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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 Martin Holladay

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 28, 2012 7:05 PM ET

Edited Mar 29, 2012 1:41 AM ET.

Installation Logistics Favor PV, WREN is Coming
by Kevin Dickson, MSME

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

Mar 28, 2012 7:10 PM ET

1 last thought
by Frank Flynn

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.

Mar 28, 2012 8:41 PM ET

HPHW in unheated basement in ME does fine
by ice rabbit

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.

Mar 28, 2012 8:41 PM ET

PV prices, Chinese modules, and the Grid
by John Semmelhack

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.

Mar 28, 2012 10:16 PM ET

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.

Mar 29, 2012 4:47 AM ET

Response to Richard Schmidt
by Martin Holladay

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.

Mar 29, 2012 4:51 AM ET

Edited Mar 29, 2012 4:52 AM ET.

Response to John Semmelhack
by Martin Holladay

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.

Mar 29, 2012 4:58 AM ET

Edited Mar 29, 2012 4:59 AM ET.

Response to Frank Flynn
by Martin Holladay

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.

Mar 29, 2012 11:26 AM ET

Fun Factor
by Kevin Dickson, MSME

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.

Mar 29, 2012 12:29 PM ET

Solar Thermal IS dead
by Don Mallinson

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.

Mar 29, 2012 12:31 PM ET

Edited Mar 29, 2012 12:32 PM ET.

Response to Kevin Dickson
by Martin Holladay

"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.

Mar 29, 2012 1:22 PM ET

Whoa... Look at where you are before you leap to this conclusion
by Leslie Baer

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:

Mar 29, 2012 1:37 PM ET

I don't get it
by Graham Irwin

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.

Mar 29, 2012 1:52 PM ET

Response to Leslie Baer
by Martin Holladay

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.

Mar 29, 2012 1:57 PM ET

Response to Graham Irwin
by Martin Holladay

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.

Mar 29, 2012 7:04 PM ET

What Error?
by Graham Irwin

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?

Mar 29, 2012 9:55 PM ET

Load reduction vs. load shifting
by Katy Hollbacher

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!

Mar 30, 2012 5:01 AM ET

Edited Mar 30, 2012 8:29 AM ET.

To Graham Irwin: about your error
by Martin Holladay

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.]

Mar 30, 2012 5:07 AM ET

Response to Katy Hollbacher
by Martin Holladay

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.

Mar 30, 2012 8:09 AM ET

Edited Mar 30, 2012 10:23 AM ET.

Energy output per square foot
by Martin Holladay

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.

PV versus solar thermal table - 3.jpg

Mar 30, 2012 12:17 PM ET

Response to Response to Robert Del Mar
by 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.

Mar 30, 2012 2:00 PM ET

No Error!
by Graham Irwin


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.

Mar 30, 2012 2:11 PM ET

Response to Graham Irwin
by Martin Holladay

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.

Mar 31, 2012 1:32 AM ET

What a great discussion!
by Michael Horowitz

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.

Mar 31, 2012 5:53 AM ET

Edited Apr 2, 2012 8:49 AM ET.

Response to Michael Horowitz
by Martin Holladay

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.

Mar 31, 2012 1:02 PM ET

Issue with the details, not the conclusion
by Michael Horowitz


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.

Mar 31, 2012 2:04 PM ET

Edited Apr 1, 2012 6:44 AM ET.

Response to Michael Horowitz
by Martin Holladay

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.

Mar 31, 2012 4:46 PM ET

GSHP and conditioned space
by Mary Florence Brink

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

Apr 1, 2012 6:41 AM ET

Response to Mary Florence Brink
by Martin Holladay

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.

Apr 1, 2012 10:10 AM ET

HPWH discussion a distraction?
by James Morgan

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?

Apr 1, 2012 12:08 PM ET

Response to James Morgan
by Martin Holladay

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.

Apr 2, 2012 12:59 PM ET

Edited Apr 2, 2012 1:06 PM ET.

Excellent article and comments
by Bob Z Rational Energy Solutions

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.

Apr 2, 2012 1:48 PM ET

Response to Bob Z
by Martin Holladay

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!

Apr 2, 2012 6:27 PM ET

Air source HP and PV
by Bob Z Rational Energy Solutions


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.

Apr 4, 2012 11:30 PM ET

Ontario says Solar Thermal is King
by Steve Dyck

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 as $8 per watt, unless you do it yourself, and avoid the structural engineering review. So it is either a $25,000 investment for the 5 kW (plus $3,000 for the heat pump water heater), or a $16,000 investment for the 2 kW plus a heat 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.

Apr 5, 2012 11:50 AM ET

By Definition
by Peter Hastings 4C

inequitable subsidy structures for renewables

Please describe an equitable subsidy structure.

Apr 6, 2012 5:20 AM ET

Edited Apr 6, 2012 5:21 AM ET.

Response to Peter Hastings
by Martin Holladay

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.

Apr 9, 2012 2:00 PM ET

Response to Graham Irwin
by Marc Rosenbaum

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?

Apr 12, 2012 10:59 PM ET

Edited Apr 12, 2012 11:09 PM ET.

Does this assume net metering
by Keith Davis

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)?

Apr 13, 2012 4:04 AM ET

Edited Apr 13, 2012 10:31 AM ET.

Response to Keith Davis
by Martin Holladay

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.

Apr 27, 2012 1:45 AM ET

Big picture time..
by Talon Swanson

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.

May 2, 2012 3:56 PM ET

New Technology in ST
by Mike S.

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.

May 2, 2012 4:27 PM ET

Response to Mike S.
by Martin Holladay

Thanks for the information; it's interesting.

For interested GBA readers, here is a better link:

Jul 27, 2012 5:59 PM ET

by Colin Dumais

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.

Jul 28, 2012 5:58 AM ET

Reply to Colin Dumais
by Martin Holladay

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.

Jul 30, 2012 7:45 PM ET

Late to the Party but Still Intrigued
by Roger Davenport

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....

Jul 31, 2012 6:10 AM ET

Response to Roger Davenport
by Martin Holladay

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.

Sep 18, 2012 10:58 PM ET

Real world data
by Bob Lemaire

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.

Sep 19, 2012 6:26 AM ET

Response to Bob Lemaire
by Martin Holladay

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."

Sep 19, 2012 10:02 AM ET

Edited Sep 19, 2012 10:11 AM ET.

Hindsight is 20/20
by Bob Lemaire


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.

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