including STHW in the late 70s, every underemployed contractor with no experience goes where the money is flowing. For years after 1985, our company made a living fixing the horrible installations by inexperienced contractors, and much of that maintenance cost is negatively associated with ST than the quality of the contractor.

    We will see the fallout of the quality, or lack there of, from the work of PV contractors soon. I expect you will have to update your low maintenance estimate of PV, not only for equipment failures, but for property damaged by poor installations from inexperienced contractors taking advantage of the government bubble created in the PV market.

    And seeing this original article is from 2009, would you have any updates to your maintenance costs estimates for PV? I'd be surprised if they haven't significantly exceeded what you anticipated when this was originally written. I know because we are experiencing a significant increase in PV system service work requests. An overwhelming majority of those systems were installed by contractors who have gone out of business. History is about to repeat itself in the solar field.

  140. User avater GBA Editor
    Martin Holladay | | #140

    Response to John Schlosser
    Whether or not a solar thermal system is cost-effective depends on the local cost of energy as well as the local cost of solar thermal equipment. The analyses in this article were made for North America; energy costs and equipment costs in China are, of course, different.

    Solar thermal equipment can be simpler in climates that don't experience freezing temperatures. However, I don't recommend that cold-climate homeowners install systems without freeze protection, with the idea that they will remember to drain their systems for the three coldest months of the year. All it takes is one cold night to freeze plumbing; if the homeowner forgets to drain the system in time, the equipment will need costly repairs.

  141. John Schlosser | | #141

    Simplified DHW question
    After travelling in China (incl northern areas) and seeing so very many residential rooftop solar hot water systems, am having trouble concluding that solar thermal for domestic hot water is impractical or relatively inefficient.
    Perhaps the key is installing a simplified system, rather than one optimized for everything all the time.
    Use the STHW system as pre-heat only; limit use to the 8-9 months without freeze risk; and use simplified overheat protection. Perhaps avoid anti-freeze and pumps altogether, routing DHW supply via the collector only with there's something to be gained thereby.

    Can you please comment on availability of such a system off-the-shelf and possible ROI for the Pacific Northwest climate?

    PS. Regarding freeze risk, a thermostatically-controlled drain valve is what I had in mind.

  142. Paul Soucy | | #142

    Solar Thermal is dead is incorrect:
    Solar Thermal is not dead:
    The SRCC OG 300 is a computer computation based on the design of the system and a properly designed system can offset 5000 kWh per year.
    Over the years I seem many poor solar hot water installations, the design of the solar hot water system and installation is everything.
    I always use a drain back system because I found over the last 30 years they are more reliable, I have systems still working over 25 years with no issues.
    If I am heating domestic hot water I use separate tank for the solar a 150 gallon polypropylene tank and a thankless gas back up or booster so if we got 3 days of cloud weather the water would first go through the solar tank and then get heated to the desired temperature.
    In the summer we have a bypass value and use only the water from the solar tank.
    I grew up in a family of 8 my father installed our first solar system in 1980 it used 2 - 4 x 8 solar collectors with a 150 gallon polypropylene tank that last a life time we had pumps fail over 30 years but that is it. The system provided all the hot water for our family. And many of my customers systems are still working to this day and are extremely happy.
    A properly designed system the quality of components used and installation has everything to do with the efficiency and payback. A properly designed solar hot water system will have next to no maintenance. Over 30 years we had a 1 pump fail the solar tank is still as good as the day we installed it. We needed to install new collectors when we did the roof over but that was not because they failed. We replaced them because we needed a new roof and the collectors where old.
    Whoever wrote the article is way off base, solar thermal is definitely not dead and a properly designed system with good equipment and installation can collect much more solar energy per square foot than P.V. system.
    On the high end our drain back solar hot water system installed runs about $8000 on the high end for 2 - 4 x 8 high quality collectors, 150 gallon solar tank and a tank-less booster or backup water heater.

  143. User avater GBA Editor
    Martin Holladay | | #143

    Reponse to Paul Soucy
    Thanks for your comments. Your comments confirm rather than undermine my argument.

    Your reported cost for a solar thermal system ($8,000) is in line with my estimated range of $8,000 to $10,000.

    Of course solar thermal systems work; I never claimed they didn't. I'm glad that you have had few maintenance problems with your solar thermal system.

    Concerning re-roofing: I also had to replace the roofing on my house. While you found it necessary (or appropriate) to replace your solar thermal collectors when your were re-roofing -- an expensive proposition -- I felt that there was no need to replace my PV modules when I re-roofed. I simply dismantled the PV arrays and put the old PV modules right back on the roof when the job was done.

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