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Solar Thermal is Dead

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

Posted on Mar 23 2012 by user-756436

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 pumpHeat pump that relies on outside air as the heat source and heat sink; not as effective in cold climates as ground-source heat pumps. 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 COP 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.”

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  2. Metro Solar Atlanta

Mar 23, 2012 8:26 AM ET

Some thoughts
by user-1095434

- 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.

Mar 23, 2012 8:41 AM ET

Edited Mar 23, 2012 8:48 AM ET.

Response to Marc Rosenbaum
by user-756436

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.

Mar 23, 2012 11:39 AM ET

too soon
by fpsco


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.

Mar 23, 2012 11:52 AM ET

Edited Mar 23, 2012 11:54 AM ET.

Response to Francis Robertson
by user-756436

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.

Mar 23, 2012 12:32 PM ET

Overpriced Solar Thermal
by Kevin_in_Denver


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.

Mar 23, 2012 12:41 PM ET

Edited Mar 23, 2012 12:42 PM ET.

Response to Kevin Dickson
by user-756436

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.

Mar 23, 2012 12:55 PM ET

Drain Water Heat Recovery
by JJ1

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.

Mar 23, 2012 1:01 PM ET

Edited Mar 23, 2012 1:02 PM ET.

Response to Jan Juran
by user-756436

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.

Mar 23, 2012 2:09 PM ET

Edited Mar 23, 2012 2:16 PM ET.

The Laws of Thermodynamics
by Ptkj254Byd

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

Mar 23, 2012 2:31 PM ET

Response to Jason Chenard
by user-756436

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.

Mar 23, 2012 4:02 PM ET

All true, to an extent
by Ptkj254Byd

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.

Mar 23, 2012 4:20 PM ET

Edited Mar 23, 2012 4:22 PM ET.

Response to Jason Chenard
by user-756436

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.

Mar 23, 2012 5:51 PM ET

Edited Mar 23, 2012 5:52 PM ET.

Is Resistance Futile ?
by 4y23MuNF7N

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...

Mar 23, 2012 6:25 PM ET

Edited Mar 23, 2012 8:02 PM ET.

PV + electrical resistance water heater
by runt05

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.

Mar 23, 2012 8:29 PM ET

Quit a good article for those
by jklingel

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.

Mar 23, 2012 9:15 PM ET

What does Thorsten Chlupp have to say???
by x5xv4K5JgN

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.

Mar 23, 2012 11:09 PM ET

Solar Thermal Is(n't) dead
by d296FrvJPL

"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)

Mar 24, 2012 12:40 AM ET

Edited Mar 24, 2012 12:41 AM ET.

"Of course the laws of
by jklingel

"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.

Mar 24, 2012 2:16 AM ET

Edited Mar 24, 2012 2:18 AM ET.

Solar Thermal DHW is Better in Sunny Places
by Kevin_in_Denver

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.

Mar 24, 2012 3:55 AM ET

Edited Mar 24, 2012 4:54 AM ET.

Response to Sean Hebert
by user-756436

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.

Mar 24, 2012 4:03 AM ET

Edited Mar 26, 2012 10:40 AM ET.

Bids from Les Dell and Kevin Dickson
by user-756436

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.

Mar 24, 2012 4:24 AM ET

Edited Mar 24, 2012 5:18 AM ET.

Response to Stephen Magneron
by user-756436

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.

Mar 24, 2012 4:28 AM ET

Response to Les Dell
by user-756436

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.

Mar 24, 2012 4:34 AM ET

Response to John Klingel
by user-756436

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.

Mar 24, 2012 1:48 PM ET

Martin: Got it. Thanks. I am
by jklingel

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.

Mar 24, 2012 8:06 PM ET

Cheers, Martin
by John Semmelhack

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.

Mar 25, 2012 5:05 AM ET

Edited Mar 25, 2012 5:17 AM ET.

Response to John Klingel and John Semmelhack
by user-756436

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.

Mar 25, 2012 4:20 PM ET

Edited Mar 25, 2012 4:24 PM ET.

don't recall
by jklingel

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.....

Mar 25, 2012 4:26 PM ET

by jklingel

semmelhack and i did some price fixing before posting

Mar 25, 2012 5:20 PM ET

large tank costs
by DrDanger

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)

Mar 25, 2012 7:05 PM ET

Stuck on thermo theory
by user-1114009

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.

Mar 25, 2012 7:53 PM ET

Edited Mar 25, 2012 7:56 PM ET.

What about the Secusol appliance
by user-914333

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?

Mar 25, 2012 8:59 PM ET

me, too, but dropping it
by jklingel

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.

Mar 26, 2012 4:49 AM ET

Edited Mar 26, 2012 8:45 AM ET.

Response to Jonathan Teller-Elsberg
by user-756436

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.

Mar 26, 2012 4:54 AM ET

Response to Rich Cowen
by user-756436

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.

Mar 26, 2012 2:04 PM ET

Lots of fuzzy math regarding heat pump water heaters
by user-1004076

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.

Mar 26, 2012 3:03 PM ET

Response to Dana Dorsett
by user-756436

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.

Mar 26, 2012 3:41 PM ET

So I guess I AM surprised...(response to mholladay)
by user-1004076

...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?

Mar 26, 2012 10:16 PM ET

Edited Mar 26, 2012 10:17 PM ET.

by wjrobinson


No soup for youze guys and your slanted studies.

Mar 27, 2012 5:50 AM ET

Edited Mar 27, 2012 5:54 AM ET.

Response to Dana Dorsett
by user-756436

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.

Mar 27, 2012 5:53 AM ET

Response to AJ Builder
by user-756436

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.

Mar 27, 2012 7:39 AM ET

by gusfhb

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:

Mar 27, 2012 8:05 AM ET

Response to Keith Gustafson
by user-756436

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.

Mar 27, 2012 8:15 AM ET

Edited Mar 27, 2012 9:30 AM ET.

by gusfhb

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

Mar 28, 2012 2:16 PM ET

Solar Thermal still has a pulse
by w5L7aPNHQx

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.

Mar 28, 2012 2:44 PM ET

Edited Mar 28, 2012 2:57 PM ET.

Response to Robert Del Mar
by user-756436

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.

Mar 28, 2012 4:15 PM ET

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.

Mar 28, 2012 4:35 PM ET

Edited Mar 28, 2012 4:40 PM ET.

When you're hot....
by user-1006269

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

Mar 28, 2012 4:49 PM ET

One more point about solar thermals systems
by user-756436

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."

Mar 28, 2012 5:19 PM ET

3 thoughts
by SxPvAwVJNy

- 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.

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