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Musings of an Energy Nerd

Solar Thermal Is Really, Really Dead

A new analysis shows how much cheaper it is to heat water with PV modules than with solar thermal collectors

Back in early 2012, in an article called “Solar Thermal Is Dead,” I announced that “it’s now cheaper to heat water with a photovoltaic array than solar thermal collectors.”

Now that almost three years have passed, it’s worth revisiting the topic. In the years since that article was written, the cost to install a photovoltaic (PV) system has dropped significantly. Moreover, I’ve come across monitoring data that allow for a more accurate estimate of the amount of electricity needed to heat water with electric resistance elements or a heat pump.

First, I’ll present my assumptions.

How much does a solar thermal system cost?

In my earlier article, I estimated that a residential solar thermal system with two 4′ by 8′ collectors and a solar storage tank with a capacity in the range of 80 to 120 gallons costs between $8,000 to $10,000 to install. I stand by that estimate.

Of course, some contractors can beat this price, while others will charge significantly more. (In a recent comment posted on GBA, an Ohio-based solar contractor named Daniel Young estimated that the solar thermal system I describe would cost $16,250.) For the purposes of the comparisons made in this article, I’ll assume that the installed cost of a residential solar thermal system is $9,000.

How much does a PV system cost?

My calculations are based on a PV system cost of $3.74/watt. The figure comes from a the “Solar Market Insight Report 2014 Q2” published by the Solar Energy Industries Association.

Some GBA readers have received quotes of $3.50/watt for a PV system, while others are still paying $4.00/watt or more. One thing’s for sure: prices for PV are still dropping.

The price comparisons made in this article do not include any incentives, rebates, or tax credits.

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  1. user-2890856 | | #1

    There is room and reason for both technologies Martin .

  2. GBA Editor
    Martin Holladay | | #2

    Response to Richard McGrath
    I'm not sure what your point is, or why I should change my opinion because you have provided a link to a company that sells solar thermal equipment (or equipment that combines PV and solar thermal equipment on the same roof).

    If anyone wonders whether this equipment works, the answer is: yes, it does.

    The only problem is that the equipment isn't cost-effective.

  3. GBA Editor
    Martin Holladay | | #3

    Response to AJ
    Every situation is different. As I wrote, my article gives you enough information to make your own calculations. If your situation favors a solar thermal system, by all means install one.

    There is no need to limit a PV system to the size required to operate a heat-pump water heater. If PV makes sense in your area -- due to high electricity rates or a favorable net-metering agreement -- by all means, install a bigger PV array. Simply do the math. If PV makes sense, install as big an array as you can afford, or as big an array as your local net-metering contract allows.

    On the other hand, if PV doesn't make sense, don't install a PV array. It's worth pointing out, however, that if a PV array doesn't make economic sense in your area, it's unlikely that a solar thermal system will make economic sense either.

  4. fitchplate | | #4 the HP heat free?
    Money for nothin', heat for free.

    Seems that no matter how many times the "capital cost" numbers are run, the PV + HP solution comes out ahead of the solar thermal. (Unless the ST is DIY). But one issue confuses me (as per AJ’s point): am I paying to heat the air the HP uses? If so, wouldn't system operating costs (not just capital costs) be considered? If the HP's COP is 1, wouldn't the cost of my central heating system and the fuel it uses to produce the BTU’s scavenged by the HP to make DHW have to be added in to compare real hot water costs of the options? At a rate of one-to-one?

    PV system + HPHW heater + prorated capital cost of space heating system + fuel

    Basement heat is not free in my house.

    Thanks, Martin for leaving out the rebates, grants and tax deductions from this article.

  5. GBA Editor
    Martin Holladay | | #5

    Response to Flitch Plate
    In my first analysis -- the one I trust -- the option with an electric-resistance water heater plus PV is 25% cheaper than the solar thermal option. That option has nothing to do with heat pumps. If your calculations show a different result, trust your own calculations before you trust mine.

    A heat-pump water heater will almost always cost less to operate than an electric-resistance water heater, even if it steals a little space heat for a few months a year. Read more of Marc Rosenbaum's articles on this topic. Again, even with stolen space heat, you're likely to come out way ahead with a heat-pump water heater.

    But if you prefer a solar thermal system, it's your choice.

  6. vensonata | | #6

    A revolution has occurred...thanks
    Martin's original article on "is solar thermal dead?" appeared everywhere. It was the shot heard round the world. Now it appears the war is over. Yes, in the comments there are a few hold outs, but the math is overwhelming. Consider off grid for an even clearer victory. I have a huge PV array of 11.4 kwh to run an off grid retreat center. It is sized to virtually eliminate diesel even in cloudy December. But the huge amount of over production 9 months a year can be used to heat domestic hot water. It renders solar thermal pointless. The production is so large that simply an electric resistance tank will do. Even in December there are days when I overproduce by 20 kwh and cannot store it in the 40kwh battery bank...might as well produce solar hot water or the electricity is simply grid to feed into.
    One more point for those who believe that heat pumps rob from the house heat. Think about it. You don't heat your house 7 months of the year! And that is the 63% fraction that Martin is talking about in useful supply.

  7. iLikeDirt | | #7

    Pretty decisive
    Heat for an indoor HPWH has to come from somewhere.

    In uniformly hot climates, it comes from the sun heating up your house so the cost is ridiculously low and you get some free AC. Score!

    In uniformly cold climates, it comes from your heating system. If you heat with electric resistance, the HPWH's COP is probably lower than 1 because you are using equipment to MOVE electric resistance heat generated from elsewhere. If you heat with gas or a heat pump with an outdoor compressor, it's as if you were generating the heat with those appliances, with a slight penalty due to the electricity from the heat pump in the HPWH. So the bulk of your water heating bill simply shows up as a small addition to the space heating component of your utility bill.

    In mixed climates, both of these things hold true and it is probably still a big net win due to the double benefit when it's hot.

    I'm not convinced that the complications of a HPWH are worth it yet, especially in cold climates where electric resistance heat is used, and considering the greater mechanical complexity. The engineer in me prefers mechanically simpler devices with fewer moving parts.

    Still, the fact that straight-up PV+electric resistance is cheaper than solar thermal speaks for itself, to me. And on top of that, there's nothing stopping anyone from installing sufficient PV and really nice electric tank like a Rheem Marathon or one of those Westinghouse things with a stainless steel inner tank, and then adding a supplementary heat pump heater, which itself could even use outside air. That's what I'm planning on doing when my gas tank kicks the bucket. Heck, you could even add a DIY solar thermal pre-heater system at that point if you wanted.

  8. Tedkidd | | #8

    Nice job Martin.
    I think

    Nice job Martin.

    I think it's really hard to justify solar thermal unless you have use for the summer btu (don't dump them) and/or you live far enough south that winter and summer production are more even.

    I suspect gravity fed systems where there is no freeze risk are still probably a better play.

    Solar hot water still has a strong place in high use situations with big program incentives, like dairy farms.

  9. fitchplate | | #9

    ... best cost not yet posted
    If you make it a DIY solar thermal + resistance HW heater, its costs even less: $3500. and Closed Loop

  10. jinmtvt | | #10

    DIY can't be beat, because you compensate inefficiency for area ( when available ) at low additional costs. But i do not believe that this blog by Martin, is intended to discuss DIY , rather options that regular folks can purchase turn-key .

    DIY PV is also cheaper by a large factor, same as with solar thermal.

    FYI, it is hard to find HPWH for less than ~ 1600$CAD in Quebec, ( Stiebel sales for 3000$+ ) whereas i can find a first quality 6 years warranty 60gal for ~ 350$. HPWH do not make much sense here, as a regular tank replacement call ranges from 500-700$ including the tank in most of QC.

    I used to think that solar thermal hot water heating was a good option for south sunny climates, but i am not sure anymore. If you use a HPWH inside the envelope with PV... would need to run numbers.

  11. GBA Editor
    Martin Holladay | | #11

    Response to Flitch Plate (Comment #10)
    You have raised an interesting point: do-it-yourself installation will lower the cost of a solar thermal system.

    But that fact can't be used in a one-sided way to favor your favorite technology. All of the numbers will need to be changed if you want to consider a do-it-yourself installation:

    Do-it-yourself PV systems are much cheaper than $3.74/watt, as Jin Kazama notes.

    Do-it-yourself installation of an electric-resistance water heater or a heat-pump water heater will also be significantly cheaper than professional installation. So all of these numbers have to change, too.

    If you know how much these jobs will cost you, make your own calculations. Don't forget the cost of the solder, flux, copper tubing, fittings, and electrical wiring.

    When you are all done installing your do-it-yourself solar thermal system (and I know, because I've done it), you are likely to find that your system has more glitches and maintenance issues than a professionally installed system. And there will be no contractor to call with your complaints.

  12. GBA Editor
    Martin Holladay | | #12

    Response to Ven Sonata (Comment #7)
    I'm glad you agree with the conclusions of this article. But I have to disagree with you on one point: in most cases, using PV power to heat domestic hot water doesn't make much sense for off-grid homes.

    Your case is unusual. Few off-grid homeowners can afford a $42,000 PV system like yours (a system rated at 11.4 kW). In most cases, off-grid homeowners who want to heat domestic hot water with some type of solar equipment are better off with a solar thermal system.

    As my article notes, this advice doesn't apply to grid-connected homes. Owners of grid-connected PV systems (unlike owners of off-grid PV systems) can benefit for the fact that they receive a credit from the local utility for their excess electricity production during the summer.

  13. vensonata | | #13

    Has the revolution come to off-grid too?
    Martin thanks for the surprising opinion on off grid economics of pv vs solar thermal. I should clarify that the 11.4 kWh system serves a 10,000 sq ft house which has a population of from 10 to 25 people. It is a retreat center. So the system is actually small for what it does. It is only because we are hyper efficient that the system is adequate. I believe all long time off grinders need to rethink the old formulas because of the drastic fall in pv prices. The old formulas of 85% pv 15% generator, was sensible. I now think close to 97% pv 3% generator is a better formula. As well 3 to 5 days battery storage can be reduced to half of that....the reason is Pv should be tripled and that gets you through a few cloudy days with the cheaper electricity of pv vs expensive battery kwh. In general it is being realized that overbuilding wind and pv is cheaper than storage up to a point. This all happened only in the last two years. Part of the new formula means one has vast overproduction about 9 months per year and off grid you have to use it somehow...enter domestic hot water...the new battery! This could all change agin within a few years if batteries take a sudden plunge in price, which looks certain...from about 20 cents kwh to 10 cents or even 5 with lithium of Aquion saline batteries ( now available at Alt E store...3000 cycles to 100% discharge, loses le lifecycle 6000 at least). Anyway the off grid readers of green building advisor are a small minority but we all need to redo the math every couple of years in this time of rapid change. I'd love to hear from thoughtful number crunchers if my math is wrong!

  14. Expert Member
    MALCOLM TAYLOR | | #14

    DIY and Costs
    DIY is the bane of any discussion of costs. Hardwood becomes cheaper than vinyl floors, metal roofs cheaper than asphalt, passive houses are cheaper than code minimum ones. It just muddies the water.

  15. user-984364 | | #15

    Are there good controls for PV hot water?
    So, even PV + pure resistance water heat looks cheap. Great!
    And a tank of hot water is built-in energy storage, also great.
    Are there tanks and controllers which can really exploit these things?
    In other words - are there any resistance (or heat pump) tanks which are willing to go up to 180F or so with a mixing valve like solar thermal would, and smart controls on the tank to only heat up when the sun is shining?

  16. GBA Editor
    Martin Holladay | | #16

    Response to Eric Sandeen
    I don't know if you are talking about grid-connected houses or off-grid houses.

    For grid-connected houses, there is no need for any type of special controls. The PV array is on the roof and is feeding electricity to the house and to the grid (through a grid-tie inverter) whenever the sun is shining. The PV array is not hard-wired directly to the water heater. The calculation made to determine whether a PV array is large enough to meet a home's water heating needs is just a mathematical calculation. The PV array continues to work just fine, even when the PV array is smaller or larger than a hypothetical array sized to exactly meet the home's needs for heating water.

    If you are talking about an off-grid house, the PV array is not wired directly to the water heater either. The PV array is wired to a battery and controlled by a charge controller. If you want to set up an off-grid PV system so that excess electricity is used to heat water once your batteries are fully charged, you need to install a controller called a dump load controller (also called a diversion load controller).

    If you live in an off-grid house and you think that you will have so much extra electricity during the summer that an ordinary water heater won't be able to handle all that electricity, you can simply buy a larger water heater or turn up the temperature control on the water heater to its maximum setting.

  17. woodreader | | #17

    Direct connect MPPT solar power
    May be worth taking a look at.
    No battery. No charge controller. No inverter........

  18. GBA Editor
    Martin Holladay | | #18

    Response to Richard Turner
    The advantage of a grid-connected PV system is that the homeowner gets credit for 100% of the electricity produced by the PV array.

    The system shown in the YouTube video that you linked to wastes electricity, since the water heater's aquastat will disconnect the power from the PV array when the setpoint temperature is reached. If the family goes away on vacation, the value of the electricity is lost.

  19. user-705006 | | #19

    What about space heating?
    The article makes a very compelling case for using PV to heat domestic hot water. What about those that have a hydronic heating system (radiators), and like to use the power of the sun to supply or support their space heating system? Would that change the outcome of the analysis?

  20. GBA Editor
    Martin Holladay | | #20

    Response to Marcus de la fleur
    For a variety of reasons, investing in solar thermal equipment to use for space heating is even less cost-effective than investing in solar thermal equipment for heating domestic hot water:

    1. During the coldest months of the year, when space heating is most needed, solar energy is at its lowest ebb.

    2. All of the energy gathered during the summer is wasted.

    For efficient space heating with some solar input, the best approach is to have a grid-connected PV system plus ductless minisplits.

    The simplest way to use sunshine for space heating is to install south-facing windows. Even this approach must be used sparingly, however, or you will find that the investment isn't cost-effective: Study Shows That Expensive Windows Yield Meager Energy Returns.

  21. woodreader | | #21

    Response to Martin
    Thank you Martin for your counter point response,
    The thought of having the least amount of PV dedicated to heat water and by passing the expense of setting up net meter expense struck my imagination. If a 4500 watt element can recharge a tank of water in 10 minutes, then 60 minutes at 500-750 watts might be an inexpensive solutions. Look like with a two 3800-4500 watt element tank that also can be plug in to the grid that you can have the top element make up for any shortage of hot water. I live in Georgia where the weather is mild and if the math work out to 500 watts PV or less might prove interesting. All view points are welcome.

  22. 4smartheating | | #22

    Solar Heat Pump - More Performance at Lower Cost
    Good article Martin you are on the right path, solar hot water really is dead because it is too expensive at $8000 upfront to save $400 year. There is a better hydronic heater alternative that combines Renewable Energy + Energy Efficiency.

    The best water heating comes from a new hybrid category called Solar Heat Pumps for greater performance and improved economics as a DHW heater, and radiant floor heating, chilled water cooling, and power. It was featured at Denver NREL in October, and is being installed from the Arctic to Caribbean. brand from Canada is like a geothermal/ground-source heat pump in that it can produce heat on demand 24/7, but it is better than water/glycol-mix because it uses refrigerant directly in new solar panels as a superior heat transfer fluid/phase change material. The cost is comparable to a ductless mini split, and so is the installation of panels that replace the outdoor fan-condenser. This could be the start of the next article idea, "Ground Source Heat Pumps are Dead" - for residential.

    Unlike current Solar Thermal or PV that only works when the Sun is shining on the panels, about 2 hours here today, the SunPump panels are a solar evaporator that produces heating/cooling on demand even in the dark. Right away, that is about 4 times the capacity and solar fraction is no longer a limitation since full daily requirements are met even in the Winter.

    Cost wise, a 12,000 BTU capacity SunPump with two solar evaporator panels and mounting hardware is between $4000-5000 installed. With a COP of 4 not including solar gains, it fits well into either of the two scenarios (62%, 75%), above if you include the 875 watt consumption to run the compressor.

    This is great news for Passive House or Net Zero homes that need small VFD DC units to do both space and hot water.

  23. fitchplate | | #23

    nothin' there
    Tania ... I went to the URL you posted and could not find any products.

  24. fitchplate | | #24

    How well developed is the "solar pump" technology
    This is a Canadian product by a company called Solaris; and off shoot of a geothermal company that is bringing this solar powered refrigerant heating technology to market. Too fast it seems as they have no R&D or product details in the public domain.

    They must be young and developmental. Canadian operation is a Lease program, there does not appear to be specific unit for sale.

    What Is The SunPump Solar Breakthrough?

    Unlike Solar PV or Thermal, SunPump is able to produce energy on demand, Summer and Winter, Day and Night, which changes everything. This solves the painful solar defficiencies of storing energy and intermittency when the Sun goes down.

    How Is SunPump Different Than Solar PV?

    Solar PV (Photo Voltaic) converts visible light into DC power, while SunPump converts a wide spectrum of light into heat. SunPump v.4 uses Combined Heat & Power (CHP) in one panel to achieve multiple times more energy from the same panel surface area.

    How Is SunPump Different Than Solar Thermal?

    The key differences are in the heat transfer fluids, solar thermal is a water/glycol mix, and SunPump uses a phase-change liquid common in better refrigeration systems like heat pumps or air conditioners. While both work well in direct Sun, only SunPump is able to work in the dark, poor weather, and at sub-freezing ambient temperatures. More…

    What Is Special About The SunPump Panels?

    The black panels are single piece seamless aluminum with complex micro-channels that circulate refrigerant in order to boil or evaporate into a gas at -50 C. The reult is a simple, reliable unglazed panel that can boil the heat transfer liquid at ambient temperatures above -50 C.

    How Does SunPump Compare to Ground Source Heat Pumps?

    Instead of using a “geo” ground loop or wells to pull heat using a water/glycol mix, SunPump uses refrigerant directly into roof mounted solar panels to pull much more energy by lowering the fluid transfer exchange temperature and adding abundant solar energy. The result is higher efficiency at less than half the cost.

  25. jinmtvt | | #25

    never heard of ...

    this is one selling/installing company with some product infos
    • COP 7 in sunshine, and 2.7 at night

    looks like either standalone aluminum condensor type or hybrid on PV .

    if the COP of 2.7@night down to -15c is true, this is something to be considered ( on their water heater brochure )

  26. user-347767 | | #26

    Zombie Technology
    I feel like there should be a memorial service for solar thermal. My head knows it's gone but my heart doesn't want to let go.

    However, the secret is out. Lennox is offering solar PV installation with some of their heat pump and AC units. I think other big companies will likely follow suit. This may mean that PV will be added to services that HVAC installers routinely offer.

    I think that Sunpump unit still has many of the drawbacks of solar liquid thermal - possible leaks, running tubes and pipes, only useful for heat, etc. The joy of using PV is that the electricity generated can be used for so many things plus export/import with grid interactive units. I think the SunPump is like building the worlds best steam 1950 (another zombie tech).

    I'm not so sure about solar air thermal. The cost is so low and reliability so good that air panels may still have life left.

    Goodbye solar thermal. You served us well.

  27. GBA Editor
    Martin Holladay | | #27

    Response to Tania G (Comment #23)
    It's way too early to trumpet the technology you are excited about. It seems that this start-up company has developed hybrid solar collectors that combine PV and thin tubing that circulates refrigerant.

    Before anyone gets excited about this technology, we need to know:

    1. How much the hardware costs.

    2. How easy it is to run refrigerant in leak-free tubing from a heat pump to collectors on your roof.

    3. How many technicians exist to install and repair these collectors.

    4. How dependable the hardware is.

    In 5 or 8 years, we may have enough information to assess this idea. For now, I'm a skeptic.

  28. user-2453173 | | #28

    Responce to comments #3,#4
    Just want to point out in a bit more detail what AJ's point #2 on PV cost really means.

    The study Martin references is for a 6kW solar PV system on your typical residential roof. $3.73/watt is a really accurate dollar figure at that size (my costing sheet tells me $3.70 as a reference). There are a lot of fixed costs in there for your typical solar contractor (permits, net-metering applications, structural engineering for the roof, the cost to mobilize an install crew, etc...) so as you take a system and make it smaller, the cost goes up. In reality a 0.73kW PV system could cost more than $8k (aka >$10/watt) if you could even get an installer to show up, while a 15kw system would cost closer to $3-$3.10/watt (we've installed a few residential system of that size this year, it happens). Martin addresses this in comment #4, but i just wanted to add some detail here.

    I think the assumption here is that if you are installing a Solar PV system on your home, you can analyze what the benefit is for you if you take the cost of 0.73kW of your 6kW whole home PV system. I don't think Martin is suggesting you install 3 solar modules on the roof so you can run a HPWH, and stop there. (Not trying to put words in your mouth Martin)

  29. GBA Editor
    Martin Holladay | | #29

    Response to Daniel Young
    You are correct.

    To my way of thinking, it doesn't make much sense to install a PV system if the PV system costs a lot more than it saves. Fortunately, in many areas of the U.S., homeowners who install a PV system are saving more money than they are spending (over the predicted life of the system).

    In some locations, a solar leasing company will put PV modules on your roof for no money down, and will begin selling you electricity for a lower cost than the local utility. That arrangement is a win-win-win situation.

    If you live somewhere where PV makes sense, I assume that you will install a system that is large enough to make the financial calculations worthwhile. If all you can afford is a tiny PV system, you'll probably be better off if you simply keep paying your local utility whatever they are charging (instead of hiring a contractor to put 2 or 3 PV modules on your roof).

    One final point: if you can't afford a $9,000 PV system, you probably can't afford a $9,000 solar thermal system either -- making this whole worry about the cost of a tiny PV system moot.

    Assuming that you are a middle-class or upper-class homeowner with $9,000 burning a hole in your pocket, I certainly think that it's worth asking the question, "What type of solar equipment should I spend my money on -- a solar thermal system or a PV system? And if I choose a PV system, how many watts of PV will I need to heat my domestic hot water?"

  30. markgimmeshelter | | #30

    Solar Thermal is indeed badly
    Solar Thermal is indeed badly wounded and really what an amazing story that PV has achieved such success. For those of us off grid ven sonata's comments are spot on concerning the new economics of off grid PV systems,
    the curse of back up generators is becoming a much smaller part of the picture as systems can be affordably over sized providing excess power for much of the year some of which might be be captured and stored as heat. The future of back up generation may well be the hybrid in your garage for the seasonal long dark December stretch.

  31. vensonata | | #31

    Reply to Mark Klein
    Yes Mark (and everybody else off grid), the new math of BIG solar arrays is upon us. It is parallel to the the "super-insulation" it used to be R 20 walls were a good choice in colder climates. Now r40 makes sense. 85% pv made sense at $10 watt installed, but surely at $3 watt it can make sense to triple your solar pv. In sunny places like Arizona you are then plus100%, in cold cloudy December Canada at 4000ft and 51 latitude like me you will hit about 97% solar. The generator may run 25 hours a year instead of 300 hours. Now what? Suddenly propane hot water makes no sense 8 months a year when you have huge amounts electricity over production. Voila suddenly you are paying off your pv system with 90% diesel reduction plus wear and tear on the generator plus propane and lifespan of the (still necessary) propane hot water system for winter use. Payback for me is ten years through dispel propane savings. But soon we will add an electric vehicle because even hot water does not use up the excess...another $2000 year gas savings. Payback goes down to six years. Etc, etc, etc. once again I say all off grid math needs to be redone.

  32. GBA Editor
    Martin Holladay | | #32

    Response to Ven Sonata
    Your argument is compelling. The new reality of cheap PV will indeed change many of the old rules of thumb for off-grid homes.

  33. wjrobinson | | #33

    My post, is being misread. I
    My post, is being misread. I thought it was clear. I changed the PV value based on the idea that the heat pump water heater will not achieve a very good COP in climate zone 6. As time keeps ticking I also realize that summer dehumidification of a basement is valuable so... who knows what is a wash and what is not. Marc knows more about this than all of us.

    Anyway my post prior has nothing to do with PV size, cost, whatever. It is to do with COP/HPWH.

  34. GBA Editor
    Martin Holladay | | #34

    Response to AJ Builder
    Thanks for your clarification.

    Just to defend the other commenters, though -- it should be pointed out that in Comment #3, you wrote, "No one is getting a small PV array installed, if they did the cost would be quite higher you would think." I think that is the sentence that Daniel Young was responding to in Comment #30.

  35. wjrobinson | | #35

    Close to making sense and also not IMO
    1-Heat pump source not calculated correctly,it just moves heat created and paid for at less than 1 COP via more than 1 COP in climate zone 6, my area.
    2-No one is getting a small PV array installed, if they did the cost would be quite higher you would think. However my attached redo of Martin's two spreadsheets uses Martin's numbers, I just changed the heat pump Kw for PV because it's COP to me is effected by it's theft of lessor COP heat.
    3-Heat pumps have many down sides so far besides where the heat is coming from, noise, lifespan, cost.
    4-Good electric tanks cost much more, such as a Marathon, more of an equal comparison to use.
    5-It has been mentioned that if we could get solar thermal installed in all homes the install price might come close to to 1/4 of what is quoted in this blog.
    6-Many in my area enjoy some form of water, like a lap pool, hot tub, standard pool. The heat from solar thermal if used fully when it generates excess radically changes the value of thermal to a positive. It has to though I have been wrong more than not when postulating off hand verses the likes of Marc R and is fantastic monitoring skills.

    Let's do a few more charts with more ways of looking at this. I need a chart that covers at least every point 1-6 and I may think of more. I'll post such.

    As to PV, I do love it and wish I was fully PV with an electric F350 fueled by it too. ;) Attached my favorite use of PV

  36. BCinVT | | #36

    Thanks once again, Martin,
    Thanks once again, Martin, for pushing the discussion forward on all the changes in energy consumption. For me contractor-installed thermal systems have never made sense economically. I still think they make all kinds of sense for DIY use.

    But Ven's posts have been great in confirming my perceptions about PV. I installed my own system this past summer, and in doing the calculations and comparing costs I came to exactly the same conclusion. I went with a smaller (and cheaper) battery bank and a bigger PV array. Even on a fairly cloudy day in December I can get back up to full charge, and only after 2 weeks with no sun did I get the generator going. I have plans for the excess power in sunnier months , so it will not go to waste.

  37. fitchplate | | #37

    Big PV simply does not
    Big PV simply does not interest me. I find the idea of a small grid tied PV array an economically appealing retrofit. I see a 0.96 Kw kit advertised for $2300 and 50 gal GE heat pump for $1200. If that supplies an annual average 60% of our DHW needs that saves on more expensive BTU’s from burning propane.

    Can you show me better price for small PV kit.

  38. GBA Editor
    Martin Holladay | | #38

    Response to Flitch Plate
    It seems that your thinking on this issue has changed over the last few weeks.

    You are suggesting a PV kit that costs $2,300, or $2.40 per watt.

    Here is a link to a PV kit that costs $2,180, or $2.18 per watt.

    If you are willing to pay $3,000, you can get a PV kit that costs $2.00 per watt.

    I don't know anything about the company that is selling this equipment, and I am not making any claims about whether the company is reputable.

  39. DavidAlex | | #39

    off grid solar
    Can I vote for Ven Sonata to write a guest blog detailing his off grid solar setup and the math he used to choose the system? I think that would be really fascinating.

    [Editor's note: On January 12, 2015, GBA published the guest blog you suggested. Read Ven Sonata's contribution here: An Off-Grid Solar Community.]

  40. GBA Editor
    Martin Holladay | | #40

    Response to David Hicks
    Great idea.

    Ven: if you are reading this, how about it?

    Contact me by e-mail:
    martin [at] greenbuildingadvisor [dot] com

  41. user-1107451 | | #41

    The End user who gets hosed by Con Ed (Conned is right)
    Hi All
    Ive been investigating the solar I can put on my roof BUT Orange and Rockland (Rockland NY utility owed by Con Ed) say I can only install panels to 110% of my electric usage, not my TOTAL usage.

    So they get to keep hosing me on gas delivery costs because I would have to replace my gas boiler with an electric one BEFORE put my request for solar system on (Wanted to do net metering so I can pay off the solar install ASAP)

    I pay more for the delivery of both eletric and gas than what I actually use so what I was thinking is that even if they hose me on the total number of cells I can put on my roof (110% worth) I could put up solar collectors to lower the hot water costs of the gas they are forcing me to buy, as they have told me once the system is approved I cannot add cells later even if I change from gas to electric, they are forcing me to do it first
    I cant wait for these Organic Flow batteries to be commercial so I dont have to deal with these mooks at all

  42. user-1006269 | | #42

    Premature death
    The demise of solar thermal is based on the economics Martin has described.
    They are fairly accurate and do tell a tale.
    The other story is that solar thermal is a simple technology and should be less expensive.
    In the 1970's and '80's the solar industry sold technology that was supported by tax credits. This stifled cost innovation because everyone wound up selling tax credits.
    As one who was in the industry back then, it was amazing to see the 50% decrease in the solar thermal industry after tax credits ended. It was within a couple months.
    Fast forward to the 2000's and we see no real innovation in solar thermal technology. No one has really brought the cost down, even though all we need to do is put something black in an insulated case and put it in the sun.
    There is no high tech science there. There is the need for decent design. That seems to be lacking to some degree.

    As one who is tangential to the solar industry now, what I see is an industry that still bases its selling practices on tax credits with not enough attention to trying to produce a more cost effective system.

    The good news is that DIY systems that work well and are quite inexpensive are being done on sites like

  43. kim_shanahan | | #43

    Thermal shared pain
    Tom Gocze, I share your frustration. At a recent forum sponsored by our Senator (Tom Udall) that convened many super-smart eggheads from our national labs, the purpose of which was to figure out ways to get the scientists thinking about ways to solve energy issues with products that could be brought to market, I proposed that an X-Prize type challenge should be brought forward around the notion of doubling the thermal output in half the panel size for half the existing cost..

    The sound of crickets ensued. Obviously not a very sexy technical challenge to the assembled. Bring on the Chinese PV panels!

  44. user-1044661 | | #44

    More accurately, SHW is dead for small households

    I agree with your overall premise, but I think you analysis is a bit over simplified. First of all, I'd be interested to see the data that points to US average household consumption being in the 40 GPD range instead of 60 as ASHRAE says(I don't doubt that it is true for European homes and probably for Canadian ones, but our experience with American homes leads me to believe the 60 GPD estimate is pretty reasonable in most cases). But even if the average household consumption is 40 GPD, surely there are households that are closer to 60, 80 or 100 GPD. SHW still makes sense for a lot of them.
    We generally tell our customers that SHW no longer makes sense for households of 2 or 3. But for households of 4 and certainly for those of 5, 6 or 7 (or for commercial applications) it still makes tons of sense, especially compared to expensive oil and propane which are the most common water heating fuels in our area. No electric heat pump water heater (or resistive electric water heater) will give you adequate recovery to meet most people's performance requirements in a home that uses 100+ GPD of hot water. Also, obviously as the load increases, the required PV system grows proportionately (while the additional cost for SHW doesn't; a three collector SHW system is only a thousand dollars or so more expensive than a two collector system). In addition, the SHW will use less roof space for the same total BTU output (compared to electric, not HPWH).
    Finally, I agree with a previous comment that I think your installed cost for HPWH and Electric water heaters are too low, assuming you are looking for a professional installation of a high quality product.

    All in all, good article. Good point. I just think it deserves a bit more nuance than you provided.

    Happy New Year

  45. rethinkguy | | #45

    California Questins
    Good article. Seems NE centric. CA Title 24 prohibits any resistance heat - water or heat -even if adequate PV. Only HP and gas allowed. Building 8 compact green homes, no need for AC, and want no ducts. I seem stuck with gas hydronic, but boilers too big for the 4 zone 1500 SF well-insulated house - cycle too much.
    Any body got CA solutions.

  46. Vwoodruff | | #46

    Not quite dead yet
    I'll second Fortunat's comments and sentiments and add a few more...

    The PV cost is a bit inaccurate. there is no way we would be able to install a 1-1.5 kW PV system for $3.50-4.00 per kilowatt. We can install at $3.50 or less per marginal kW for a residential PV system, but to get guys on the roof, run conductors, grid interconnect, etc. a system with 4-5 modules results in a much higher unit cost. It would be more accurate to let readers know that the initial cost also assumes that the owners will drop another $9-12k on the rest of the PV system to get the $3.50-4.00 per watt cost.
    Fortunat's mention of the per sf output is critical. In certain markets (including ours here in Maine) we are often limited by available roof space when sizing PV systems either due to shade obstructions or the size and orientation of the roof. Adding in the cost to build a structure for a ground mounted array comes into play in enough instances to mention and changes the numbers significantly. When ground mounting is not possible, there is incentive to maximize roof production. As more jurisdictions adopt the minimum perimeter clearances for PV arrays, this issue will be exacerbated.
    An owner of a solar water heating system will never wake up one morning to find that politicians or regulators have cut the value if his/her system in half. One of the issues rarely talked about in articles like this (and unfortunately too often within solar sales circles) is the significant impact of utility policy on the long-term value of the energy produced from the PV system. The entire analysis is built upon the assumption that every kWh from the PV system will be compensated at retail value. There are states where this is in question, and the last election cycle did not help the industry's cause.

    The reason that so many SWH enthusiasts come out on articles like this is because you are intentionally being provocative amongst a readership that respects what you have to say. To bury a technology based upon major assumptions or for specific applications borders on irresponsible. How many engineers and architects with limited exposure to SWH technology will read this post, trust what you have to say, and write off SWH even in applications where it makes much more sense than PV? This is a huge deal, since SWH is highly underutilized in commercial applications where engineers and architects have a significant amount of influence in what technologies are included.

    It may serve to make for a provocative title and get readers to the site, but the ultimate question should be whether the tact taken in an article like this serves to push us (builders, designers, etc.) in the right direction as responsible, community-minded professionals.


    Vaughan Woodruff

  47. user-1006269 | | #47

    X prize
    I am not sure SHW is worthy of an X prize, given it is such a simple technology.
    And it seems that an array size would not halve, but cost should.
    Years ago, I worked with NREL to develop a low cost all polymer solar collector for freezing climates.
    The intent was a much lower cost.
    There had been a lot of prognostication about this over the years.
    Prototypes were made in the early 1980's at Brookhaven National Labs, using millions of R & D money from the Feds.
    The collectors went to Colorado for testing. They all failed. The Brookhaven guys felt they could do it with more money. Their work was based on very inexpensive, low temperature collectors developed at Rutgers university for greenhouse heating.
    Our very very small project proved some interesting concepts that were very low cost but it was a very limited project, which also borrowed on the Rutgers work. We worked on it on our own a bit.
    At some point, I realized that all polymer collectors for DHW really needed years of field testing. They could bring costs down significantly, but they need to endure a lot for a long time.
    Conventional flat plates do that very well.

    Technology will improve. Polymers and/or design will improve and the cost of SHW will come down.
    I am not prepared to call the technology dead.
    Slightly infirm, yes. As mentioned earlier, there are times and places where it works now.
    Our company has some small role to play.

  48. jinmtvt | | #48

    - 1.7KW @ 250w/panel = ~7 panels , i don't see why 6-7K$ would not be a reasonable PV system cost, and you surely can get this kit installed for lower than 9K$, which is the solar price point and the goal of the comparison.

    - Please explain how area becomes a critical criteria for hot water production purposes? 7 PV panels = ~ 150sqft of installation area is not difficult to find on most residential buildings.

    - lastly, what does electricity costs have to do when comparing solar VS PV cost for a fixed water consumption ?

    I'm not ranting against your post but, why would informing people that currently and at future PV prices, solar does not make any sense ( unless sentimental or DIY) would be irresponsible ? You come up with some numbers for a specific location in which solar thermal has a good enough +value compared to PV , enough to offset possible maintenance costs and additional gains of grid-connected pv ( aka 25%+ ) and we analyze it. For now, PV wins.

  49. ecdunn | | #49

    Solar Thermal died a long time ago
    I participated on a panel at the ASES convention in 2010 and suggested this and they have never invited me back to speak. Some folks love the technology too much and do not want simplicity or true sustainability.

  50. vensonata | | #50

    Guest blog request for vensonata
    I'd be happy to describe the whole system. We have spent many years off grid and feel many could benefit from our experiments. I think it will particularly make people realize that you can live well, economically, and on a large scale in a fairly severe cold climate zone completely off grid. I will contact Martin to find out how one submits an guest article. By the way, the battery hit 100% by noon today, with fifteen people in residence, happy new year!

  51. GBA Editor
    Martin Holladay | | #51

    Response to Tom Gocze (Comment #43)
    You wrote, "Solar thermal is a simple technology and should be less expensive."

    I've heard this a lot -- especially from people who have never worked as a plumber. They know that the prices I quote are correct, but it just bugs them.

    But it is what it is.

    You also wrote that there are "DIY [solar thermal] systems that work well and are quite inexpensive." That's true. But if you are going to switch to DIY pricing for solar thermal, you also have to switch to DIY pricing for PV, or the comparison is unfair.

  52. GBA Editor
    Martin Holladay | | #52

    Response to Kim Shanahan (Comment #44)
    Your reference to Chinese solar panels sounded to me as if you hold out hope that Chinese manufacturers might help bring down the cost of solar thermal collectors.

    It's worth pointing out that many Chinese (and Spanish, and Israeli) manufacturers are already manufacturing solar thermal collectors. These efforts have not yet brought down the price of the technology significantly.

    Everyone who loves solar thermal technology seems to be hoping for a technical breakthrough, or a drop in manufacturing costs. But this is a very old, very well understood technology. It's time for solar thermal boosters to admit that the costs aren't likely to drop.

  53. GBA Editor
    Martin Holladay | | #53

    Response to Fortunat Mueller (Comment #45)
    In your comments, you (like a few other commenters) have explained that you know about circumstances where the costs or some other factors are different from the numbers in my table.

    As I wrote clearly in the article, if your numbers are different, do your own calculations. If your calculations show that a solar thermal system makes economic sense, I encourage you to install a solar thermal system.

    You seem especially interested in the case where a family uses 64 gallons of hot water per day instead of 44 gallons of hot water per day. Such families certainly exist. So let's do the math.

    In my original table, these were the results:
    Solar thermal = $10,200
    Electric resistance + PV = $7,595
    Heat pump + PV = $5,132

    If we bump up the daily hot water use from 44 gallons per day to 64 gallons per day, we get:
    Solar thermal = $10,200
    Electric resistance + PV = $10,472
    Heat pump + PV = $6,091

    So in this case, the solar thermal approach and the electric resistance + PV approach are basically a tie. But you can still save 40% by installing a heat pump water heater plus PV.

    One final point: You wrote, "Your installed cost for HPWH and Electric water heaters are too low, assuming you are looking for a professional installation of a high quality product."

    If you live in an area with high labor costs, feel free to add $500 (or whatever) to the assumed cost for these two items. But note that each of the three systems under consideration (House A, House B, and House C) needs one of these water heaters. Adding $500 to each column doesn't change the difference between each column by even a penny. So my conclusions concerning which scenario is cheapest remain unchanged.

  54. GBA Editor
    Martin Holladay | | #54

    Response to Vaughan Woodruff (Comment #47)
    Like a few other commenters, you feel that it's unrealistic to think that a homeowner can find a contractor to install a small PV system for $3.74 per watt. You're right.

    But my point is that most homeowners don't install very small PV systems. They install PV systems in areas where these systems make economic sense. Once they see the math, they get excited by the savings, and they install the biggest array they can afford, or the biggest array they can put on their roof.

    The main point that I have been emphasizing is that $9,000 is the number to beat, because that's what it costs to install a solar thermal system. How big of a PV system can you install for $9,000?

    At $4/watt, you can install a 2.25-kW system.
    At $4.50/watt, you can install a 2.0-kW system.

    These hypothetical PV systems are significantly larger than the PV systems needed to meet a family's needs for domestic hot water. So my numbers aren't unreasonable.

  55. GBA Editor
    Martin Holladay | | #55

    Another response to Vaughan Woodruff
    You wrote that my writing is "intentionally being provocative."

    That's not true. My article lists the reasons why many people like solar thermal systems. I repeatedly urged readers to do their own calculations; if your calculations result in a different conclusion, I wrote, go ahead and install a solar thermal system.

    The main point of this article -- and many articles I write on many topics -- is that (a) we need to consult monitoring data collected by researchers, and (b) we need to do the math.

    All too often, decisions are made based on emotions, or salespeople's exaggerations, or rules of thumb, or obsolete advice.

    Times change. Cheap PV is a challenge to old assumptions, and it's time to sharpen our pencils and do the math. Sometimes the results of these calculations are surprising.

  56. GBA Editor
    Martin Holladay | | #56

    Response to Ed Dunn
    Thanks for your comments. I agree that "Some folks love the technology too much and do not want simplicity or true sustainability."

    Old habits die hard. Some of the people who "love the technology too much" are nostalgic and sentimental, while others are worried that their business interests are threatened.

    Solar thermal systems have their place. But these days, solar thermal systems rarely belong on a residential roof.

  57. Vwoodruff | | #57

    Response to Martin

    To your first point, I do run the numbers since I own a business that installs both PV and SWH systems. Since I live in an area where I have been for a long time and intend to be until I die, the process of determining cost effectiveness and providing the right solution for my clients is critical - otherwise I would get a poor reputation and have very uncomfortable interactions at the grocery store, gas station, etc. My argument about the numbers isn't some baseless argument.

    Here's a real world example:

    A 32 sf/80 gal SWH system with a SS tank w/ electric backup (all high quality components) costs $8,000. Installing a system with an electric backup is the least cost system in the northeast (keep in mind that in some parts of the country, this system would be a 40sf / 80 gal system, which would cost even less). This is essentially what you arrived at, though you put in a duplicative cost of $1,000 for an auxiliary water heater when in the case you presented it would be far more common to use a single tank system.

    That $8,000 will get you about 1.5 kW of PV after you add in the cost of adding a comparable electric water heater, such as a Marathon. As Fortunat mentioned in his post, once you get to an average 4+ person household, the numbers will bear out for residential SWH in some markets (depending upon energy costs). We have even seen them bear out for smaller households in our market where the installation of a SWH system addresses a major heating deficiency, such as a tankless coil in an oil boiler.

    You are correct that most homeowners won't go for the small PV system due to the economy of scale, so you can get to a better marginal cost for the additional PV. This then adds two major factors to consider - budget and roof space. As I mentioned in my post, these are real world considerations that are important to take into account.

    These points aren't meant to argue that SWH makes sense in a majority of cases, but rather to rebuff your claim that the technology is fully inappropriate for residential applications ("really, really dead") and that people who suggest otherwise are promoting their own self interests or are being sentimental.

    This gets to your defense that you are not being intentionally provocative. Let's take a look:

    The title of your article from two years ago is "Solar Thermal is Dead."
    The title of your recent article is "Solar Thermal is Really, Really Dead."
    The image shown at the top of your article in the both the e-newsletter and the blog posting show a tombstone proclaiming the death of an entire industry - "Solar Thermal."
    You refer to SWH enthusiasts who don't agree with your analysis as having an "emotional" attachment to their technology. Your last response is pretty telling - calling your detractors "sentimental", "nostalgic", or putting their own interests above those of their clients.

    I write for a number of magazines, so I will give you the benefit of the doubt that an art director for GBA ran with the tombstone idea. If that is the case, you have to admit that GBA is being provocative. If the graphic was your idea or your work, I am interested how you could argue this choice is not being intentionally provocative.

    If you are going to proclaim something really, really dead, there better not be any life left. As many people have mentioned, there are a number of exceptions to your arguments. We are not arguing that SWH systems have the same residential applicability that they had 8 years ago nor that they are more universally appropriate than PV in many markets. Instead, we are responding to an article that has proclaimed the technology as not viable. This doesn't make us sentimental or emotional - all of that wears off when you're crawling around in an attic or crawlspace putting these systems in.

    I'll repeat a huge assumption that you made in your article - that of the value of electricity produced by a PV system. One can avoid the issues of utility policy and roof clearances with SWH but not with PV. This isn't a minor argument, and in states where utility commissions side with the utility power play of charging solar customers (efforts that are taking place across the country), you will see far different numbers - even with heat pump water heaters.

    Solar thermal is not dead, it is simply the nerd over in the corner at the Junior High dance where all of the cool kids are dancing the night away with the pretty girls. SWH may not be flashy, but it has significantly more applicability than you suggest (talk commercial applications and your argument of its death is laughable) and there are plenty of girls that will find it to be a much better choice than the cool kids when it comes time to commit to one or the other.

    Vaughan Woodruff
    Insource Renewables

  58. user-1006269 | | #58

    Response to Martin
    As one who has installed hundreds of systems over the years and is still in the business (albeit as a manufacturer), my original point is that there is a lack of innovation in design and an inertia to change things.
    These are founded in the desire as a contractor to not get "that call" that something is not working.
    A reasonable aspect to being in business. Call backs are always expensive and a pain to both the customer and installer. Hence things get complicated and innovation tends to stifle.
    A PV system with a heat pump or even resistance HW is simple and correlates to current lower cost.

    Solar thermal will always have a market. It will come down in price. At present it is not keeping pace with the electronics that has driven down PV costs or the mass marketing of HPWH.

    I do see the title of this piece as being somewhat sensational, but realize you are not trying to sell magazines here, but are perhaps tweaking the (us) solar thermal guys. Not a bad thing.

  59. user-705006 | | #59

    Follow up question for Martin (and everybody else)
    Martin, thank you for your response to my comment (#20 and #21).
    I keep looking at solar space heating options and still scratch my head. Shouldn't the space heating system be based on the peak load (or the level to which the building envelop is air tight and insulated?). Ductless minisplits (or south facing windows) may work for a pretty good house or passive house. But that is a real small market out there.

    I am looking at our major urban centers with an existing old masonry building stock, where you consider yourself lucky with R-30 in the walls and where south facing windows with good solar access are not a given. Keep in mind that there are existing buildings where adding the additional R-10 to get to R-40 in the walls would be more expensive than covering that extra heat load with solar thermal.

    Considering these limitations in our urban centers, are ductless minisplits really viable and/or cost effective? I have been looking for a while, but haven’t come across a convincing PV solution yet. But I would love to hear what you and other readers would suggest.

  60. Vwoodruff | | #60

    Wisconsin Electric
    As chance would have it, I received a post this afternoon from a colleague in a LinkedIn Group that has news from Wisconsin:

    "Our utility, Wisconsin Electric is charging $3.78/per KW solar installed per month, increased meter charges $10/month (this may reduce kwh charges), and changed yearly net metering in winterery Wisconsin to monthly, basically is killing grid tied solar's ROI. Worse I hear this team could be on the move to a utility near you. We are looking to install grid back up systems, similar to generator back up systems, any thoughts?"

    This is a striking example of the regulatory issues that I mentioned in a previous comment.

    Vaughan Woodruff
    Insource Renewables

  61. Expert Member
    MALCOLM TAYLOR | | #61

    "basically is killing grid tied solar's ROI. Worse I hear this team could be on the move to a utility near you."

    So you have information that BC Hydro is considering the same policy? Or Hydro Quebec?
    I don't have any fixed views on this debate, but if you are going to accuse Martin of being deliberately provocative you should probably refrain from what appears to be similar behaviour.

  62. GBA Editor
    Martin Holladay | | #62

    Response to Vaughan Woodruff (Comment #58)
    The figures you provide are very close to the ones that I suggest in my article, so I think that we agree more than we disagree. As I recommended in the article, you have done your own calculations; that's good, of course. Your calculations show that the cost of a residential solar thermal system basically ties with the cost of a PV system. When this happens, a homeowner has to decide whether the maintenance costs of a solar thermal system are likely to be higher than the maintenance costs of a PV system, and then to make a choice based on all the available information.

    I'm not the only person who has concluded that "solar thermal is dead" for residential customers; many other experts, some of whom have commented on this blog, agree with me. You are, of course, fully entitled to your opinion, and your comments are well reasoned and are welcome here. I appreciate your perspective, and GBA gains in usefulness when all opinions are fully aired. Thanks for that.

    I'll leave it to other GBA readers to decide whether the tombstone art was appropriate or inappropriate.

    The most interesting question you raise is a political one, and it has to do with how local utility regulators with rule on future changes to net-metering agreements. I have followed this issue closely, and it will be interesting to see how it plays out. My own guess is that American homeowners who own PV systems are unlikely to see their net-metering contracts change for several years, but that some utilities will change the rules for future (not-yet-installed) PV systems.

    If local utilities impose fees that reduce the cost-effectiveness of PV systems, we may see more homeowners choose innovative approaches, including wiring PV modules directly to electric resistance elements in their water heaters (a method that does not require a battery, an inverter, or permission from your local utility -- but which may not be in compliance with electrical codes), or going off-grid (once the price of batteries drops). Stay tuned.

  63. GBA Editor
    Martin Holladay | | #63

    Response to Marcus de la Fleur (Comment #60)
    You wrote, "Ductless minisplits ... may work for a pretty good house or passive house. But that is a real small market out there."

    While ductless minisplits are point-source heaters (like wood stoves), and therefore raise questions concerning heat distribution, the economics of a ductless minisplit does not depend on having a "pretty good house" or a passive house. If your local fuel costs show that a ductless minisplit will save you money (compared, for example, to a heating system that uses fuel oil or propane), then installing a ductless minisplit will always lower your fuel bill, whether the envelope of your building is leaky or tight.

    You wrote, "I have been looking for a while, but haven’t come across a convincing PV solution yet."

    PV systems aren't appropriate for every building or site.

    1. A PV system only makes sense if your building has an unshaded south-facing (or perhaps west-facing) roof, or a yard with an unshaded location with good solar access.

    2. A PV system only makes sense if electricity costs are high enough in your region to make the installation cost-effective. Do the math.

    3. A PV system only makes sense if your local utility offers a reasonable net-metering agreement.

  64. Vwoodruff | | #64

    Response to Malcolm

    I was simply relaying the response of an installer in Wisconsin. Those aren't my words - that is simply the full text of yesterday's unprompted posting from a colleague. You'll notice in my comment that I didn't say any gloom and doom about these utility policies being inevitable across the continent. Instead, I am pointing to assumptions in this article - and a previous one written by Mr. Holladay - that draw into question the thesis that SWH is dead. It is safe to say that in a state like Wisconsin the numbers in this article are quite irrelevant, which puts a declarative, universal statement like "solar thermal is dead" on a pretty shaky footing.

    That said, the issue of pushback from utilities related to solar policy is not unique to Wisconsin. If you haven't seen this white paper entitled "Disruptive Challenges" and written by the Edison Institute (a utility trade group), it is a compelling read about the tactics utilities are encouraged to pursue related to the looming "threat" of PV.

    Again, I am not arguing against PV - I work with it on a daily basis in sales, design, installation, and policy capacities. I am against hyperbole, and that is exactly what the title and artwork for this article is. Our work should be to provide the best long-term solutions for our clients and proclaiming the death of an industry and referring to those who embrace SWH as emotional doesn't serve this end. In fact, it may serve to do the opposite.

  65. dankolbert | | #65

    Thanks, Vaughan
    Printed out the report - looks like interesting, if depressing, reading.

  66. n7ws | | #66

    Solar Thermal died a long time ago at my house(s)
    I've been a lurker for some time but this is my first comment. I'm in sunny Tucson, AZ and as I write at 9 AM, it is indeed sunny. It's also 30 degrees outside and I'm looking at a snow covered mountain from my window.

    With the two ST systems that I've had on two different houses, under these circumstances, I would be worrying about the collectors freezing. Both of these systems were installed by previous owners; I personally would have never installed them.

    System A was a single-axis tracking collector that heated potable water and stored it in a parallel standard water heater used as a reservoir. Rooftops in the neighborhood were festooned with these after a company, who shall remain nameless, convinced gullible folks that these were a good idea. I repaired mine three times, twice for freeze damage and once to replace the lead screw in the tracking mechanism. I had plenty of spare parts since several neighbors offered me their broken systems if I would remove them for them. I sold that house and moved to house B, where I am today.

    House B had a flat plate ST collector, feeding a standard 80-gal electric water heater, installed by the previous owner. An electrically-driven circulating pump activated when there was energy to collect and a place to store it. In the summer there was plenty of collectable energy, but really no place to store it. Once you get the tank to temperature you're done. Period. (Some in Tucson want to call it the solar capital of the world because we have so much sunshine. but they never consider that availability is different from usability)

    On days like today, I would be pumping electrically-heated hot water back up to the collector to keep it from freezing. Great economy, huh? After two freeze ups and ruptured collectors due to circulating pump failures I gave up. While having a new super-duper two-speed, two-compressor heat pump installed on the roof (another mistake) since a crane was here, I had the contractor remove the collector. As part of the deal he replaced the 80-gal tank with a 50-gal standard water heater.

    I did consider replacing the holding tank with one that had a heating loop and used food grade antifreeze as a working fluid, but after analysis, I concluded that there was never going to be a ROI. As a retired engineer living on investments, I don't spend money to feel good or worry about "sustainability." I'll feel good if my money sustains me until I die.

  67. GBA Editor
    Martin Holladay | | #67

    Response to Wes Stewart
    Your experience mirrors that of many Americans who live in homes with solar thermal equipment. When something breaks, either (a) they can't locate a local contractor willing to work on the system, or (b) the anticipated repairs are too expensive to justify, so they abandon the system or have the collectors removed.

    It won't be long before someone posts a comment along these lines: "Your system should have had freeze protection! If better equipment had only been installed, everything would still be working fine."

    That's half-true. But the fact is, these systems require maintenance, and the return on investment is so marginal that it's really hard to justify paying a few hundred dollars every time a tank starts to leak or a pump burns out.

    By the way, problem (a) -- the difficulty of finding a local contractor willing to repair a broken solar thermal system -- will be getting much worse over the coming years, as the phenomenon I am describing ("solar thermal is dead") makes solar thermal experts increasingly rare.

  68. n7ws | | #68

    Response to Martin Holladay
    Thank you for your comments. In my case, I was doing my own repairs, so finding someone else to do it was a non-issue. And yes, today we would have better freeze protection, but at the time, these systems were "state-of-the-art." The "art" was simply too primitive.

    I took a course in solar heating and cooling at the U of AZ back in 1970 something, so have had an interest for a long time. I'm not anti-"alternative energy" but I think it still needs to make sense. Often it does not, but in the interest of feeling good, we do it anyway.

    I enjoy your writings. Happy New Year.

  69. Marc Rosenbaum | | #69

    Some thoughts on this good discussion
    - The numbers Martin quotes from me on energy used by electric water heaters and HPWHs are based on best rather than typical equipment - Marathon water heaters, and Stiebel Eltron HPWH. They include all energy, meaning that standby losses as well as actual heating of water are covered.
    - There is no question in my mind that as installers we roll trucks back to solar thermal jobs much more than PV jobs. Fortunat's experience may be different.
    - HPWHs in heating climates do take heat from the house. In the online course I teach on Zero Net Energy buildings, the energy model spreadsheet provided to students includes an algorithm to calculate that additional extracted energy. As an example, for a 44 gpd house in a climate with a 6 month heating period (similar to Boston for example) that has a HPWH with a COP of 3 and is heated by a minisplit heat pump with a COP of 2.5 I calculate an additional energy usage of about 350 kWh/year, which lowers the effective COP of the HPWH from 3 to 2.3.
    - I lived with solar thermal hot water for 30 years in NH, the first 5 years with a high performing batch heater and the last 25 years with a thermosyphon system. Both were designed into the houses from the get-go and were passive so they cost less and had no controls or motorized components, so they had very long service lives. The more conventional pumped systems have more components to fail.
    - I'm one of those folks having a hard time letting go of solar thermal but I also don't like to go back to installations, and the combination of PVs and minisplits has been so reliable for us. The jury is out on the lifespan of HPWHs, so that's where the analysis is uncertain.

  70. PAUL KUENN | | #70

    Keep it flowing folks!
    Come on brainiacs, let's get that PV and thermal working together. What would it really take to come up with a non-conductive substrate (flexible and durable for the expansion differences) on the back of a PV module that can help cool it with a water/glycol mix? Most of the inefficiencies of PV are because of the heat factor. I know my modules get too hot to touch. Electrons just don't behave when it's so hot. Let's cool them down a bit and make some hot liquid in the process. Hazards?... all homes I know have electric wires running right over water pipes throughout the floor joists. Few inspectors ever catch that. Gee, we have folks flying around in space drinking their own urine... hmm.

    Most know I would never give up floor heat but will now truly push mini splits and DHW heat pumps with PV, only after they get the house built or re-built to my satisfaction. Mortgages will be paid back in double time:-)

    Just thinking out loud!? ...

  71. GBA Editor
    Martin Holladay | | #71

    Response to Paul Kuenn
    Several companies have developed equipment that combines PV with solar thermal collection. Here's one: SolarWall.

    In February 2005, I wrote about a company in Brentwood, N.H. called Dawn Solar. (The company is now out of business.) Dawn Solar has developed a roof system consisting of PEX tubing behind standing-seam metal roofing; the metal roofing was covered with Uni-Solar peel-and-stick PV modules. There were problems: the temperature of the circulating fluid was lower than the temperature of fluid from a conventional solar collector, and the amorphous PV had a lower output per square foot than crystalline PV modules.

    In spite of several start-ups that have had to fold after several years, entrepreneurs keep dreaming that this type of solar collector will have a future. There's even a Wikipedia article on the topic: Photovoltaic thermal hybrid solar collector.

    These efforts are similar to efforts to create a flying car. You can manufacture this product, but the result is neither a good car nor a good airplane.

    There are lots of problems. The biggest one is probably maintenance. If something goes wrong, these panels are complicated to fix.

    Ultimately, though, this technology fails for the same reason that ordinary solar thermal collectors are in decline: while it's possible to build and install this type of complicated equipment on your roof, it's not possible to gather enough energy to make the expensive equipment cost-effective.

  72. TomLane | | #72

    Response to Martin Holliday
    Several Points I would like to make . First Water Heater Heat Pumps are a non serviceable throw away item . If they fail there is no service network or repair items to acquire from a distributor to repair them . They only make good sense when used in a garage in a Southern Climate ., like South Florida . I would agree if there are only 1 to 3 people AND you are installing a grid connected system say 6,000 watts and have enough room that I would put up 2,400 more watts of PV power rather than a solar hot water system . If there is a large family ( 4 to 6 people ) and electric rates are high , and a single pump Drainback system can be installed with 120 gallon tank and 75 to 96 square feet of collector area , which we do for $7,400 then a solar hot water system is feasible AND will be around 5 times longer than a water heater heat pump . Also many situations for two to three people WITH LIMITED ROOF SPACE say only 70 to 100 square feet would make more sense to use solar hot water than put in less than 1000 watts of PV modules . As of 2015 wearever installing reseed tail PVsystems at $2.84 a watt to $3.00 per watt . I Agee that that dividiving $7,400 by $2.84 for an extra 2,600 watts is a better deal for 1 to 3 people . However , if they need a new water heater or have an old one that will need to be replaced soon and there is 4 or more people especially that use hot water for laundry then if you can get a 120 gallon single pump Drainback installed ( unbelievably low maintenace and extremely long life systems ) with about 80 square feet of collector area that solar hot water is worth while . Most of the reports on solar hot water HAVE BEEN DONE BY PEOPLE WITH ONLY 80 gallon tanks and 64 square feet of collector area when a 120 gallon tank with 50% more storage than an 80 gallon tank cost only $150 more AND two 4x10 collectors cost only $128.00 more than two 4x8 collectors with 25% more collector area . The installation cost is essentially the same and even if three 4x8 collectors are used the installation only goes up about $850 and adds 50% more collector area . Pressurized glycol system have far more maintenance , do not work as well in collecting BTU'S and do not last as long as Drainback systems . Tom Lane

  73. GBA Editor
    Martin Holladay | | #73

    Response to Tom Lane
    For GBA readers who don't know Tom Lane, he is the author of Solar Hot Water Systems: Lessons Learned, 1977 to Today, the best book written on the topic. If you want to install a solar thermal system, you should buy Tom's book. Here is the link: Solar Hot Water Systems: Lessons Learned, 1977 to Today.

    Tom, thanks very much for your comments.

  74. user-968787 | | #74

    PV for DHW
    I saw earlier in this thread mention of using PV wired to a heater element in a hot water tank. Aside from the fact it could potentially conflict with local building codes, wouldn't that be reasonably economical from a payback point view? Seems you would need to pay for the panels and some wires but relatively speaking keep installled costs quite low?

  75. user-705006 | | #75

    Thank you Tom Lane...
    ...for your comment (#73). You touched on a number of items that made me scratch my head. The numbers, rationales and scenarios you mentioned begin to make sense.

    Martin, re. #64: So may be ductless minispits can be used, despite a leaky and poorly insulated building envelop. I question if at this point the PV+ductless minisplit system still is more cost effective compared to SHW.

    The "solar hot water is dead, really dead" statement makes sense in the specific scenario you used (DHW only). Not that convincing though if one begins to add functions (such as space heating) and change some of the variables.

  76. n7ws | | #76

    To Martin re: Tom Lane Book
    The links within the page that you linked to are broken. In other words, things like Tom's bio, the TOC, etc are inaccessible.

  77. jinmtvt | | #77

    solar and future ..
    We need to keep the near future in mind when discussing solar vs PV water heating.

    First off, HPWH all in one such as the ones sold in North America right now, will be coming down in prices probably down to near 1000$USD as competition ramps up a bit. If you can get ~ 10 years out of it , at 1000$ in any cooling southern climate, and replacement being just as easy as replacing a regular water heating tank , i don't see any disadvantages here.

    Then, it is only a matter of time before the larger brands bring about a NA adapted version of split how water system, which has already been on sale in Some Europe and asian countries for quite a while, with up to 7 of COP @ 20c+ and possibility to replace /fix separate components , just like regular mini-splits. If they can adapt for heating climates as they did with space heating mini-splits, we could be looking at pretty high yearly efficiency.

    Then there are already a few chinese type products, that someone is ought to bring to market in the coming years ... example :
    This panel type is less costly ( ~500$ ) then full compressor/evap split type units ( which are going for around 1000-1200$ on chinese sites ) And is only good for hot climates.

    Every nation is on the lookout for energy efficient in everyday usage ( hot water and building climate ) .

    start from here and navigate

    Newer fujitsu powerstage euro units work down to -25c now .

    High leaving water temperature 60°C kept down to -20°C outdoor temperature without using backup heater.
    from their doc

  78. GBA Editor
    Martin Holladay | | #78

    Response to Darryl in Winnipeg (Comment #75)
    The suggestion you discuss -- wiring a few PV modules to an MPPT controller, and connecting the MPPT controller directly to an electric resistance heating element -- was suggested by GBA reader Richard Turner in Comment #18.

    This is a DIY approach, and like all DIY approaches, it will be less expensive than hiring a contractor to install a conventional PV system. The main advantage to any DIY approach is that you can save money. The disadvantage to this approach is that you won't get credit for all of the power produced by your PV modules, since the element will sometimes be shut down by the water heater's aquastat.

    There is a long history of DIY installation of PV modules by homeowners who don't feel like negotiating with their local utility. The most common approach, called "guerrilla solar" by Home Power magazine, is to purchase a PV module equipped with a micro-inverter (in other words, a PV module that produces AC electricity) and to simply plug it into a wall receptacle. The method works, but it isn't legal.

  79. GBA Editor
    Martin Holladay | | #79

    Response to Marcus de la Fleur (Comment #77)
    You wrote, "I question if at this point the PV+ductless minisplit system still is more cost effective [for space heating] compared to SHW [solar hot water]. The 'solar hot water is dead, really dead' statement makes sense in the specific scenario you used (DHW only). Not that convincing, though, if one begins to add functions (such as space heating) and change some of the variables."

    You seem determined to experiment with a space heating system that includes solar thermal collectors. If you want to install such a system, go ahead. But I stand by my analysis.

    There is no way that adding more solar thermal collectors (out of a mistaken belief that the collectors are a cost-effective way to produce heat during the winter) to an already-not-cost-effective solar thermal system will make the system more cost-effective than a plain-vanilla solar hot water system for domestic hot water. Because you plan to use these solar collectors for space heating, and because the output of these extra solar thermal collectors isn't needed during the summer (when their thermal output is highest), the installation of these extra solar thermal collectors guarantees that the cost-effectiveness of your equipment will be lower, not higher, than a system designed to supply domestic hot water. (There is a demand for domestic hot water during the summer; however, there is no demand for space heating during the summer.)

    If you want to use the sun for space heating, install a ductless minisplit and a grid-connected PV array.

  80. GBA Editor
    Martin Holladay | | #80

    Response to Wes Stewart (Comment #78)
    You have noted that some of the links on Tom Lane's web site aren't working. I'm not exactly sure why your comment is addressed "to Martin," since I am not responsible for maintaining Tom Lane's web site. (Tom, if you are reading it: fix your broken links.)

    If any readers are having trouble purchasing Tom's book, you can purchase it from several sources. Different editions are sold for different prices. The 2005 edition is available on Amazon for $38 new or $30 used.

    The book is also offered for sale by the Midwest Renewable Energy Association.

  81. SolarRon | | #81

    RIP ST? Really?
    Martin, There is no doubt your article is designed to be inflammatory as was suggested.

    I think the vast number of responses are pretty much all on the money however they mostly address the given sub-text of your article and not the main claim.

    If this title were true we would all freeze up and die. (Same sort of out of line focus.)

    1. Fact: ST is not dead, perhaps this myopic focus is.
    2. Fact: The article is focused on one simple area of ST and not the overall picture. So if we are talking only retrofitting SHW into small boxes it will be much as you describe, but with a few caveats, important caveats.
    3. Fact: It misses several salient points:
    a. No comments on GHG (Greenhouse Gasses) savings or offset. As the world becomes less and less habitable, ST may be a real legislated option.
    b. Will it be better to heat the same amount of water with PV, perhaps, but very few home designs these days allow for the volume of collector space required, and PV is faced with fractional shadow effect: i.e. one small shadow along say the bottom of a panel can and will drop the performance dramatically. Trees for instance become problematic.
    c. Many times we have installed both ST and PV where PV drives DC pumps in remote locations only to find the PV must be sized many times bigger to prevent the ST from overheating in the early day or evening when linear direction PV output is negligible, but ST is still "on the boil".
    d. A professionally designed and installed ST system even at this small size is still viable, especially when combined with a, and especially when installed during construction with full integration. The larger the better.
    e. If ST was completely dead then why would they spend large to build power systems in some of the coldest areas of the county. And this is just a s small version:
    f. Solar EVT panels operated very well at -40 when designed and installed properly.
    g. ST is alive and well in any location where lots of heat is required but there is limited aperture. For example Laundries, Car washes, pools etc.
    h. ST is alive and well in production process pre-heating

    OK I will stop here, but you get the point. I agree: It is not as financially competitive in a small box.

    Check out
    It seems to me this system will provide the innovation Tom Talked about, the original ground-up design concept Marc talked about, and more. They are focused on new homes with installations throughout Australasia, and are adapting the technology to work here. Storing summer heat for use in winter is the Holy-Gail of ST and it is being done. Once you increase the size of the ST installation to full-on heating, then hot water as per this article becomes ancillary; it is hard to duplicate with PV.

    EE Warehouses, schools, recreation centers, shopping malls all become viable in northern climes. And
    Yes, I believe both ST-EVT combined with some PV are the best combination when all aspects including things like GHG are taken into consideration.

    And now the huge tariffs on imported may change the financial dynamics of PV.

  82. morganparis | | #82

    Free heat for life
    Omigod. Right up there with the insulating paint.

  83. jinmtvt | | #83

    Ron Theaker
    You dare link thins kickstarter ?? :p
    Do you understand how much energy is required for the total heating season in a regular, average house up here in zone 6-7-8?? i don't think so ...

    Read Martin's post #80, and read it again.

    Solar thermal might cut in in a few select applications, but even there it is slowly loosing margin to heat pumps etc... that still provide with heating all through the night. Most industrial "water pre-heating" applications are now best served with by a combination of daily solar thermal and heat pumps, that still work throught the night without additional area and or storage (read $$$$ ).

    To sum it up, there are alot of opinions expressed here, but not many are backed up by numbers.

    As far as i know, Martin's proposal of a PV+resistance water heating is still the most efficient and future proof installation for now. Not very expensive to replace that 3-500$ tank once it breaks or new technology comes out.

    Not so much with an installed solar thermal , that will still work at the same efficiency in 20 years,
    when new and more efficient techs will be out .

    "C'est un probleme a deux faces."

    We need to address new construction/upgrade, and it needs to be future proof, which only PV is able to provide as of now. ( be it PV + resistanceWH for cold or HPWH for hot climates ).

  84. GBA Editor
    Martin Holladay | | #84

    Response to Ron Theaker
    I think that the only thing GBA readers need to know about your posted comment is that you are trying to use this web site to raise money for your Kickstarter project that promises investors that they will get "free heat for life."

    I don't know what country you live in, but (for your sake) I hope that your local statutes don't consider it to be a crime when someone exaggerates to potential investors in hopes of achieving financial gain.

  85. dankolbert | | #85

    Energy too cheap to meter!
    To quote David Byrne:

    Facts are simple, facts are straight
    Facts are lazy, facts are late
    Facts all come with points of view
    Facts don't do what I want them to.

  86. Expert Member
    MALCOLM TAYLOR | | #86

    More David Byrne
    Letting the days go by
    Let the water hold me down
    Letting the days go by
    Water flowing underground
    Into the blue again
    After the money's gone
    Once in a lifetime
    Water flowing underground

  87. tmoag | | #87

    solar thermal is alive and well and growing.
    As the CEO and founder of a vertically integrated solar thermal company I was surprised to learn my industry is dead. This year marks our 10th consecutive year of double digit sales growth. Our process heat sales alone were 2.8 million US $ and we have a solid year of signed projects going into 2015. I suggest the author consult AEE INTEC, an Austrian research institute on solar thermal energy. I also suggest checking out the online database of solar process heat at for some perspective. Photovoltaic panels simply do not produce sufficient energy to be practical in commercial process heat applications. Thermal technology will always have a place in the renewable landscape. Regarding the authors points concerning residential applications 50% of our projects involve space heating support in shoulder seasons and pool heating via heat exchanger in summer months. There is a small percentage of our sales dedicated to simple residential water heating and we do install SUN BANDIT, an off grid packaged pv water heating system as part of a diversified strategy. More often our customers choose conventional closed loop pressurized glycol systems based on a long history of proven reliability. Thermal technology installed by skilled labor is a beautiful thing. Suffice it to say the author has likely not experienced life outside the blog sphere or had the opportunity to work with skilled labor in our industry who are passionate about this technology and understand the value proposition to the marketplace. Thankfully my company has an opportunity to explain and prove that value proposition every day to our customers. In terms of innovation there have been enormous advances in plc based fluid flow control. We routinely integrate geothermal, chp and waste heat technologies into our processes. Heat exchangers, coils and thermal storage relate well to solar thermal fluid dynamics. A pv based system is normally open or normally closed, that's it. Voltage on voltage off. No platforms for temperature based btu get the point. It is unfortunate the author does not understand the diverse spectrum solar thermal functions in or how cool the applications for thermal are from an engineering perspective. There is a huge market for thermal. Maybe Platos allegory of the cave would better illustrate my point. Alas the author appears to be watching shadows on the wall........

  88. GBA Editor
    Martin Holladay | | #88

    Response to Terrence Moag
    I'm glad to hear that you have developed a profitable business selling solar thermal systems. As I have written several times, there are many solar thermal applications that make sense, and we need good solar thermal installers like those who work for you.

    My article focuses specifically on single-family homes. In my original article (you can find the link in the first sentence on this page), I wrote, "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. ... Unless you’re building a laundromat or college dorm, solar thermal is dead."

    All of my examples in the article on this page concern residential hot water systems. I repeated that point in Comment #57, when I wrote, "These days, solar thermal systems rarely belong on a residential roof."

    Your references to "process heat" applications -- that is, heat used for manufacturing purposes -- is interesting but irrelevant to this discussion. Congratulations on serving this niche. You wrote, "There is a small percentage of our sales dedicated to simple residential water heating." That doesn't surprise me; I don't doubt this percentage of your sales would be small.

    Your reference to my limited experience and Plato's Cave is an ad hominem attack that does not advance your argument. I certainly agree with you that "Thermal technology installed by skilled labor is a beautiful thing," but I never argued otherwise. Beauty was important to Plato, as it is to me, but it doesn't change the math I outlined in my article.

    By the way, your are wrong on one technical point. You wrote, "A PV-based system is normally open or normally closed, that's it. Voltage on voltage off." In fact, my PV system ramps up gradually as light intensity increases. In the early morning, or in overcast weather, the voltage is constant but the amperage output is low. As the sun rises in the sky or the clouds burn off, the amperage ramps up.

  89. GBA Team | | #89

    Post deleted by author
    [The author of this post has requested that his comments be deleted.]

  90. GBA Editor
    Martin Holladay | | #90

    Response to Michael Levitz
    1. You are correct that some, but not all, tanks used in solar thermal systems include an electric resistance element. These so-called "one tank" systems are usually less efficient than "two tank" systems. (The storage tank installed in my solar thermal system does not include an electric resistance heating element.) If you know the cost of your solar thermal system -- and if you know that you are planning to install a "one tank" system -- then use your own installation costs when making your calculations. That's what my article advised.

    2. I have addressed these issues repeatedly in my comments. You think that solar thermal costs should come down; you may be right, but your are making a prediction. We'll see. Concerning the cost to install small PV systems, see my previous comments on the topic. PV costs have been dropping dramatically. It's possible that costs are now bottoming out -- but many experts predict continued cost reductions for PV. My estimates are based on current costs, not predicted price drops.

    3. My oldest PV panel is now 34 years old, and has shown no degradation in electrical output. See my article on the topic: Testing a Thirty-Year-Old Photovoltaic Module. You wrote, "Yes, there is maintenance involved in SHW but nothing to the tune of $3000 during its useful live." Well, maybe you are right, and maybe you are wrong. My brother Peter has paid $2,200 in maintenance costs on his solar hot water system in just the last 6 years. Read his story here: Solar Hot Water System Maintenance Costs.

    4. Of course grid-tied PV systems depend on the grid. (That's true for most home appliances and most pieces of HVAC equipment.) Many studies have shown that the benefits of PV to utilities (peak load reduction and reductions in the need for building new power plants) are greater than the lost revenue to the utilities.

    5. My experience is the opposite of yours. My PV modules shed snow faster than my solar thermal collectors, but that may be due to the fact their mounting angles are different.

    6. You are right that if someone doesn't have enough room on their roof for PV modules, they may want to install a couple of solar thermal collectors. But remember, these output-per-square-foot calculations are usually flawed, because not all of the energy output of a solar thermal collector is usable.

    7. My energy use data for heat-pump water heaters is based on monitored performance of water heaters in several Massachusetts houses. The monitoring was performed by Marc Rosenbaum, a conscientious engineer and long-time energy nerd.

  91. vensonata | | #91

    The grid as storage: advantage pv over solar thermal
    The plain advantage of electricity over hot water is versatility. After you have filled your hot water tank on a splendid sunny day by noon, then what? I presume you have sized your system for overkill on optimal days so that you can squeak by on less abundant solar run out of storage and the excess is wasted. This never occurs with pv on grid. All excess is fed into the infinite maw of the grid, cleverly to be recovered on a dismal winter day from that very grid. Presuming a fair feed in tariff it is really something that distinguishes the two systems: solar thermal is one dimensional heat, pv is multidimensional electrical power. The advantage is plain. Even off the grid excess electricity is a wonderful problem to have!...shall I charge my car? How about run the well pump to storage? Cook electric today, vacuum, electric chainsaw, electric wood splitter, water the garden...on and on the possibilities are limited only by imagination. Solar hot Electricity...unlimited.

  92. CorGBA | | #92

    There is no way that so little PV can produce the power
    I have installed solar hot water and PV and I know the output of both.
    My system was at the latitude of south Canada and my 3kW PV at optimum angle and direction onmy produced 2100kWh/y so you numbers for solar output at higher (Canada) latitude appear to be off by a factor of 3, which seriously skews the outcome in favor of PV.
    Now, I love PV and the simplicity and efficiency it carries, but I had a 6x6ft thermal system surrounded by approx 250 sq feet of PV and that was covering all the available buildable surface, so there would have been no area left to add PV. I know your calculation for the heat pump would require the same area for PV as my hot water collector, but the output of that PV would be insufficient since it appears to be undersized by 3x.
    Another smaller factor was that I used a smart "Hot Top" boiler which already contains the heat exchange coil to use the existing building heat source, so I did not need the additional electric water heater - another save compared to your overview.
    Lastly, the cost figures you present appear to be for a retrofit. Installing solar thermal at construction (or remodel) time will greatly reduce the install costs, since the system designer and plumber can do everything in one go, compared to a contractor coming in later to retrofit pipes and collectors to an existing house. So, my contention is that if you live in higher latitudes, thermal is by no means dead, especially if you have it installed at an appropriate moment.

  93. vensonata | | #93

    Cor van de Water...accurate pv production numbers
    You seem to be getting 700 kwh per year per kw of pv. That is less than half what you should be getting at optimal fixed angle. The best place to get accurate prediction is a wonderful calculator by the national renewable energy lab, the best source of all such info. Type in "pvwatts" into google and you can calculate from your address exactly what your array will be producing including system losses of 14%. You can enter any angle or single tracker or double tracker and it will spit out the production for every month of the year based on national weather statistics for your local area. I am sure you will find it reports that your 3 kw system should produce about 4400 kwh per year. If it does not produce that much perhaps you have some shading or obstruction or a wiring fault. The site is free and easy to use and deadly accurate.

  94. jinmtvt | | #94

    Cor van de Water
    I fail to see how you can get less than ~1000Kwh/kw installed in southern Canada usuing NRC maps :,4240&SETS=1707,1708,1709,1710,1122&ViewRegion=-2508487%2C5404897%2C3080843%2C10464288&title_e=PV+potential+and+insolation&title_f=Potentiel+photovolta%C3%AFque+et+ensoleillement&lang=e&lang=e

    Use your municipality here, look for your setup angle and fix your stuff or call a pro.

  95. vensonata | | #95

    Michael Levitz redo your pv and inverter math
    Many pv panels have warranties for 25 years. Their actual degradation are tiny and and there is not the slightest reason to expect them not to last 35 years and still perform at 90%. Most of the older estimates were cautious, pv in general has turned out much more robust than initial expectations. Micro inverters now come pre installed on panels with the same 25 year warranty as the panel. Or you can buy them separately and add them...things have come a long ways quick in that area. Prices now for micro inverters about 60 cents per watt, panels can be found for 85 cents per watt. When you enter a pv array on the NREL site "pv watts" they will give you a standard price per wattt for your system. This must be computed to the area and average prices I suppose. $3.30 per watt commonly is their estimate. At these prices, lifespans, and versatility it would be very difficult for solar thermal to compete. However there is one thing that solar thermal lends itself to that pv does not. You can actually build a complete solar thermal system in your garage as a hobby and they really work. They are cheap as long as you don't count your labor! You can install your own pv but it is another thing to build a pv panel so advantage d.i.y to solar thermal,

  96. user-1033003 | | #96

    I apologize if I missed this
    I apologize if I missed this in the comments:
    The article fails to factor in storage efficiency. Most people are going to generate their hot water during the middle of the day, and most of it will be used for showers the next morning - what's left of it after sitting in the tank overnight. Even a very highly insulated tank is going to lose heat. On the other hand, PV generates electricity at the middle of the day and if you don't use it, you put it back into the grid and take it out the next morning when you need it. This seems like an added advantage of PV. I'm not sure how the variable power rates factor into this, but I suspect that they are highest at midday when PV is generating and relatively lower in the morning when people are showering.

  97. trevdawg122 | | #97

    Calculating Water Heating Energy
    Martin, I enjoyed your article. I read your original article in 2012 and did my own calculations in the fall of 2013 for a house that I'm finally building now! Well after reading this follow-up article I decided to go with a HPWH and add some extra PV panels. I'll avoid giving my opinion because I've seen plenty of that already. Also I apologize if this has been covered already in previous comments, but after reading through the first hundred my eyes rolled into the back of my head.

    What I'd like to know is how you calculated the amount of energy required to heat the water. I struggled with this in my analysis. Obviously it depends on climate zone because the incoming average water temperature will vary based on region. Thank you.

  98. GBA Editor
    Martin Holladay | | #98

    Response to Brian Godfrey (Comment #97)
    I agree with you.

    Your points were noted in my original article (Solar Thermal is Dead), where I wrote: "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. .... Unlike owners of a grid-connected PV system, who can be credited for their excess electricity production, ... owners of a solar thermal system can't sell the excess production of their hot water systems."

  99. GBA Editor
    Martin Holladay | | #99

    Response to Trevor Bowden (Comment #98)
    Q. "What I'd like to know is how you calculated the amount of energy required to heat the water. I struggled with this in my analysis. Obviously it depends on climate zone because the incoming average water temperature will vary based on region."

    A. You're right that incoming water temperature affects this calculation. As I noted in my article, the calculations made here are based on monitoring data from Massachusetts, where Marc Rosenbaum determined that the annual average energy required for domestic hot water is 0.21 kWh/gallon of hot water for an electric resistance water heater or 0.07 kWh/gallon of hot water for a heat-pump water heater.

    In Arizona or New Mexico, the required energy will be less.

    In my first article on this topic (Solar Thermal is Dead), I used a different approach. I based my calculations on the fact that it takes 0.0855 kWh to raise the temperature of a gallon of 50°F water to 120°F. (This is based on the fact that it takes 8.33 BTU to raise the temperature of water 1 degree F.) The problem with this approach is that it doesn't take into account the thermal losses that occur when the tank loses heat through its jacket, or distribution losses.

    There are many ways to make these calculations, and all methods have limits and caveats. Choose which method you prefer, but be humble about the accuracy of your results.

  100. girwin | | #100

    Interesting analysis, but debatable conclusions
    Your analysis is informative, but I find your conclusions to be based on assumptions that serve to confuse, rather than clarify, the issues at hand.

    First is the notion of the power grid as a “big battery” that can accept renewable energy at one time and “store” then “return” it at another is a myth. In reality, the US grid is, for the most part, a coal and natural-gas-fired generator that never turns off and delivers electricity at about 33% efficiency. Adding renewable energy to the grid from on-site photovoltaics is environmentally beneficial, but it does not undo the impact of energy drawn from the grid at other times. It’s a benefit that’s ultimately unrelated to the environmental impact of the building. Equating zero fossil fuel use with grid consumption matched by renewable energy generation on a yearly basis is an artificial construct. From an environmental standpoint, there is no functional difference between this and offering another “offset” such as riding a bicycle, walking or taking public transit in place of driving one’s car, eating less red meat, planting trees, etc. All these things are environmentally beneficial; none of them “erase” the environmental impact of grid energy use. Like these examples, surplus energy fed into the grid does not represent a reduction in the environmental impact of the building on which the PV panels are mounted.

    Viewing PV energy as an “offset” can cause confusion about the environmental impact of a water heating system. For example, one can reach the conclusion that switching from natural gas to electricity, and adding PV panels, is environmentally beneficial. In reality, the PV panels provide the environmental benefit, while the electric water heater has an environmental impact roughly three times greater than a gas heater of the same efficiency. A heat pump water heater is a more environmentally benign choice than electric resistance, but it must operate with an average annual COP of about 2.7 to match the impact of a 90% efficient gas water heater, since the grid supplies energy to run the heat pump water heater at about a 66% loss. Whether coal or gas is burned on-site or at a power plant is immaterial to climate change, the only relevant factor is the quantity. Unpacking the generation and the consumption makes the analysis quite clear.

    As such, a description of PV panels as being “a cheaper way to heat hot water” than solar thermal is not accurate on a yearly-average basis, at least from an environmental perspective. A solar thermal system produces “negawatts,” meaning it eliminates the carbon emissions typically associated with domestic hot water heating. A PV system can produce “negawatts” as well, if the output is applied directly to water heating, but an equivalent demand reduction takes about four times the area of PV as solar thermal panels since the collectors operate at about 15% and 60% efficiency, respectively. Further, unless the PV-powered water heater is set to superheat the tank at hours when there is output from the PV panels, one could very well be burning coal and gas to heat domestic hot water on summer mornings and evenings before the PV system has the opportunity to provide energy for that day or the next

    On a cost basis, the relative benefit is dependent on the vagaries of the utility billing scheme in question. In regimes where the customer may only reduce his or her electric bill to zero, one is actually incentivized to use more on-site electricity, regardless of the relative efficiency and/or harm to the environment. Where the customer is permitted to sell surplus power to the grid in any quantity, it is irrelevant whether the PV system is used to heat water. In fact, it might be financially beneficial not to consume such a salable commodity and to rely something other than electricity for water heating.

    The very existence of a market for surplus PV energy is dependent on there being relatively little of it in any given grid. Since PV systems in a particular region have peak outputs at nearly the same times and dates, each new system reduces the market value of all of them. While local solar availability and peak demand conditions vary, it doesn’t seem to take much to throw the market into disarray. Both Germany ( and Hawaii ( have seen negative wholesale electric prices at times of peak solar output, meaning they can’t give it away. This has occurred with about 20% PV grid penetration. California has not yet experienced this, but they are hypothesizing about the coming “duck curve” to match Hawaii’s “Nessie.” While there will likely be storage systems, seasonal uses and other schemes to mitigate the overabundance, it seems likely that the renewable energy market may drive homeowners out of the generation business, and that the economic value of a residential PV system will be analogous to that of a solar thermal system, i.e. a good way to offset the home’s energy use, at time of generation and within the capacity of its energy storage system.

    I do agree with you that solar thermal systems are often too expensive in this country. In many cases, they are seen and offered as a “boutique” solution, custom built by expensive specialists on a project-by-project basis. In many other parts of the world, this is not the case, and solar thermal systems are nearly ubiquitous. Whether this difference is due to our superior intelligence and common sense or to our access to cheap and abundant fossil fuel I shall leave to others to ponder.

  101. GBA Editor
    Martin Holladay | | #101

    Response to Graham Irwin
    I'm happy to concede one of your points -- that "On a cost basis, the relative benefit [of a PV system] is dependent on the vagaries of the utility billing scheme in question."

    But cost is almost always a factor in almost all construction specifications. After all, the reason that most of us don't install gold roofing (the type of roofing installed on the State House in Montpelier, Vermont) has to do with cost, not performance. Your observation that cost is a factor is not particularly original, and does not disqualify my arguments.

    These costs are subject to change, of course. Not only are PV modules getting cheaper, but some electric utilities want to reduce the credit they now provide to homeowners who sell them PV-generated electricity. It's up to each homeowner to determine where they stand in this shifting landscape, and whether an investment in PV makes sense.

    In some U.S. states, including Wisconsin and Arizona, net metering contracts are PV-hostile. In other states, including California and Massachusetts, net metering contracts are PV-friendly. It's hard to predict which approach will prevail in the end. If this uncertainty is enough to sour you on PV, then you shouldn't invest in a PV system.

    Your statement that "the US grid is, for the most part, a coal and natural-gas-fired generator that never turns off and delivers electricity at about 33% efficiency" muddies the facts. The percentage of electricity that is generated by coal varies widely from region to region. Where I live, in Vermont, the contribution from coal-fired power plants is 0%. In fact, the carbon intensity of the grid is falling, due in part to the growth of PV, but mainly due to the increase in wind-generated electricity and the substitution of natural gas for coal. This reduction in carbon intensity should be celebrated; moreover, we all have a chance to take part in this grid transformation by installing a PV system.

    Right now, PV systems supply less than 1% of America's electricity, which is a far cry from the 20% that makes utility executives (and you, evidently) quake in their boots. When a PV system feeds electricity into the grid, all of that electricity displaces electricity that would otherwise have to be generated by conventional power plants or other means.

    The electricity grid is imperfect and polluting, but most of us use it. I imagine that you use it, too. Although my own home is off-grid, my lifestyle (like the lifestyle of almost all Americans) depends on the grid in many ways. Since we depend on this grid, pointing out its imperfections is useful, but only up to a point. Encouraging the installation of PV systems, in hopes that a much higher percentage of our nation's electricity will be produced by PV in the future, is a good thing, with more benefits than most analysts realize or admit.

    You wrote, "I do agree with you that solar thermal systems are often too expensive in this country." This fact (which we agree on), coupled with the falling cost of PV, will soon make the debate on these pages irrelevant. Americans aren't particularly eager to spend $8,000 (or more) for a solar hot water system, but PV installations are booming. Economics will trump any debate over plumbing vs. electricity; fortunately for the planet, the PV boom is part of a necessary transition to a low-carbon future.

  102. girwin | | #102

    A couple Gedanken (“thought experiments”) while I'm ruminating on the topic:

    1) Suppose a solar thermal system is designed for a house where the yearly output of the collectors matches the estimated domestic hot water demand. The solar tank contains a backup heating element with the thermostat set to 120ºF. The system has a sensor to prevent overheating that directs the solar energy to a pool, or a spa, or the domestic hot water for an in-law unit, etc. This sensor is also set to 120ºF. Assume that all of the thermal energy collected by the system is put to use. What percentage of the domestic hot water for the house is heated with solar?

    2) Imagine two identical homes, side by side, identical occupants, identical schedules and usage. Both homes use electricity for water heating, identical water heaters. One home has a PV system installed, the other does not. After morning showers, dishwashing, etc., we watch the electrical meters on each house. Is there any difference between the rate of change of the readings of the meters on the two houses as the water heaters draw electricity from the grid to replace the hot water? In the late morning to early afternoon (no one is home) the house with the PV system sees the meter spin backwards, the other house does not. Evening comes, and people come back from work and school and use more hot water, then the water heaters draw electricity from the grid to replace the hot water. Once again, is there any difference between the rate of change of the readings of the two meters? The house with the PV system has made an environmentally beneficial contribution to the grid at mid-day, but how can one say that the PV system heated their water?

  103. GBA Editor
    Martin Holladay | | #103

    Response to Graham Irwin
    Concerning question #1: the solar fraction of a solar hot water system depends on the climate and the size of the system. Most two-collector systems for single-family homes end up with solar fractions in the 60% to 70% range. Increasing the solar fraction from (for example) 65% to 75% or 80% is possible but expensive -- and as you try to reach 90% or 100%, you end up needing lots of collectors and a very large storage tank in all but the sunniest climates.

    In your scenario, a system that produces the same amount of hot water on an annual basis as the family uses will obviously not result in a 100% solar fraction. When there are three weeks of cloudy weather in November, the solar fraction will drop to 0% or 5% for those weeks. When there are three weeks of brilliant sunny weather in June, the system will make more hot water than the family can use, and the system will dump lots of unneeded hot water into the dump loop.

    Your second question -- about "the rate of change of the readings on two electrical meters" -- confuses me. What do you mean by "the rate of change"? On a partly sunny day that starts out cloudy, a 2 kW PV system will go from producing 20 watts at 11:00 a.m. if it is cloudy, and, in 5 seconds, when the cloud blows away, the system can suddenly start producing 2,100 watts. That is a very fast rate of change -- from 20 watts to 2,100 watts in 5 seconds. Speedy Gonzales.

  104. Darin427 | | #104

    remove existing system???
    We moved into our house 2 1/2 years ago (northern climate). It had an existing solar thermal system just hooked up to the water heater (electric backup). The previous homeowner installed this himself and is no longer in the area. I can't find anyone in the area to work on it or tell me if it's working. I now need to replace the roof in a few months, so it would be the ideal time to get this hideous thing off my rooftop. So, knowing that I didn't really pay anything out of pocket for the I just keep it? Or get rid of it now to avoid expensive maintenance down the road?

  105. GBA Editor
    Martin Holladay | | #105

    Response to Darin Smith
    Ideally, you would find an experienced solar contractor to evaluate the system and tell you whether it's working. If that's impossible, I can't think of a single reason why you would want to keep it.

    You may be able to sell the used collectors to a hobbyist or do-it-yourselfer.

  106. girwin | | #106

    PV panels are great, but they aren't "magical."
    Please explain to me how the PV panels in your example are "heating water." It seems to me that in most situations, hot water is drawn and replaced with cold water in the morning before there's any significant output from the PV system, and the water is immediately reheated with (mostly) coal and natural gas burned at the power plant at ~30% efficiency. When the occupants come home from work and school in the evening, the process repeats. Yes, the PV output in the middle of the day is environmentally beneficial, but it's probably powering commercial buildings, correct?

  107. GBA Editor
    Martin Holladay | | #107

    Response to Graham Irwin
    If you're thinking of an off-grid house, you're right, of course. You would need a battery to store electricity generated between 9:00 a.m. and 4:00 p.m. if you needed to use the electricity in the evening or early morning.

    But that's not how things work in a grid-connected house. You can't actually attach a label to an electron at 10:00 a.m. and figure out where it goes. Electricity isn't like that. Every building connected to the grid draws electricity from the grid as it is needed, and all of the PV arrays and coal plants that are connected to the grid feed electricity into the grid.

    If you wanted to connect your heat-pump water heater or electric-resistance water heater to a timer, so that it only operated between the hours of 9:00 a.m. and 4:00 p.m., you could certainly do that. If you did, you'd probably need a bigger water tank. This hypothetical system would behave somewhat like a solar thermal system -- although of course it would be mechanically simpler and much more reliable.

    If PV-generated power ever reaches the point where it is supplying 18% or 20% of the electricity needs of the U.S., utilities may need to develop new strategies to handle the PV-generated electricity. But we're not at that point yet -- and if we do reach that point, it will be good news for the planet.

  108. girwin | | #108

    Magical thinking

    I think there's a fairly strong correlation between grid energy used and environmental impact, i.e. grid electricity consumed = fuel burned @ power plant = environmental impact. There is no "big battery" (except hydro-electric in some situations) in the grid so what's really happening is that the atmosphere is being used as "storage" in your proposal - carbon is emitted during water heating, at a later time, the PV system is reducing potential emissions from another use in (likely) another location. Even this "storage" analogy is a bit of a stretch as the PV doesn't sequester carbon, it relies on synchronized potential pollution to offset.

    Why is this anything other than a semantic difference? Because it can lead to illogical conclusions. For example, if the "net zero" water heating approach has no environmental impact, it would be impossible to reduce the environmental impact. In fact, if the identical PV system were retained, and the water heater exchanged for something that ran on gas (or even coal) at an efficiency greater than the grid (~30%) x the COP of the heat pump water heater, there would be less environmental impact, no? This doesn't take away from the benefit of the PV system, it merely delineates the two activities rather than conflating them.

    Beyond that, if one counts avoided emissions at another time as equivalent to emissions from the activity itself, why is PV the holy grail? Could one not, for example, convert one's garage into a home office, get rid of the commuter car and telecommute, and then claim "credit" for that activity as an offset against one's water heating? How about forgoing red meat? Both of these carbon reduction approaches are extremely cost-effective. If someone published an article stating that "veggie burgers are a cheaper way to heat water than PV" I think it would likely be met with ridicule, yet it follows the same logic.

  109. GBA Editor
    Martin Holladay | | #109

    Response to Graham Irwin
    If you can find a way to live off the grid, be my guest. I'm doing it.

    If you are using grid electricity, welcome to the club. We all want a cleaner grid. One way to get there is to increase the percentage of PV-generated electricity that is contributed to the grid. Right now, that percentage is extremely low in the U.S.

    I never claimed that PV is magic -- although it's close. (The first time I hooked up my PV modules to a 12-volt grain grinder, and watched the sun turn wheat into flour on a sunny day, it certainly seemed like magic to me.) Nor did I claim that PV is the holy grail, or that the grid includes a big battery.

    I will continue to report, however, that every kWh of PV-produced electricity that is fed into the grid displaces electricity generated by other means -- which in the U.S. includes electricity generated by coal, natural gas, and nuclear power plants.

    Of course we need to reduce our energy use as much as possible, and live environmentally responsible lives. If you can reduce your carbon footprint by working in a home office and eating rice and beans, I support you 100%.

    People who live in a grid-connected house in a location that allows homeowners to sign up for net metering can reduce or eliminate their electricity bill when they install a PV system. This investment in a PV system yields better energy savings, and a better financial return, than an investment in a solar thermal system.

    These facts don't prevent you from installing a solar thermal system. You can have one if you want. I do.

    And if you remain seriously hung up about whether or not your PV production is simultaneous with your electricity use, I strongly suggest that you put your electric water heater on a timer so that it only operates from 9:00 a.m. to 3:00 pm. or 4:00 p.m. Then you can stop bugging me about simultaneity.

  110. girwin | | #110

    Put the heat pump water heater on a controller so it raises the set point when there's output from the PV panels, add double the area of PV than comparable solar thermal panels (assuming an average COP of 2) and you may have an interesting proposition there. If you want to use electric resistance, make it 4x the area.

    If you stop making questionable claims that "bug" me, I will refrain from commenting here and use my time more wisely - deal?

  111. GBA Editor
    Martin Holladay | | #111

    Response to Graham Irwin
    I'm glad that you found my suggestion interesting. I'll take that as a positive note, so that we can end our conversation on a note of agreement.

    If you take my interesting suggestion, there will be many benefits. In addition to reduced maintenance costs, a huge benefit would be the fact that once the aquastat on the water heater is satisfied (because the tank is up to temperature), the PV array would begin spinning your electric meter backwards. That's a trick that no solar thermal collector is capable of.

  112. Darin427 | | #112

    existing system
    Thanks Martin for your quick response yesterday on my existing system that the previous homeowner installed. Is there an easy (safe) way of turning this off for a month so I can compare electric bills? Short of that, I don't know how else to really determine if this is saving me money or not.

  113. GBA Editor
    Martin Holladay | | #113

    Response to Darin Smith
    I don't know whether your system circulates a glycol solution, or whether you have a drainback system, so it's hard to know if it can be safely shut down.

    The best way for you to shut down the system would be to get up on the roof and to cover the collectors with a heavy tarp, secured so that it can't be blown away. That's safe.

    If your system has glycol, you don't want to close any valves in the collector loop, because the glycol can overheat and be ruined. Nor do you want to cut electrical power to the circulator, for the same reason.

  114. Jansigirl | | #114

    Great article, stimulating comments, BUT
    In the hottest part of California, in the Coachella Valley desert, solar thermal for DHW is indeed dead. I called every company in the area, and all respond about maintenance hassles due to the intense heat on rooftops in the summer. On the other hand, at a recent Home Show, there were 4-5 vendors selling solar coil collectors for pools -- a simple solar thermal system that tends not to break in the heat.

    Another deterrent to solar thermal, from the only company I found in a 100 mile radius, is their requirement for a 1500 gallon storage tank to supply enough hot water for those pipes and DHW use in the winter. Where does one find room for a 1500 gallon tank? Not in anything I'm going to build.

    We are already getting backlash from utility companies. I am planning a residence in the high desert where heating costs tend to equal cooling costs, propane is $4 a gallon, yet the local electric cooperative can't accept any more net metering because the AZ company that sells them electric requires they purchase a certain quantity to continue to get their low price.

    I'm still hoping to find a solution that allows me to harness as much solar power as possible for all my needs. The solar gurus discourage battery storage due to the short life of batteries in the heat and say their clients living off grid pay $5000 every 2-3 years to replace them. The dominance of PV net metering sure makes it hard these days for someone in a sunny climate to make use of all that free energy. Hopefully a solution will arrive soon.
    (nb if this is a duplicate, not intentional but I kept losing comments before posting)

  115. GBA Editor
    Martin Holladay | | #115

    Response to Janice Bell
    I understand your frustration. A couple of points:

    1. If you really want to install a solar thermal system -- and considering the very real issues with maintenance in your climate, I don't recommend it -- I don't see why you would need a 1,500-gallon tank. Most residential systems have tanks with a capacity of 120 gallons or less. The only reason to install a 1,500-gallon tank would be for a space heating system.

    2. The fact that some utilities are not accepting new net-metering customers is, indeed, frustrating. My advice is to plan a roof that is capable of receiving a PV array, and sit tight. Within a few years (I predict), one of two things will happen: (a) Due to political pressure, utilities will allow homeowners to connect their PV systems to the grid, and will provide credits that make sense, or (b) The price of batteries will drop to the point where grid connection is irrelevant.

  116. girwin | | #116

    Response to Martin Holladay
    To clarify, I was not seeking your blessing nor you approval to pursue a vegetarian or a car-free lifestyle, I was questioning whether these approaches are any less valid "credits" to claim against water heating than PV electricity supplied to the grid at an unrelated time.

    To heat water with PV, I make it that you'll need 2x the area in PV of the solar thermal panels and a heat pump with high enough capacity to utilize the output of the PV panels at an average COP of 2. This system would be costed against a solar thermal system with electric resistance backup because times with low solar energy are likely to coincide with minimal heat pump performance (though this should be verified.)

    If space were constrained, one might conclude that the less efficient but more versatile output of the PV panels is better sold to the grid or used to charge electric cars or the like and use it in conjunction with solar thermal for its space efficiency.

    In any case, you face a steep uphill climb against the laws of thermodynamics to convert heat into electricity and back into heat again, all to eliminate an incremental bit of plumbing, but that's just physics, not "magic."

  117. GBA Editor
    Martin Holladay | | #117

    Response to Graham Irwin
    Your description of the needed comparison -- comparing a PV array and a heat-pump water heater to a solar thermal system with electric-resistance backup -- is of course exactly the comparison that was made in my article. You're right that the solar thermal system requires a smaller area; but as my comparison shows, the PV equipment costs less and is more dependable. So homes without much area that can be devoted to solar panels may prefer to install a solar thermal system, as you point out, although such a system will cost more and be less dependable.

  118. tomcat513 | | #118

    Is solar thermal for pools dead too?
    Interesting article.Thanks for giving me a new perspective!

    Does this argument scale up for pool heating as well? Please forgive me if this has been answered already, I didn't see an answer in the comments.

    I also wonder if the type of thermal makes a difference. I'm using evacuated tubes and I have enough heat for hot water year round and the pool > 80 for about 6 months of the year. I also have a lot of PV. I ask because evacuated tube and flat panel costs are converging, and evac tubes gives you more independence from local temperature.

    But it never occurred to me to price it against pure PV plus heat pumps though. I know there are heat pump pool heaters, so maybe efficiency scales up and gives the same answer.

    I live near San Francisco, so I have an unfair advantage on climate. :-)

  119. GBA Editor
    Martin Holladay | | #119

    Response to Tom McReynolds
    The economics of heating swimming pools with solar thermal collectors is entirely different from the economics of heating domestic hot water with solar thermal collectors, due to the fact that the target temperature for swimming pools (as low as 80 degrees, as you point out) is generally much lower than the target temperature for domestic hot water (120 to 140 degrees).

    The type of solar collector used to heat swimming pools is usually a simple low-temperature collector consisting of unglazed black plastic tubing. Because this type of collector is cheap, and because the desired water temperature is relatively low, this type of collector is cost-effective -- compared to heating pool water with fossil fuels, of course, but not compared to diving into the unheated swimming pool regardless of the water's temperature.

  120. nospam32 | | #120

    Problem with your math
    Your PV system is making a wrong assumption and is undersized. If you used your numbers in an off grid situation you would be taking a lot of cold showers.

    I have a HPWH and have measured that it uses 3-5kWhr of electricity IN ONE HOUR to heat a tank of water during normal daily usage. I am also averaging 10-15kWhr/day electricity to heat water for a family of 4. You seem to assume that you can use all day to heat the water. A 0.57kW system wouldn't even work.

    Look at the math - assuming somebody in Toronto, they can get an "average" 3kWhr of electricity generated per day per kW of solar panels used. So, at least, I would need a 2kW system = 2000Wx$4/W=$8000 system. In reality you will find that you need a 5kW system with batteries to early keep up with your water heaters demand - the would cost $20,000. PV is not cost effective for heating water as its efficiencies are way to low.

  121. GBA Editor
    Martin Holladay | | #121

    Response to No Spam
    No Spam,
    You are correct that it makes no sense to use an off-grid PV system to heat water. I never advised off-grid homeowners to do that.

    The economics of grid-connected PV systems are entirely different from off-grid systems, however. My math assumes a grid-connected system, and my math is accurate.

    I have lived in an off-grid house for 40 years. I heat my water with a solar thermal system in the summer and a thermosyphon loop off of my wood stove during the winter. Both systems use a propane-fired tankless water heater for backup.

  122. whitenack | | #122

    Run a similar comparison for GSHP?
    Hey Martin. I really enjoy these mental exercises. Could you do a similar comparison between GSHP and PV powered ASHP? I have read your articles in the past where you say that GSHP use a lot of electricity compared to ASHP and in one article you flat out recommend not using GSHP. If I take the money I would save in not installing a GSHP plus the extra cost of operating one each year, how much money would I have to buy PV, and could that come close to generating the energy needed to power an ASHP?

  123. GBA Editor
    Martin Holladay | | #123

    Response to Clay Whitenack
    Many energy consultants have performed the analysis you suggest, and air-source heat pumps (usually ductless minisplits) plus PV always beat a ground-source heat pump -- whether you are talking about a small residential installation or a larger commercial/institutional installation.

    One such consultant is Andy Shapiro, who designed an extremely energy-efficient gymnasium (the Field House) for the Putney School in Vermont. Here is a link to an article that describes Andy's analysis:

    "The decision to utilize heat pump electric heating was made based on the decision to achieve net-zero performance with on-site electricity generation. A spreadsheet analysis was created to compare the annual performance of a VRV air-source heat pump system to a ground-source heat pump system. This model predicted an annual coefficient of performance (ACOP) of 2.3 for the air-source heat pump (inclusive of indoor fan power), based on Concord, NH weather. This compares to an assumed (based on experience) ACOP of 3.5 for the ground-source heat-pump (including ground water and distribution energy). The size of the system was calculated to be approximately 28 heating tons.

    "For the ground-source system the cost of the boreholes was estimated to be $106,000 and the cost of the water-source heat pump system inside the building was estimated to be $200,000, for a total system cost of $306,000. The cost of the air-source system was estimated to be $200,000. The cost of the added PV array required to offset the added electrical load of the air-source system compared to the ground-source system was $38,000, based on the additional electric consumption of 6,400 kWh/yr. The calculated net savings by going with air-source heat pumps instead of ground-source heat pumps was $68,000 including the added PV area required. Thus, the decision was made to proceed with design of the air-source system."

  124. doverturtle | | #124

    PV versus SHW System pricing
    I have been planning for two years now to install a solar hot water system, and just now stumbled across this website. There is some excellent information here, but it seems to me that the cost of a solar hot water system has been grossly overstated. I have found it difficult to price out systems, as most are intended to be installed by companies dealing in these systems and I am planning on installing one myself. I have found the following systems:
    The one that most fits my needs would be the 30 tube/80 gallon. The cost of the system is $2,499. Add in the turnkey option (for all the needed hoses, fittings, etc.) for another $395. The site claims to have a good deal on shipping, but I haven't checked into it yet. I will assume for now that it is around $200, which brings the total cost to $3100. The collectors are eligible for the 30% tax credit, so that brings the cost down to $2170. As hard as it has been to find any other system prices for SHW, it is just as difficult for PV. Can anyone tell me if I can purcahse a PV system that I can install myself for anywhere close to the SHW price? I am not committed to SHW, so if PV can be done as cheap I would definitely like more information on where to purchase a PV system. One drawback to PV that I have found in my area is that the utiltiy company makes people jump through hoops to get them tied into the grid.

  125. GBA Editor
    Martin Holladay | | #125

    Response to Adam Coffman
    Just because you can purchase most of the hardware needed to install a solar thermal system for $2,499 + $395 = $ 3,894, doesn't mean that the system can be installed for that price. If a solar contractor has to come to your house, discuss the installation, get up on your roof to install the collectors, use a licensed plumber to install the equipment, and provide a warranty (and maybe have a callback occasionally), that work will cost $7,000 to $9,000.

    Whether or not you can install a PV system on your house depends on (a) whether or not you are a licensed electrician, and (b) your local building regulations. Call up your local building department to find out what regulations apply in your town.

  126. doverturtle | | #126

    Response to Martin Holladay
    I work with heat transfer fluids on a daily basis. The plumbing is not an issue for me. I may need to obtain a permit to install the system, but I have been unable to determine that so far. I will need to check with the township. I am certain that I would not need to hire an installer. What I am looking for is information on PV systems. I can easily do the electrical work myself. It just needs to be inspected by an electrician. I am planning on using a lower roof on my house (it is the most robust as the main roof was built in the late 1800's), but that roof is only 12' x 8', so it may not be large enough anyway for PV.

  127. GBA Editor
    Martin Holladay | | #127

    Response to Adam Coffman
    If you can easily do the electrical work yourself, then you are all set.

    If you don't know enough to do the work yourself, consult an electrician.

  128. Expert Member
    MALCOLM TAYLOR | | #128

    When trying to compare alternatives it simply muddies the waters to include DIY as part of the discussions. In individual cases of course it may influence the costs significantly, but that should in no way alter the apples to apples comparison of two technologies.

  129. charlie_sullivan | | #129

    DIY comparison
    DIY grid tie solar kit prices are about $2/W, and the cost is probably similar if you buy the pieces. Permitting is more complex because you need the utility involved, but let's put that aside for now. Using $2/W and Martin's estimates of 2.05 kW or 0.68 kW for resistance or heat pump water heating, you need $4.1k or $1.36k worth of PV hardware. The latter is cheaper even after you buy a heat pump water heater, especially if you opt for the GE unit at the low end of the cost range at $1k. And it's easier to fit the smaller array on your small roof, if you go with the HPWH. That appears to come in a little cheaper than your $3100 solar thermal option, but it's not the slam dunk win for PV that Martin finds for professional installation. So it seems to me that you could go with whichever seems more manageable in terms of your DIY skills and the permitting hassles.

    Here's an example of a 2.04 kW PV kit for $3900, or $4260 with a mounting kit

    For lower power and smaller roof area, you could buy the components separately, or ask them for a custom kit with 3 panels instead of 8, and 3 microinverters.

    Edit: reply to Malcolm's comment above. Yes, it is confusing to include both DIY and professional costs in the same discussion. But which one is the "right" one to consider depends on the individual homeowner's plan. What's important is to be clear about which comparison you are making and to avoid overgeneralizing the results. So to anyone considering a professional installation, ignore my numbers above and go to the top of the page to Martin's comparison.

  130. jpdortmunder | | #130

    Solar Thermal BS
    I am sick of this nonsense. I have spent 30 years in the solar business and every company's sales force looks like a scene from the Saprano's. It's a giant ripoff scam and all of you morons are using the ridiculously inflated prices that keeps these bastards wearing Rolex watches and driving expensive cars. The approximate equipment cost of your 2-4x8 panel, 120 gallon tank system is $3000. A good two man crew will install this system in about five hours and be paid about 300 bucks. So your installed price is $9000. You stupid dishonest bastards.

  131. jpdortmunder | | #131

    Martin Holiday
    $2499+$395=$ 2894 not $3894. The installation will cost the contractor about five hundred bucks. This industry is ruined by moronic thieves. Six grand is a one hundred percent mark up. Let alone $9000. It's a friggin gold mine for these vermin.
    Adam, don't go with vacuum tubes. They are unreliable and have a high rate of failure. They lose their vacuum.

  132. gurugoa | | #132

    Completely different calculation for India
    I am amazed at the calculations. Here in India we use Solar Thermal extensively. If we use a 2 Panel Solar thermal that gives us between 200 to 250 Litres of hot water per day. (around 60 gallons). The same setup gives 63% of 44 gallons (about 105 litres in the climate that you have assumed in your calculation).
    All these calculations apart.; a two collector system with insulated tank with a 10-15 year life costs INR 50,000 versus a 2 KWp PV (grid fed - although that is not yet available for us everywhere) which cost upwards of INR 200,000 (4 times that of a thermal system).
    As for space - a 2 panel thermal system will need about 1/4 the space for the 8 numbers of 250Wp panels that a 2 KWp system will need.

  133. Dana1 | | #133

    Latitude & climate matter! (local economies do too)
    india is blessed with a lower latitude, which results much higher insolation than in the US. Only in the desert southwest (the sunniest part of the US, outside of Hawaii), does the sunshine exceed the average in India. Most of the populated US receives much less. In my city in the northeastern US there is a large number of immigrants from Ghana, some of whom complain about how dim the sun seems to be here.


    Also, in most of India there is no risk of the solar collector or pipes freezing, whereas in most of the US systems have to be designed to manage those issues, which the solar thermal system more complex & expensive. Photovoltaic (PV) panels don't have those problems, though it's important to mount solar panels (any type) so that the snow can slide off easily.

    The cost of PV is falling everywhere. Under the current Indian administration policies the size of the industry in India will grow by more than an order of magnitude in the next five years, which will drive the installed price of rooftop systems down rapidly. Whatever else people might think of Narendra Modi, he is not a puppet of Coal India, and he managed to bring a LOT of solar power to Gujarat while he governed there. (But it was primarily large utility scale arrays, not residential rooftops.) Rooftop solar pricing should drop by half in the next five years, but it's not clear whether heat pump water heaters will become cheap & available in India any time soon.

  134. 4smartheating | | #134

    HPWH Assumption COP 3 is Too Optimistic by Double
    Martin's math has a couple fatal flaws.

    1. Martin assumes Rosenblum's is correct that a typical HPWH has a COP 3.0 annual average, this is a significant error that cannot be justified. The NREL testing found the average HPWH unit was under COP 2 with averages closer to 1.6. Add in the Canadian CHMC study on the Parasitic load a Heat Pump Water Heater puts on the Furnace, and the net gain was close to 1.2. The size of a PV system is more realistically in the range of 1.5 kw or higher.

    2. The installed cost of 1 kw PV is about $6000-7500 here. It is a mistake to use any $x/watt estimate because 4 panels, roof hardware, inverter, and install starts at $6k to show up. Try the math at $6-7/kw and for 1-1.5 kw and the article would be more credible.

    3. Energy Factor used for HPWH is not the same as COP used for Air Source Heat Pump. Even ASHP and GSHP differ on COP, leaving out the fans or pumps to boost the numbers. The Sun Pump solar hot water heater has closer to field tested COP than in Lab calculated that omits required electrical consumption.

  135. GBA Editor
    Martin Holladay | | #135

    Response to Tania G
    As I wrote in my article, "Using the information in this article, GBA readers can perform their own calculations." I clearly stated my assumptions and the basis for these assumptions.

    Everyone's costs are different. COP data and equipment specs are both moving targets. Feel free to plug in your own numbers and reach your own conclusions.

  136. Expert Member
    Dana Dorsett | | #136

    Residential PV in the US is ~ $3.50/watt, not $6-7/watt

    This is consistent with the installed cost bids I've reviewed for 4-15 kw systems in MA in the past 12 months. There are locations in the US where the installed cost is well below that average, and locations where it's well above, but $7500 /1kw would be more than 2x the US average. Even for a sub-2kw system that's quite an up-charge!

    Out of curiosity, where is "" in the statement "The installed cost of 1 kw PV is about $6000-7500 here." ?

  137. GBA Editor
    Martin Holladay | | #137

    Second response to Tania G
    As I wrote in the article, "PV plus electric-resistance approach is about 25% cheaper than the solar thermal route." What this means is that my argument is valid even if you totally discount the idea of using a heat-pump water heater.

    But, as I said in my earlier response, perform your own calculations and choose the equipment you prefer.

  138. Jon_Lawrence | | #138

    I am afraid I passed up on
    I am afraid I passed up on the deal of the century. I got a quote back in December of 2012 from a major solar company for a 10KW DC system for $17,600. So $1.76/W DC. That was a prepay plan, so I believe solar company got the tax incentives and SREC income. That came out to $.076/kwh over the 20-year life of the contract. I will be getting a quote from them in the near future for my upcoming PH.

    I also got a quote from a regional company at the same time for a 7.2 KW DC system for $24.4k after ITC, or $3.4/W DC. They estimated $19k of SREC income over 15 years and a 20 year production of 160,712 kwh, for a cost/kwh of $.031. Even if the SREC's went to $0, the cost would be about $.15/kwh, which compares to my cost at the time of $.167/kwh.

    A big part of the marketing pitch by the salesmen at the time was the ability to lock in the rate now because the utility rates were going to increase 4% per year. I have TOU now, but just checking the tariff rate in effect for my area, averaging the summer and winter rates, the $/kwh is now - $.167, the same as 2.5 year ago.

    Dollar and sense aside, if I had pulled the trigger, I would have reduced my carbon footprint quite a bit. Won't make that mistake on the next house.

  139. charlie_sullivan | | #139

    RECs and carbon footprint
    Jonathan, if you had sold the RECs, you would have sold the right to claim you reduced your carbon footprint. You would have given someone else buying renewable certified electricity from the grid the opportunity to reduce their footprint, which is a good thing, and you would have saved money, which is also a good thing, especially given that (based on your other comments) you tend to invest that money in worthwhile ways. So I'm not against it, but I think that people who participate in the REC market should respect it.

  140. Jon_Lawrence | | #140

    Charlie,The SREC pricing is

    The SREC pricing is NJ is currently $280/MWH. That will help to pay for those triple-pane windows, batteries and other PH improvements. So this could be a win/win where I reduce my energy consumption by 80% and the utility gets to add solar generation to its network to be bought by someone else. However, it will be interesting to see what the utility does when my energy flow is one way 99% of the time. I am sure they will send me a nasty gram eventually. I might end up selling my power directly to my neighbor.

  141. kevin_in_denver | | #141

    Historic Thread
    I was pleased to see comments above from well-known experts in solar thermal: Tom Gocze, Tom Lane, and Fortunat Mueller.

    Another person worth mentioning is Jay Burch of NREL, who spent many years searching for low cost high reliability solar DHW. He never really found it, and is now retired.

    In 2012, I built a prototype solar sytem for my house using some of Fortunat's basic research, low cost Chinese evacuated tube collectors, and my 33 years of experience. The goal was a sub $3000 domestic water heating system. It worked pretty well for a couple years but alas it did freeze and break. I still think that cost goal could be met with an intense failure-mode-elimination-effort , but I'm also convinced it's not worth it, PV has won.

  142. Ken_Sayers | | #142

    PV & Solar H2O
    I have a 2.1 kW PV system and an 82 gal. Rheem solar hot water heater with one collector, using a closed loop system. The whole thing, together, cost me less than $8,000 after rebates. The collector provides me with 82 gallons of water at about 160°F about 85% of the time. This allows the PV to assist with all my other appliances, TV, computer, and heat pump. I keep the house around 76°F and my electric bill runs me, on the average, less than $70/month. The electric company charges me 19¢/kWh and pays me 5¢/kWh for what I generate in excess of what I use.

    I would have to say that it works for me. I must say, however, that my 1600 sq foot home is really energy efficient.

  143. john3885 | | #143

    Counter Points
    The article incorrectly inflates the cost of the solar thermal system. Adding the cost of a separate electric resistance HW heater to the system cost is incorrect. An electric element backup is normally included in the solar hot water tank at no extra cost. Most systems have no electric backup but simply "pre heat" an existing gas, oil, or propane fired HW tank thus reducing use
    of inefficient fossil fuel-based HW systems.

    The analysis disregards current incentives. For the PV options, the tank cost would not qualify for a 30% federal tax credit, while with solar HW, the tank does qualify, since it is integral to the system and stores solar heated water. Also, rebates vary by state and need to be accounted for. In several states, rebates are very generous for solar thermal systems- up to 40% of the system cost!

    Roof space required for a PV option of equal production is 2-3 times that required for solar hot water collectors. That extra space is not always available. For maximum energy harvesting, best use of roof space is a combination PV panels and solar thermal collectors.

    No way a PV installer is going to install a tiny 5-6 panel PV system for
    $3.75/Watt. Expanding an existing PV design to accommodate HW load can make sense, but again roof space can be an issue.

    The jury is still out on heat pump hot water heaters in northern climes. They rarely are in heat pump mode in cold basements in the winter and if in heated spaces, they make space heating systems work harder and burn more fuel. Also they are loud, have unproven life, and require an expensive refrigeration expert for repairs and maintenance. Solar thermal systems consist of robust hardware and if installed and maintained properly, have consistently demonstrated system lives of 20-30 years.

  144. GBA Editor
    Martin Holladay | | #144

    Response to J. Moore
    I compared the cost of the equipment required for a solar thermal domestic hot water system with the cost of the equipment required for a heat-pump water heater.

    You are apparently talking about a one-tank solar thermal system -- a type of system with many disadvantages, which is why such a system is rarely installed. A quality system is a two-tank system. (You need an electric-resistance backup heater because the sun doesn't always shine.) The main problem with a one-tank system is inefficiency -- the electric resistance element keeps the water in the tank warm, so there is never any cold water available to send to the solar collectors. (The efficiency of the solar collectors depends on the delta-T -- the colder the water you send to the collectors, the higher the efficiency of the system.)

    You're correct that I disregarded incentives in my analysis. (That point was clearly stated in the article: "The price comparisons made in this article do not include any incentives, rebates, or tax credits.") Incentives vary from state to state and from country to country, and it's possible to cherry-pick a location to prove almost any point you want to prove. In many locations, contrary to your statement, incentives and rebates have long been in place for purchasers of heat-pump water heaters.

    You are absolutely correct that a solar thermal system usually takes less area than a PV system of comparable output. If your roof is tiny, you should go ahead and install a solar thermal system (if you can afford one).

    The listed price for a PV installation of $3.75/watt is, indeed, obsolete -- but the price change is in the opposite direction from your implication. PV is cheaper, not more expensive, than noted in this article. Here at GBA, we're getting many reports from U.S. residents who have installed systems for $2.75/watt, which only makes my argument stronger.

    There is no need to install a tiny PV system. PV is so cheap, and the return on investment is so attractive, that you should install the biggest PV system you can afford. This investment will outperform any stock portfolio in your retirement account.

    If your point is that there is a minimum reasonable size for a PV system, you're right. Depending on your needs and expectations, that minimum size might be 1 kW, 2 kW, or 3 kW. Readers who conclude that an $8,000 PV system is unaffordable won't be able to buy one, of course -- but they won't be able to afford an $8,000 solar thermal system, either.

    In general, I've heard of more maintenance issues with solar thermal systems than with heat-pump water heaters. It's possible to have good luck or bad luck with either type of system. For readers who are wary of heat-pump water heaters, I recommend that you re-read my article, and note my calculations on an electric-resistance water heater coupled with a PV system.

  145. Expert Member
    Dana Dorsett | | #145

    What a difference 30 months can make (in PV pricing)
    The average cost of sub 10KW rooftop PV in the US is now closer to $3/watt (all in, no subsidy), than the referenced $3.74/watt. ( ) In some local markets it's closing in on $2/watt.

    The value/flexibility of PV output is greater than the value of thermal output of solar thermal of equivalent rooftop real estate too.

    The EF efficiency of heat pump water heaters has now climbed into the low 3s, up from the low 2s back when that analysis was done, even as the average retail pricing has slowly declined. (eg: ) The improved water heater efficiency reduces the size of the PV array necessary to support it by about a third, so instead of 570-680 watts in the examples used in the article, it's now only takes ~380-450 watts of panel.

    In Massachusetts a 55 gallon or smaller HPWH with an EF of 2.3 or greater qualifies for a $750 rebate, down from $1000 a couple of years ago, to reflect the drop in the hardware pricing. For bigger tanks the rebate is smaller (probably reflecting the projected higher income of those buying larger water heaters.)

  146. Evan Ravitz | | #146

    ... unless you have an attached solar Greenhouse.
    If you just run say 50 feet of black hose through your attached solar Greenhouse to the input of your water heater or pump, you'll save plenty. In many places, no freeze protection or transparent collector cover is needed, so the real cost is the black hose.

    As to maintenance, with a simple thermosiphon design, that should be limited to water leakage. But in a greenhouse with a dirt floor, that should be a benefit: free plant watering!

  147. dcp123 | | #147

    I know you're right, but...

    I know you're right that the most cost-effective way to use the sun for water heating or space heating would be to buy some PV panels and use them to run a heat pump or two, but the sun is so nice and toasty warm that there is an intuitive belief that there must be some way to use it that isn't just flushing money down the toilet relative to some other means that would reduce fossil energy consumption more at less cost.

    So, here's my whatabout question. Whatabout if a person were about to replace his composite shingle roof with a metal one, was expecting to use metal purlins on top of continuous insulation outside the roof anyway, knew of a place that advertises (and maybe actually sells... maybe) purlins with a little rounded groove to hold a pex line up against the metal roof to be warmed by it. Is there maybe some tiny chance that it would be economically rational to set up a couple of zones on different areas of the roof where the heat during the day could be used to warm glycol that could be used for water preheating, maybe a little subfloor heating in winter, and (bonus) subfloor cooling on summer nights to chill the house down to minimize AC use the next day.

    Yes, I know that the water wouldn't be as hot as with a well-designed thermal collector, that the system would require at least a couple of pumps, several automated valves, a big insulated tank, and the homeowner (OK, it's me we're talking about) would have to program a controller himself (let's say I consider that challenge a bonus, even though I don't really have time).

    Now comes the part when you explain that the roof would be cold when space heating was needed, that even in summer, it probably wouldn't get water quite warm enough for domestic hot water use, that parasitic losses from the pumps would exceed any energy savings, and that only crazy people try to chill there houses to 65 at night in the summer to avoid running the AC the next day. However, we already try this just using open windows and fans and it really does seem to minimize cooling except on a handful of days when it never gets that cool at night. And, no, we don't ever really manage to get the house down to 65 at night in the summer, but with a stonking big radiator on the roof, I'll bet we could.

    Please ignore the fact that my house isn't plumbed for subfloor heating and cooling and that it's a bad choice for me to pay to install that when mini splits work better. Hypothetically, if I already had the subfloor heating set-up, could it maybe make a little sense?

    I'm not actually going to do this if it doesn't make sense, but I really wish it did because the idea just appeals to me at a non-rational level. I think the space heating probably makes no sense because the roof can't get that warm on the days when I'd most want heat. Also, once I get the house decently insulated, I won't need heat on the days when the roof does get fairly warm. Water heating might work during the warmer half of the year, but not necessarily at a reasonable cost. Also, I feel like the nighttime cooling during the summer could work and I like it cold when I sleep, but would it justify the cost? I guess not.

    Facts: Northern California (East Bay), much more heating than cooling, roof gets pretty much sun, but only a couple of areas almost never get shaded by trees, and our sunny hours are somewhat limited by being in the bottom of a valley.

  148. GBA Editor
    Martin Holladay | | #148

    Response to Domenico Perrella
    This is an old story. Several companies have marketed systems like the one you describe. In all cases, the value of the heat gathered by these systems is too low to justify the high cost of the equipment required to collect the heat.

    Surprisingly, this experiment is engaged in every 5 years by a new enthusiast. At the end of the experiment, the enthusiast adds up the cost and headaches and realizes that very little heat was gathered, and the cost is high.

    I suppose you can run the experiment one more time, expecting different results, if you want. When you are all done, report back.

  149. dcp123 | | #149

    Thanks, Martin

    Apologies for the belated response and thank you for your response to my question.

    You know, I really wrote that post with the idea that you would save the day by talking me off that ledge. It is really hard for me to overcome the intuitive belief that all the heat I can feel from the sun must be usable in some way to heat my house. The tricky part is that most of the nice warm sunlight occurs in the months when I don't need heat and there's a limit to how hot an exposed metal roof can get if it's 45 degrees out. That's an advantage that an enclosed collector has over a big exposed roof, although it has higher costs per square foot to go along with that advantage.

    But, what about my effort to avoid the results of prior experiments? Can we also totally dismiss the ability to chill the house during summer nights? Even on a warm day a lot of heat radiates out into space from a warm object like a roof that is being kept warm by cooling something large and warm, like a slab (if my house were built on one) or maybe cooling the air in the house. I have to believe that, with a little insulation in a house without many windows exposed to summer solar heating, chilling the house to about 65 at night would keep it below 77 or 78 all day, which I can live with. But I guess I can't get much of a cost offset with that since I've already admitted I don't spend a lot on cooling.

    Well, you've rained on my parade, but that's what I actually wanted you to do.

    Thanks for being a spoilsport.


  150. GBA Editor
    Martin Holladay | | #150

    Response to Domenico Perrella
    If outdoor temperatures are cool at night, and you want to take advantage of those temperatures to cool your house, use a whole-house fan. For more information, see Fans in the Attic.

  151. user-6733106 | | #151


    Just curious, is there any guidance available or discussion on utilizing PV for supplementing a hot water baseboard heating system for a home? I'm likely showing my lack of knowledge on this topic (obviously a homeowner and not a builder). Is it even possible to supplement baseboard hot water heat with a PV array? Our home currently utilizes a Utica oil burning boiler to heat water for three baseboard zones + the fourth zone going to a hot water "Super Stor" tank. I don't mind paying my oil bill for warmer months to heat water we're using, but in the winter, the oil bill is steep! It seems there would be more information available on this if it were feasible... so perhaps I've just answered my question...

    1. GBA Editor
      Martin Holladay | | #152

      Assuming that you house is grid-connected -- most are -- then a PV array simply runs your electric meter backward. (This assumes that your local electric utility provides net metering contracts; the actual credit provided by your local utility may vary from the scenario I'm describing.)

      As a simple example: If your household uses 1,000 kWh of electricity in a typical month, and your PV array produces 600 kWh of electricity that month, then you only have to pay your local electric utility for 400 kWh. The PV array has lowered your electricity bill.

      If you are able to afford the installation of a very large PV array, either on your roof or as a ground-mounted array in your yard, then you may want to switch from oil heat to electric heat. One of the most efficient ways to heat your house with electricity is to use a ductless minisplit -- a type of air-source heat pump. For more information on ductless minisplits, see the articles indexed on this page: "Ductless minisplits and ducted minisplits."

      Some homeowners who heat with oil leave their old heating system in place. That way, if they install a ductless minisplit, the oil heating system is still available if needed for very cold weather.

  152. user-6733106 | | #153

    Thanks for the quick response. I actually had a contractor in the house last year to assess if it made sense to go with a minisplit system (with the hot water baseboard heat, we have no duct-work for central A/C, so I thought we could potentially have benefits 3-fold in our multi-level home: 1) A/C cooling in the summer months, 2) A/C would provide the added benefit of dehumidification (currently running a dehumidifier in the "basement" section of our multi-level, and 3) potentially more efficient (or supplemented heat in the colder months). The contractor (who's pretty reputable in the area) actually said the layout of our house just isn't conducive to a mini-split system, and it would likely require two systems... one system could handle the upper/lower level rooms (to the left of the entryway when entering), and a second system would be needed to handle the mid-floor (entry/living/kitchen/dining).... at that point he thought it was cost prohibitive, and that I was better off continuing to run with dehumidifiers and window A/C units when desired.

  153. 218P | | #154

    My tax free annual rate of return on my solar hot water was 25% (DIY). On a comparable basis my rate of return on my PV system was barely 5%.
    The SHW had more frequent repairs, but the PV system had massively more costly repairs e.g. $2400 inverter failure; 24 panel manufacturing defect causing massive efficiency declines.

    1. GBA Editor
      Martin Holladay | | #155

      Thanks for sharing your anecdote. It's interesting -- but your case is very unusual. The vast majority of homeowners who have installed both a solar hot water system and a PV system have come to the opposite conclusion. The factors that affected your result seem to be the following: The cost of you solar hot water system was unusually low (because you installed it yourself), and you had unusually bad luck with your PV system (due to the inverter failure and defective PV modules).

      Sorry to hear about the problems with your PV system. Here's my advice to GBA readers: Before choosing a PV installer, research the company's reputation, and ask about the warranty.

    2. Expert Member
      MALCOLM TAYLOR | | #157


      As I commented back in 2014 (post #15), it is very difficult to make any meaningful comparisons between materials or technologies if you bring DIY into the discussion. For example, discounting my labour, firewood costs me around $6 a cord, making all other forms of heating uncompetitive. However bought from a supplier, a cord here on Vancouver Island is around $270 - which changes the equation dramatically.

  154. 218P | | #156

    Thanks for your response to #155. The PV did have decent warranties (Kyocera Plates, SMA inverter),
    the failures were the result of time, most solar warranties do not extend beyond 15 years.
    The SHW is (with repairs) still going strong after 43+ years; I have less faith in PV longevity (only 22 years)

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