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Solar Thermal is NOT Dead

Solar thermal systems produce hot water in an environmentally responsible way

Roof-top solar thermal collectors can be used for domestic hot water or space heating.
Image Credit: Bob Starr

An article written by Martin Holladay, “Solar Thermal is Dead,” was published by GBA on March 23, 2012, and another article titled “Solar Thermal is Really, Really Dead” followed it on December 26, 2014. The premise of these articles is that solar thermal is dead because “It’s now cheaper to use a photovoltaic system to heat domestic hot water.” These two articles have been very widely circulated and remain very much with us today. As one example, I recently Googled “solar domestic water heater” and these articles came up #2.

It is my duty, as a fervent solar thermal advocate, to offer the other side. [Editor’s note: The author owns a company that sells equipment for solar thermal systems and radiant floor heating systems.]

Martin’s articles miss the mark in some important areas.

First of all, Martin’s broad statement that “Solar Thermal is Really, Really Dead,” based upon one domestic hot water comparison, goes way too far. Even if the analysis was correct (and it is not), solar thermal encompasses production of domestic hot water (DHW), space heating, radiant underfloor heating, snow melting, root zone heating for gardens, compost production, crop drying, pool heating, and more. Martin is speaking for the talents and creativity of many people, and while I personally admire Martin’s many contributions to green energy in this space, these two articles are not among them.

Let me first discuss the comparison between solar thermal and PV and then discuss the potential of solar thermal in general.

Solar thermal is more efficient

To begin with, solar thermal collectors are about five times more efficient than PV panels, and that is a very important consideration. There are many different ways to view the relative efficiencies of PV panels and thermal collectors. One method is to look at peak performance, or what the collectors do under ideal conditions. The other way is to apply a “performance factor” which accounts for the fact that collectors do not always operate under ideal conditions.

The peaking performance of the typical PV panel is about 16%. The peaking performance of the typical flat-plate solar collector is about 76%, according to Wikipedia. That is 475% better.

If we apply the performance factor then we will see what will happen in the real world. The performance factor for PV is about 65% and the performance factor for solar thermal is also 65% at fairly low residential operating temperatures. All of that means that we will get to keep about 10.4% of the solar resource with PV panels and about 50% with thermal collectors.

Either way, solar thermal collectors harvest about five times as much energy as PV. It means that if we want to make useful energy from the sun, it will take almost five times the solar panel area with PV as it will with solar thermal. This matters because there is only so much good south-facing area on a typical building.

If solar thermal panels cost about the same as PV panels, it will cost almost five times as much money. These simple efficiency observations are not the whole story of the comparison between PV and solar thermal but they are certainly the beginning. Then, we must consider the money value of the energy produced, the effectiveness of its utilization, the ability of the energy to be stored and other important factors. But this snapshot of efficiency tells us something about what PV is up against.

It is also clear that we do not get a tremendous amount of energy from solar collectors of either kind. The sun is a gentle form of energy to begin with and we only get 11% of it in the case of PV. We must use it wisely and efficiently, but it does work, both economically and technically.

PV panels produce electricity; solar thermal collectors make heat

To be fair, PV panels produce electrical energy and electricity is wonderful stuff. It has a much higher value than simple thermal energy. It can be used for all kinds of things and our lives would be much poorer without it, but my point is that PV is just not a good way to make bathwater or space heat. It is inherently wasteful.

The conversion efficiency or so called “heat rate” of electrical utility generation is about 33%. The only reason that electric cars make sense is because the efficiency of the internal combustion engine is much less than that (20%).

Thermal energy is also useful to us and we need a lot of it. Between space heating, domestic warm water production, clothes drying, and cooking, a major portion of residential energy consumption is thermal. The remainder that really requires the higher quality of electricity is actually quite small. Environmentalists are telling us that “If you want electricity, then PV is a good way to do that, but if all you want is simple thermal, then you should go with simple thermal solar collectors in the first place because they are much more efficient and environmentally appropriate.”

Much is being said about PV costs having come down. It is true and a wonderful thing that PV costs have come down dramatically in recent times, but they still cost more than solar thermal collectors and they produce so much less useful energy. There is really no indication that PV costs will continue to come down. In fact the current low costs of PV seem to be related to predatory pricing practices in the world markets and they may not be sustainable, but let us hope.

Martin used a system cost of about $10,000 for a solar thermal water heater in his first article. That is too high. In a later analysis he offered the option of using $6,000. I find that more reasonable. It is arguable what a two-panel solar thermal water heater should cost, but if you give me your credit card number, I will send one out for $5,361.48. This will buy you the materials, in kit form, for a glycol-based solar domestic water heater, which is the preferred method.

Antifreeze-based systems are more reliable

In my humble opinion, a solar thermal system should run on antifreeze, even in Florida. Key West, Florida is the only city in the continental United States that has never suffered a freezing event. Antifreeze will prevent freezing, boiling over, and corrosion. Glycol systems cost a little more than drain-backs and drain-downs, but they are worth it because of the avoided problems and higher efficiency.

Antifreeze in a solar thermal system does not need to be replaced every two years as in an automobile because there is no exposure to ambient air. The solar thermal system using glycol has only one moving part, the pump, and the glycol lubricates the pump for very long service life. Glycol solutions have been lasting more than 25 years. If a glycol solar thermal system fails (or if the power goes out), it goes to a fail-safe condition. The only true maintenance item with solar thermal systems has been exterior pipe insulation which seems to last about 20 years if it has UV inhibitors. Glycol systems can also be used for other things as well as DHW.

Installation should cost about $1,500. If no one will do it for that, then do it yourself. If you buy a good kit and you have a good installation manual and you are a reasonably competent do-it-yourselfer, the results will be excellent. Martin seems to have counted the cost of the storage tank twice in his comparison between solar thermal and heat-pump powered DHW.

Heat-pump water heaters take heat from the ambient air

There is at least one efficiency problem with heat-pump water heaters: They rob Peter to pay Paul. The First Law of Thermodynamics informs us that energy can be neither created nor destroyed (but it can be moved). Accordingly, the heat that is produced for domestic water is moved from the ambient air and must eventually be replaced by the space heating system.

Martin’s analysis compared the performance of solar thermal systems, making domestic hot water only with PV panels operating at 20% of tilt and just making energy. They are not comparable. PV panels at this tilt will be covered with snow all winter and would do little even if they were not. We must compare PV systems with solar thermal systems operating under the same conditions in order to learn anything.

Maintenance in solar thermal is minor

Martin mentioned maintenance problems in his comparison between PV and solar thermal. Here, we have to be careful not to confuse true maintenance with the cost of correcting mistakes. We all know of solar thermal systems that freeze up, boil over, leak, etc. But this is not the fault of solar thermal in general.

If you select a proven design and install it according to the manual and with care, a solar thermal system will be a pleasure to own for the life of the building.

On the other hand, if you create something experimental, and don’t consider every single thing, and then cut some corners, it can and will bite you. Your solar thermal system can be like a good refrigerator; it will run quietly and without maintenance for a very long time.

Combined uses help utilization

Martin correctly notes that solar energy must be effectively utilized or it is not very useful. A major advantage of PV systems is their utilization rate which can approach 100% because the electricity that exceeds the immediate demand can be sent back to the utility where it can be used for other purposes.

The benefits of solar thermal are “site-specific.” If you cannot use the solar thermal energy at the time that it is produced on site, it may be wasted at best and may even be a nuisance. But it is true that you can (and should) improve utilization of the solar energy with end uses that can be valuable and even fun.

It is incorrect to compare PV panels with utility-combined uses with solar thermal without combined uses. One of the most attractive combined uses for solar thermal energy is for underfloor radiant heating, and we will use it as one example where solar thermal is certainly “not dead.”

Radiant underfloor heating with solar thermal: a major improvement

We have time in this space to look at one particular solar thermal application that is ideal for residential applications and in combination with DHW uses. I ask you to set aside what you may have heard previously about solar thermal and solar space heating and take a new look. It is just possible that when solar thermal is combined directly with radiant underfloor heating, there are significant benefits that have been underappreciated in the past.

A glycol solution which is warmed in the solar collector flows to the DHW heat exchanger and also to a radiant panel floor. When space heating is no longer wanted, a thermostat turns off the flow to the floor. Simple as that.

The main reason that radiant underfloor heating goes together so well with solar thermal is because it is a very low temperature use and low temperatures lead to higher efficiency in the solar collector because less heat is lost to the outside air. Solar collectors that can operate at lower temperatures will harvest significantly more energy than if they must run at higher temperatures.

What we are proposing here is a hybrid solar space heating system that combines the relative advantages of active approaches (solar collectors and pumps) with the advantages of passive designs (using the natural materials and design) and yet leaves their respective disadvantages behind. It might be called “an Active Charge/Passive Discharge Solar Heating System” where the solar energy is collected and brought into the building in an active manner and then stored and delivered in a passive manner.

Active design approaches can collect the solar energy with excellent efficiency and do not lose energy during periods when they are not operational (as passive collectors do). But the usefulness of purely active systems has been compromised by the cost and complexity of the various mechanical systems needed to collect, store, and distribute solar energy.

Passive approaches reduce cost and complexity by using conventional building components to collect, store, and distribute solar energy, but the overall usefulness is compromised when the collection element is a part of the building envelope and loses heat at night. Passive approaches can be very demanding in terms of architecture and orientation.

It can be seen that the hybrid design will be low in cost because of its simplicity and highly efficient because of its low operating temperature. “The solar collectors operate at the highest possible efficiency because they operate at the lowest possible temperature” (second stage evaluation by National Bureau of Standards).

The very high efficiency caused by the low operating temperatures enables a small amount of cloudy day performance and a considerable improvement in partly cloudy and morning and afternoon performance. The “slab on grade” construction element provides enormous thermal storage at almost no cost.

A DOE report

The U.S. Department of Energy (DOE) thoroughly studied one such design and published a report titled “Technical Evaluation of a Solar Heating System Having Conventional Hydronic Solar Collectors and a Radiant Panel Slab.” The report concluded that the hybrid design which costs only half as much as referenced active systems collected about twice as much solar energy and delivered it into the building. From that point, the building’s very low heat loss and the building’s high thermal storage ability enabled high solar heating fractions in the difficult Northern Vermont climate.

The radiant heat will be appreciated with all floor types, but if the flooring and the structure beneath is capable of storing lots of heat energy (slab on grade construction), a significant solar heating fraction can be the result.

In comparison with any other active or passive solar heating system, the use of this hybrid solar heating method greatly increases efficiency, greatly lowers cost, improves performance, and improves architectural flexibility. Investment performance is improved accordingly.

Investment value in Boston considering energy saved and reduction in the cost of the conventional heating system and not considering tax credits was calculated to be 14% per year tax free.

A layman’s language description of the hybrid design was provided by Fred Langa, Senior Editor of New Shelter magazine.

The effectiveness of the passive solar storage element (the radiant floor) will be dependent upon what it is made out of and where the insulation is placed and how energy-efficient the building above is. Certainly slab-on-grade construction will store a great deal more heat than wooden floor joist construction. If you construct a building with very low heat loss (superinsulated), and then if you incorporate a high level of thermal mass (slab on grade), you can expect to see a solar heating fraction in excess of 90% in a poor climate like Northeastern Vermont. You will need 7 or 8 solar panels for a 1,500 square foot house. If you want wood joist floor construction, or if you only want 4 or 5 solar panels, you can expect a 50-60% solar heating fraction. On most days of the year, you will have all of the heat that you want and you will enjoy radiant comfort every day. On most days of the year, you will have all of the DHW that you want.

Bear in mind that Northeastern Vermont is about the poorest place in the country for solar heating. If you live anywhere else, these numbers will change for the better and you do not need to go to the superinsulated level.

A warm floor will lose more heat downward than an ice-cold floor, so proper insulation is important, but the floor will not lose more heat downward than other radiant floor systems.

Other solar thermal uses

Let’s think about some of the other benefits of solar thermal.

If you have a solar DHW heater it will produce more heat than the absolute minimum that you require on many days. This is not a bad thing. It is called luxury and abundance. Most people cannot have all of the hot water that they want without guilt.

You can water your garden with extra warm water and not set your plants back a week with ice cold water. Speaking of gardening, you can put a couple of extra heating loops in the garden. Your garden will get off to an early start and last a couple of weeks longer. Your flowers and vegetables will be larger than everyone else’s.

You can put a loop in the compost heap and get more compost sooner. You can put a loop in the driveway or walkway for deicing and snowmelt. It will work efficiently because of very low operating temperatures. You will track less salt and sand into the house and it will be safer. You can use excess solar energy to heat your pool.

The environment

Now, we are at the end of this economic analysis. I hope that I have made the case that PV-produced electricity is not really a less expensive way to make domestic hot water. I also hope that I have made the case that solar thermal is anything but dead and can be efficient, cost-effective, reliable, low-maintenance and even luxurious.

But now, I want to make the case that solar thermal can help us meet important social responsibilities. Environmental concerns are hard to quantify in terms of money. But just because they are arguable and hard to quantify does not justify scoring them at zero. Where and how shall we score the fact that in a few decades, Glacier National Park will have no glaciers? I do not mean to belabor these environmental and moral issues, but I cannot ignore them totally either. (I am writing for Green Building Advisor.)

Environmentalists are nearly unanimous in their perspective that we must not use utility-provided electrical energy frivolously. They say that the production of electrical energy has serious profound consequences now and well into the future. They note that most of our utility-generated electrical energy is made out of either coal or uranium. It may be OK to use electrical energy in small amounts for end uses that cannot be readily made by lesser fuels. But it is not OK to use electrical energy in large quantities to perform simple thermal tasks that could be done with any other fuel at lower cost and lower environmental impact.

It is important to understand that unless you live in the woods, off the grid, we are all interconnected. The PV energy that we make goes into a common pool. If we use utility-provided electricity for any purpose, we will cause some utility somewhere to burn coal. If we do not consume wantonly, the utility can reduce coal-generated pollution. It means that even if you have a photovoltaic panel, you cannot honorably use utility-provided electrical energy to make bathwater. After all, if you did not use electrical energy to make bathwater, that energy could have gone back to the utility to reduce pollution or could be used for another more appropriate purpose.

Good electric vehicles are available now. Elon Musk, the green business magnate, tells us that attractive PV roofing materials will be available in October 2017 (but they will not be cheap). So use PV electricity to run your car, or run your computer, or your lights or your television, but use solar thermal to make your bathwater.

Either way, free solar energy is yours; take it.

Robert Starr is a solar thermal advocate who lives in northeastern Vermont. He is president of the Radiantec Company, a supplier of underfloor radiant heating and solar heating products.

New Shelter magazine story.pdf

Robert Starr et al – DOE report from 1984.pdf


  1. GBA Editor
    Martin Holladay | | #1

    Perspectives on solar thermal systems
    First of all, I’d like to thank Bob Starr for his contribution to GBA. We’re happy to publish an article with a different perspective on solar thermal systems than reflected in my articles on the topic.

    I first met Bob Starr back in the early 1980s, when Bob was installing solar thermal equipment on a house on Route 5, between Lyndonville, Vermont, and West Burke, Vermont. Spotting the solar collectors from the road, I was intrigued. I stopped my car, walked up to the job site, and chatted with him about the system, which included seven roof-mounted solar collectors designed to provide space heating.

    Bob sells solar thermal equipment, so it’s no particular surprise that he wants to promote the sale of solar hot water systems. His business is located in Lyndonville, Vermont, about 8 miles from my house. It’s very convenient for me to have easy access to a local solar thermal dealer. My own house has two solar thermal collectors that I bought from Bob.

    As Bob explains in the article, solar thermal systems still have many applications. To provide just one example: a solar thermal system makes a lot of sense for anyone who lives in an off-grid house, as I do.

    That said, I stand by the economic analysis I made in my 2014 article, “Solar Thermal is Really, Really Dead.” As noted in the article, the analysis applies to grid-connected homes with access to net metering. If anything, the economic argument in favor of PV has only strengthened in the three years since that article was written. While that analysis assumed that the installed cost of a residential PV system in the U.S. is $3.74 per watt, that cost has dropped to between $2.75 and $3.00 per watt since my article was published.

    Bob doesn’t try to refute my economic analysis. Instead, he introduces several red herrings, including the fact that solar thermal collectors have a greater power output per square foot than PV modules. That’s true. So what? Unless you have a very limited area of south-facing roof, power output per square foot is irrelevant. What you really care about is power output per $1,000 invested. Moreover, the electrical energy produced by a PV module is much more versatile and therefore valuable than the thermal energy produced by a solar thermal collector, as even Bob admits. (Bob wrote, “PV panels produce electrical energy and electricity is wonderful stuff. It has a much higher value than simple thermal energy.”)

    I’ll readily agree with Bob that if you have a very small south-facing roof, or limited room in your yard for PV, you might want to install a solar thermal system — assuming, of course, that you can afford the steep installation price.

    Bob writes that he will sell you a kit with the parts needed for a solar thermal system for $5,361, and that homeowners can hire a contractor to install the kit for $1,500, for a total installed cost of $6,861. If you are able to do that, great. The price estimate is on the low side, however, for a variety of reasons: (1) Contractors have to add 10% or 15% to the cost of parts, to cover their labor in picking up the parts and handling any parts warranty issues. (2) Bob’s kit of parts probably isn’t complete, because Bob doesn’t know the length of tubing needed to connect the rooftop collectors to the solar tank, or exactly what type of fittings are needed to make the connections to your existing plumbing system. (3) The team required to install this equipment —a team which may include an electrician, a plumber, and a roofer — will generally charge you more than $1,500 for this work. So I stand by my estimate that such a system generally costs about $9,000.

    That said, even if you are able to install a solar thermal system for the rock-bottom price of $6,000 — which is even less than Bob’s optimistic estimate of $6,861 — and even if you live somewhere where PV systems are expensive ($3.74 per watt, which is on the high side), the PV + heat-pump water heater option is still $1,657 cheaper than a solar thermal system, as my 2014 article noted.

    I’ll use a story to explain why contractors add 10% or 15% to the cost of parts. When I bought two solar collectors from Bob a few years ago, I picked them up at his place of business and brought them home in my truck. Once the collectors were uncrated, I discovered hidden damage: one of the collectors had dents in the aluminum frame caused by a forklift. So I had to drive the damaged collector back to Bob’s place. Bob’s service was great — he accepted the return, no questions asked, and ordered a replacement collector, which I picked up a few weeks later at no charge to me. This is an example of the type of routine hassle that contractors have to deal with all the time. Contractors cover the hourly cost of dealing with these hassles by adding 10% or 15% to the cost of parts they install.

    Long story short: this anecdote explains why the installed cost of a two-collector solar thermal system is more likely to be $9,000 than $6,000.

    On the question of who should install a solar thermal system, Bob is inconsistent. On the one hand, he writes, “Installation should cost about $1,500. If no one will do it for that, then do it yourself.” Bob apparently assumes that do-it-yourself installation will be easy.

    On the other hand, when Bob discusses maintenance issues, here’s what he has to say: “We have to be careful not to confuse true maintenance with the cost of correcting mistakes. We all know of solar thermal systems that freeze up, boil over, leak, etc. But this is not the fault of solar thermal in general. If you select a proven design and install it according to the manual and with care, a solar thermal system will be a pleasure to own for the life of the building. On the other hand, if you create something experimental, and don’t consider every single thing, and then cut some corners, it can and will bite you.”

    So, how many homeowners are going to do a perfect job installing their solar thermal system, and how many will perhaps “not consider every single thing” and end up with a system that comes back to bite them?

    Bob is concerned that snow will reduce the output of a PV system; he wrote, “PV panels at this tilt [20%] will be covered with snow all winter and would do little even if they were not.” But who said anything about a 20% tilt? In fact, increasing the tilt of a PV array in Boston from a 20% tilt to a 45% tilt not only increases the annual output of the PV system by almost 3%, it also encourages snow to slide off the PV modules. And as far as I know, the PVwatts data that I used to calculate the annual output of a PV system include the effects of snow — which is to say, the reported annual production data aren’t dependent on the existence of a snow removal system. Moreover, snow reduces the output of a solar thermal system just as it reduces the output of a PV system. During the winter, my solar thermal collectors are almost always covered with snow. In my experience, snow tends to slide off PV modules faster than it slides off solar thermal collectors.

    Bob is a big fan of using a solar thermal system to provide space heating. He’s right that these space heating systems work. The only problem is that the value of the heat collected by this type of system isn’t enough to justify the high cost of the solar thermal equipment.

    To bolster his case, Bob cites a DOE-funded report that he authored in 1984, back in the heyday of the solar-house boom. (Bob and his co-authors received a $52,960 grant from the DOE in 1982 to evaluate an active solar space heating system that Bob designed. According to a DOE summary of the research project, “Energy savings were essentially as predicted. Some sales have been made, but generally [the] ‘solar’ market is slow.”)

    Suffice it to say that in the 33 years since that report was written, equipment costs have changed, fuel costs have changed, payback calculations have changed, and building scientists’ recommendations on the best way to provide space heat have changed.

    One other thing has changed since 1984: the house in Lyndonville where I first met Bob no longer has any solar collectors on the roof. The collectors were removed in 2003. (On the other hand, the PV module I bought in 1980 is still on my roof, and is still producing power matching the factory specs.)

    Bob ends his article with a plea for environmental responsibility, and I wholeheartedly endorse his plea. Homeowners with deep pockets who can afford to install a solar thermal system are doing the planet a favor by lowering their levels of fossil fuel use. There are many energy-saving investments with a much faster payback or better financial yield than a solar thermal system — including an investment in PV. But if you don’t care much about payback, a $9,000 solar thermal system will definitely reduce your use of fossil fuel.

    1. BiggestG | | #57

      There are many good points on both sides. I have both systems thankfully. Our utility company would not allow me to have 36 PV panels. Fortunately they expedited my application by reducing the request to 32 panels. I added 4 thermal panels for my hot water which took 400 to 500 Kwh of load off my demand.

      These panels are tilted steeply towards the southern horizon. I am in Western MA and can totally lose my PV production for weeks when covered with snow. My solar thermal panels are usually 100 percent clear in a day and my electric demand drops to around 20Kwh with no electric load for hot water.

      It has allowed me to be net negative in electric supply since installation.

      There can be no argument that energy capture is more efficient in thermal panels.

      Also, there is not any potential future supply issues of raw materials as is necessary to produce PV panels and related components. The propylene glycol transfer fluid is readily available, recyclable, and lasts for years without changing.

      If the ultimate use of the captured energy will be for heating of the building or water, a strong case can still be made for thermal panels.

      Next year a south facing sunroom will be added for passive solar collection.

      As I type this, it is in the single digits (degrees F) and my PV panels will still not produce for at least a week looking at the 1o Day forecast. My thermal panels are clear and my 160 gallon storage tank is at 140 F.

      For me and potentially many in New England and the snowy north, thermal solar is not yet dead.

  2. Jon_R | | #2

    I think it would be useful to
    I think it would be useful to avoid dramatic titles and generalizations and provide spreadsheets such that people can get the right answer using assumptions appropriate for their situation (eg, Hawaii vs Vermont, low net metering prices, etc).

  3. GBA Editor
    Martin Holladay | | #3

    Response to Jon R
    You wrote, "I think it would be useful to ... provide spreadsheets such that people can get the right answer using assumptions appropriate for their situation (eg, Hawaii vs Vermont, low net metering prices, etc)."

    That's exactly what I did in my article. I explained the source of my information for every input, and explained the math behind my calculations.

    The purpose of laying out all the information in such detail was precisely to make it easy for readers to perform their own site-specific calculations. I finished with this advice: "Everyone's numbers are going to be different. ... Using the information in this article, GBA readers can perform their own calculations. For some readers, the cost of a solar thermal system will be higher than either of the above analyses. For others, the cost will be lower than my lowest assumption."

  4. RedDenver | | #4

    wrong debate
    Doesn't this entire debate center around the grid and net metering, not PV vs thermal? With net metering, Martin is correct. Without net metering, then the argument shifts to your grid electricity costs vs solar thermal/PV costs. And if you need batteries (off-grid), then solar thermal is the winner.

  5. Expert Member

    As soon as someone includes DIY in their analysis of a product or building assembly the discussion becomes essentially meaningless.
    The restaurant at a resort down the road from me sells a prime rib entree for $35. The food costs are probably around $10 if I cooked it at home. What meaningful information about the cost of a prime rib dinner does this yield?

  6. GBA Editor
    Martin Holladay | | #6

    Response to Jason D
    Whenever the net metering debate comes up, I usually concede -- I've made this point repeatedly -- that the economics of PV systems are ALWAYS dependent on the details of a customer's net metering agreement (or whether the local utility offers any kind of net metering agreement at all). In a response to a comment posted by Graham Irwin in 2015 (a comment that Irwin posted in response to my article titled "Solar Thermal is Really, Really Dead"), this is what I wrote:

    "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.' ...

    "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."

  7. RedDenver | | #7

    Response to Martin #6
    My objection is more about your sensational headlines than your content. By your own admission, solar thermal is not dead. Your analysis and evaluation are much better than this article, but the headline of this article is more accurate.

    I understand why you want more clicks and debate, but you're casting the debate as PV vs solar thermal, when it's really solar thermal vs grid prices. If you can sell PV electricity to the grid, then you're just driving down your grid price.

  8. RedDenver | | #8

    Why aren't solar thermal systems site-built (or built at the solar installer's site)? It seems counter-intuitive to ship something so big and bulky when it's made of simple materials and processes. If Thorsten Chlupp and Gary Reysa can build these, surely a solar installer could figure it out.

  9. GBA Editor
    Martin Holladay | | #9

    Response to Jason D (Comment #7)
    If you talk to any solar contractor, you'll discover that (in most states) PV installations are booming, while solar water heater sales are flat or in steep decline. I don't think that my statement that "solar thermal is dead" is far from the truth. Residential solar thermal installations are way down, and the main reason for this is cheap PV.

    Of course headlines don't tell the whole story. Journalists like me want you to read the subtle arguments in the article, not just the headline.

  10. GBA Editor
    Martin Holladay | | #10

    Response to Jason D (Comment #8)
    Promoting site-built solar collectors is not going to solve any of the issues discussed in this article. A site-built solar collector isn't going to be cheaper or less prone to maintenance headaches than a solar collector built in a factory.

    Moreover, even when a solar contractor buys a solar collector and installs that solar collector along with associated hardware -- a controller, a pump, a tank, a check valve or two, and an expansion tank -- you still end up with a site-built system. The fact that these systems can be problematic is due in part to the fact that they are site-built.

  11. cussnu2 | | #11

    So the reality is
    That the whole basis for saying Solar PV is better/cheaper than solar thermal is the process whereby a wholesale commodity (electricity production) is sold, by force or fiat, at a retail price.

  12. GBA Editor
    Martin Holladay | | #12

    Response to But Why
    At least in theory, you can connect PV modules directly to an electric-resistance water heater, without involving your local utility or the grid. That approach is already happening in Australia. My next blog (scheduled for publication on July 7) will discuss that approach.

    Of course, systems that use an electric-resistance water heater don't produce as much hot water as systems that use a heat-pump water heater. Moreover, if you don't take advantage of the grid, the entire system becomes inefficient because once the water heater is up to temperature, the PV output is wasted.

    But if PV is cheap enough -- prices are still dropping -- and if utilities turn PV-hostile, the Australian approach will become increasingly popular.

  13. RedDenver | | #13

    Response to Martin #12
    I'm looking forward to it.

    PV direct to a water heater (resistance or heat pump) as compared to solar thermal would be interesting. Although that's only for DHW, as you couldn't use a HPWH for space heating.

  14. Expert Member
    Dana Dorsett | | #14

    The age of "alternative facts"???
    "To begin with, solar thermal collectors are about five times more efficient than PV panels, and that is a very important consideration."

    Yes, that's a very important consideration.

    The standard lowest-cost PV for utility-scale solar arrays might be 16%, but that's not what's going on houses. Typical residential rooftop PV panel installations in my area use panels test at about 20% efficiency under standard conditions. About five times 20% would be about 100% efficiency. Even discounted for inverter losses and an efficiency hit on higher temperature days 15-17% net AC efficiency out of the PV isn't a stretch. So to compete the "..five times...." figure means the solar thermal "only" has to be 75-85% efficiency, even after pumping energy as well a distribution & standby losses are factored in.

    That would in the realm of fantasy for anything but the absolute lowest temperature solar thermal application, not a radiant floor, and definitely not domestic hot water.

    Hauling out some economic valuation study from 1984 for solar thermal with energy at 1984 pricing, and an unverfiable panel efficiency claim on that always-accurate-and-up-to-date Wikipedia entry with unstated test conditions isn't exactly a convincing case. Yes solar thermal radiant floors work, but as a system is it really cheaper heat or more efficient as a system than rooftop PV leveraged by even a minimum-legal HSPF heat pump (that also air conditions)? Is a hybrid solution really cheaper/better than using the available solar real estate for more PV?

    If yes, make the case, but at least start with a baseline of agreed upon relevant facts, not silly straw man in a clown costume arguments, and not a nostalgic trip back to the when Reagan was in the White House. It's not 1984 anymore. Over the past 33 years solar thermal has only improved ever so marginally, with no reduction in inflation adjusted cost. But both PV and cold climate heat pumps have come a LONG way in both efficiency and pricing.. The real valuations can be made, but they need to include the levelized costs, including realistic maintenance & replacements based on something other than waving arm, and reasonable discount rates in present valuation using realistic upfront costs (net any incentives or subsidies.) Showing the math doesn't hurt (a screen shot of a spreadsheet tool of the analysis would be a good start.)

  15. user-1006269 | | #15

    More stuff
    I have followed Bob's work since his original report way back when.
    Fast forward to the 2000's: I have spoken to a number of folks who, here in Maine, have built DIY solar thermal panels to service a radiant slab for a garage. The goal is to keep the garage, which, of course, is insulated, above freezing. No other heating cost.
    This is anecdotal, but is similar to Bob's work back in the 80's.
    Vertical mounted collectors on the south face of a garage that are site built (
    can do a reasonable job of heating that slab.
    The cost is low. Yes, there is sweat equity, but even compromised performance from a DIY person's first attempt will perform reasonably with some basic information when heating a radiant slab to keep the building above freezing.

    So, there is one aspect of solar thermal that is not dead. It is not one size fits all, but it is also rather inexpensive. I think even paying someone to site build a proper design is quite inexpensive in lieu of something like an oil fired trailer furnace in the garage.

    The other solar thermal gambit that is a no brainer is solar pool heating.

    I have a tremendous amount of respect for your years of work at the EDU and GBA.

    I do get nervous when you paint with such a broad brush Martin.

  16. rockies63 | | #16

    Another Option
    The marvelous thing about experts challenging each other over the relative benefits of a technology is that they always have a wealth of articles, publications, studies, reports and conferences to refer to to back up their points.

    Rather than arguing over whether solar thermal is worth it or not I think the first question that should be asked is: "Is there any other way of heating water with the sun that doesn't involve running pipes of water or anti-freeze up onto the roof and then getting it back down to a tank?" Because that is the really problematic part of solar thermal (especially in a cold environment).

    I found a company that uses PV panels to heat the water and thus avoids having to run any piping up to the roof. I mentioned it in a thread on this site last year.

    If I am using an out door wood boiler through the late fall and into the early spring then I only need solar thermal in the summer (when I don't want a fire). This type of system seems to be a perfect solution. Why not include it in your discussion?

  17. walta100 | | #17

    To my ear any talk of efficiency in terms of the percentage of light converted into the output screams baloney, as we are not suffering from a shortage of sun light.

    Most homes do have a shortage of BTU and dollars. So let’s talk about the install system cost divided by annual BTU produced with 1 watt = 3.412 BTU installed in Lyndonville, Vermont


  18. Expert Member
    Dana Dorsett | | #18

    Efficiency matters...
    ...but the levelized cost of the net-energy delivered over the lifecycle of the system is what most people are interested in.

    When the available real estate is limited to the rooftop efficiency matters even more, since the rooftop area isn't nearly as available as total sunshine. That is part of why most people opt for 20% efficiency PV panels rather than lower cost 15% efficiency panels- they can fit more watts on the roof.

    The output of PV, electricity, is also a premium product compared to low-grade low temp heat and has more in-home uses. Solar thermal delivers it's greatest output during the seasons when it's needed least, whereas electricity is useful year-round.

  19. GBA Editor
    Martin Holladay | | #19

    Response to Tom Gocze (Comment #15)
    It's really hard to evaluate the economics of the do-it-yourself projects described on These projects are fun, and many of them work. In most cases, though, there is no accounting of labor costs or other costs associated with operating a contracting business, so evaluating costs and payback are impossible.

    You wrote, "The other solar thermal gambit that is a no brainer is solar pool heating."

    Both of my articles were specifically focused on domestic hot water systems for single-family residences -- not swimming pool heating. Here was my response (in 2015) in the Comments section to the question concerning swimming pool heating:

    "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) 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."

  20. GBA Editor
    Martin Holladay | | #20

    Response to Scott Wilson (Comment #16)
    You asked about the Sun Bandit system -- a packaged system that includes an electric-resistance water heater paired with PV modules. As I noted in my Comment #12, "At least in theory, you can connect PV modules directly to an electric-resistance water heater, without involving your local utility or the grid. That approach is already happening in Australia. My next blog (scheduled for publication on July 7) will discuss that approach."

    So stay tuned. The discussion in my July 7 blog will include a discussion of the Sun Bandit system.

  21. user-1119494 | | #21

    interesting article and so many great comments!
    I must agree that one can often get more usage from things when they are used in multiple ways, just like the coal power plant near me using its waste heat to keep the city center ice-free in the winter.

    Of course, the article would be even better if there were numbers. "In God we trust, all others bring hard data".

    And I have NO idea why the author would mention "The conversion efficiency or so called “heat rate” of electrical utility generation is about 33%."

    Got to agree with Dana, of course. We don't usually need much extra heat in summer, but power needs shoot up when the AC runs.

    Martin points out that dumping power into a water heater ends when it reaches its set point, but systems that use water heaters for demand flexibility allow the water temperature to drop fairly low when power supply is low and the "set point" can go nearly up to boiling when power is surplus.

    Almost everything in housing is "site-built", to some extent... But the more that can be turned out in identical units in a factory, the more reliable things will be. I'd take Tom's kits over my own creations in a minute, were I installing a system for a client: far less likely to get a call-back.

    Great stuff! Thanks, all!

  22. GBA Editor
    Martin Holladay | | #22

    Response to Nate G
    Of course DIY jobs cost less than professionally installed jobs. The point is that if you say that the cost to install a solar thermal system is zero, because it's a DIY job, you have to compare that solar thermal system to a PV system that was also installed for zero dollars.

  23. iLikeDirt | | #23

    Lots of assumptions about the future
    Utilities are trying to phase out net metering everywhere. It seems naive to ignore this trend or assume that the favorable net metering environment will continue throughout the lifetime of the equipment. By contrast, the economic feasibility if solar thermal doesn't change with your local political and regulatory climate.

    If we're going to talk about equipment popularity, then HPWHs are a dud. People don't want them. This product space just isn't taking off, for a lot of reasons--many of them good, like noise, high price, unreliability, contractor unfamiliarity, and blurry lines regarding which trade is responsible for maintenance and repair (your HVAC guy? Does he even know what it is?). These units generally cost $1,500 and up (not including installation), and nobody can even pretend that they will last for 25 years. The same charges levied at solar thermal equipment are even more applicable to HPWHs: They are electromechanical devices that require maintenance, in addition to which we can add the longevity issues of refrigeration equipment and digital electronics. That's not even mentioning the robbing-Peter-to-pay-Paul aspect for HPWHs installed in conditioned space. Anyone serious about heating water with electricity will probably settle on a Rheem Marathon electric resistance tank, or some other heater made with materials that yield the potential for essentially indefinite use, with simple and cheap maintenance and repair procedures. Anything else is just a recipe for high equipment replacement costs on a regular basis.

    DIY'ing a solar thermal system comes up so often because it's low-hanging fruit. is chock-full of effective systems that have been built and self-installed for $800-1,500. On this subject, Martin wrote: "These projects are fun, and many of them work. In most cases, though, there is no accounting of labor costs or other costs associated with operating a contracting business, so evaluating costs and payback are impossible."

    This is absurd. If I build a solar thermal system myself, there are no labor costs. If I don't run a business, I incur no costs of operating a contracting business. If my system costs $1,200 and operates as well as a $9,000 contractor-built system, I have saved $7,800 and my payback period is reduced by 86%. The whole point of doing the work yourself is to reduce the payback period.

    I look forward to the article on the Australian approach, as that seems like it will become increasingly common as solar PV prices fall. Tank + PV is definitely simpler and potentially cheaper than tank + collectors + pipes + pumps + freeze protection.

  24. GBA Editor
    Martin Holladay | | #24

    Second response to Nate G
    You assert that people don't want a heat-pump water heater -- they prefer to install an electric-resistance water heater, because electric-resistance water heaters are more dependable.

    In my article, I addressed that issue. My analysis compared the cost of a solar thermal system to a system that includes PV and an electric-resistance water heater, as you suggest. The PV + electric-resistance water heater option was $2,605 cheaper than the solar thermal option, even if you assume an unrealistically high price for PV of $3.74 per watt.

  25. iLikeDirt | | #25

    Response to Martin Holladay
    Martin, the key difference is the DIY-feasibility of the systems. Grid-connected PV is not generally a DIY project for reasons of safety, legality, and utility policy. Even if you could, the savings are not high because of the (relatively) low cost of the system in the first place. A zero-labor-cost DIY solar thermal system faces none of those hurdles and presents a much larger potential savings opportunity for the handy homeowner compared to a zero-labor-cost DIY grid-tied PV system.

    Probably things change when you start talking about self-contained solar PV water heating systems, which I imagine are much easier to DIY.

  26. wisjim | | #26

    DIY PVs
    I know lots of people who have only basic skills who have installed their own PV systems, most of them grid tied. They may have hired an electrician to do the connections but the basic installation of the panels themselves, whether roof or ground mounted, don't require special skills, just some figuring things out and labor. I also know at least one person with a PV+ electric resistance water heater that works well for minimal cost. He added relays to control the DC energy from the PVs so they wouldn't damage the water heater thermostats, and he has built a Maximum Power Point Tracking controller specifically for this kind of installation, which adds to the efficiency but isn't really necessary. The water heater can be set for a higher temperature so it stores more energy as heat and then of course a good mixing valve is needed so the water used in the house isn't excessively hot.
    One recently installed rooftop grid tied PV system came in at about a dollar a watt, after the 30% Federal tax credit, and the expenses included an electrician and the rental of a lift to get the panels on the roof. Lots of owner labor helped reduce costs, along with careful shopping for panels, mounting system, inverters, and other components.

  27. Jon_R | | #27

    For many calculations, post
    For many calculations, post US tax credit pricing (as low as ~$2/watt) should be used. The price trend is great - 9% drop in the last year.

    I suspect that the 20 year savings figures don't account for net metering policies (or storage costs).

  28. GBA Editor
    Martin Holladay | | #28

    Response to Nate G (Comment #24)
    You wrote, "Grid-connected PV is not generally a DIY project." From my experience, a solar hot water system is also generally not a DIY project -- although it can be.

    Like Jim Erdman, I've been involved with several PV installations that included homeowner labor. I've installed the PV system as well as the solar thermal system on my own house, and I would definitely say that the PV work is easier.

    A PV module is smaller than a solar thermal collector, and weighs a lot less, so it is much easier (and safer) to carry up a ladder, especially if installed on a two-story house. Making electrical connections on the roof is easier and safer than soldering pipes on the roof.

    I've been soldering copper fittings for more than 40 years, and I still get an occasional fitting that leaks and has to be done again (after the fluid is drained, of course). Most homeowners are much worse than me at soldering. Bleeding out all of the air from the collector loop, and charging the collector loop with the antifreeze/water solution, at the proper pressure, was a challenge.

    Expansion tank? How big? Where does it go? Check valve? Vacation bypass valves? Dump loops? Hmmm... I'll figure it out. And I did.

    Anyway, PV is easier than solar thermal.

  29. Expert Member
    MALCOLM TAYLOR | | #29

    Beating my own dead horse
    The ease of DIYing either system, or for that matter any other part of the construction process, is only significant for the very small subset of people who get involved in the building at all. I can't see how it has any place in the discussion of these things from an energy or policy perspective. Looking at the North American housing industry, DIY is essentially something that effects the decorative renovation market. In the big picture it is just background chatter.

  30. GBA Editor
    Martin Holladay | | #30

    Response to Malcolm Taylor
    Thanks for your comments. I agree with you.

    This argument -- "Solar thermal systems should be cheaper than what contractors charge to install them, so let's use the DIY cost instead when we try to prove what a good payback you can get from installing a solar thermal system" -- is an argument that just won't die. It's time to put a stake through the heart of this argument.

  31. Expert Member
    Dana Dorsett | | #31

    More "alternative facts"
    "The conversion efficiency or so called “heat rate” of electrical utility generation is about 33%. The only reason that electric cars make sense is because the efficiency of the internal combustion engine is much less than that (20%). "

    That is true for low ramping coal plants of the 1980s or earlier, but in Mr. Starr's ISO-New England grid region the vast majority of fossil-burner derived power is from combined cycle natural gas, at a fleet average north of 50%. That's about half of all power going onto the regional grid annually, with another quarter to a third coming from legacy nuclear power, and variable amount from hydro (large & small.) There is a growing slice of the regional pie being delivered by wind power and utility-scale PV, but also a growing '"invisible" slice of behind the meter PV that the grid operator only experiences as "missing load", much the same way energy efficiency appears. The fraction from 33% coal or oil or fast ramping gas peakers is now less than 10%.

    The average heat rate of the power sourced from the ISO New England grid hasn't been 33% for over 40 years.

    Other states and other regional grids have undergone even greater transformations, but the theme of combined cycle gas replacing 33% heat rate coal is strong, and in the Midwestern ISO utility scale wind power is garnering a VERY significant slice. In many individual states wind alone is covering more than 20% of the load, in Iowa it's more than 35%.

    So if you cherry pick a couple of ultra-coal states such as West Virginia or Wyoming you might be able to say the heat rate of grid power is roughly 33% in those isolated cases (not that they're isolated from the regional grid), but for the US as a whole or any US regional grid that's would only be true from long since bygone era, never to be seen again.

    BTW: I've yet to read about a car that can be powered by solar thermal panels. (Not that it's impossible, only impractical.) Electric cars make sense because they're local-emissions free (even in West Virginia), and have a low lifecycle operating cost. And as the grid grows ever greener, so does the car. In some parts of the US solar carport car chargers have become a cost-rational niche market for individual homes, but make economic sense for retail parking lot grid-tied applications too (even without net-metering.)

  32. ROBERT STARR | | #32

    More Alternative Facts
    No, I did not offer any alternative facts. The number of 33% conversion efficiency (heat rate) for electrical energy generation, has good provenance. It is what happens when you “google” “heat rate of power plants” or look in Wikipedia for the same subject.
    Regardless, the point that I am making is that according to the 2nd Law of Thermodynamics, if you change one form of energy into another, you will lose most of it along the way (to waste heat). That’s what cooling towers are for. It means that if you burn gas get heat, and use the heat to make power, and then use the power to generate electrical energy, and then lose another 10% to line loss on the way to the fixture, and then use that wonderful electricity to make heat for bathwater, it is inherently wasteful.
    You could have just used a gas water heater at the outset and saved money and polluted a lot less. Electricity should be reserved for the uses for which its qualities are appropriate (like electric cars) and not used for simple thermal tasks.
    I hope that helps.

  33. GBA Editor
    Martin Holladay | | #33

    Response to Robert Starr
    You're right about the general principle -- burning fossil fuel to create electricity results in waste heat. But you're wrong about the percentage. Dana Dorsett is correct that your 33% percentage is decades out of date.

    You have compounded your error by mentioning that we can expect to "lose another 10% to line loss on the way to the fixture." To quote the most authoritative source on this issue, "The U.S. Energy Information Administration (EIA) estimates that electricity transmission and distribution losses average about 5% of the electricity that is transmitted and distributed annually in the United States."

    The overarching issue that you neglect is that green builders in the U.S. are building all-electric homes. This is deliberate, not foolish: we are preparing for a fossil-fuel-free future, and are doing our best to decarbonize the grid by adding PV where possible (and, I hope, by supporting politicians who favor a transition to renewable energy sources).

    Environmentalists in the 1980s thought that it was better to burn natural gas than to use an electric heat pump. But times have changed.

  34. ROBERT STARR | | #34

    Speaking of EIA, go here to
    Speaking of EIA, go here to see current heat rates, not decades old.
    Anything above 10,000 is only in the 30s percentile
    Regardless, it may be pointless to quibble about whether heat rates are 33 or 50 percent and whether line losses are 5 or 10 percent. Either number puts tremendous amounts of CO2 in the air that doesn't have to go there. All of our nuclear plants are too expensive and dangerous. They are being closed for economic reasons. We will burn lots and lots of coal for many, many years.
    Google "How many people die per year from air pollution."
    It is looking too far into the future to plan for a no carbon grid.

  35. Expert Member
    Dana Dorsett | | #35

    Whither the nukes, hydro & wind? @ Robert Starr
    Carbon emissions only about the fossil burners, not the average for power drawn from the grid. From a carbon point of view the heat rate of a nuclear power plant is irrelevant, and the heat rate of a wind farm, PV, or hydro plant isn't a concept worth chasing.

    Legacy nuclear power in the ISO-NE delivers more than 1/4 the total TWh/annum for your region, hydro & utility scale renewables another double-digit fraction of the monitored & metered power that the ISO can "see". This is what it was in 2015 (note the share relegated to coal- it's even smaller now):

    The grid operator has a web site by which anyone can observe the quasi-real-time condition of the grid (updated every 5 minutes) that includes the regional fuel mix, localized marginal price, congestion, etc. It's a very slow movie to watch, but enlightening:

    Behind the meter PV can't be tracked in real time by the ISO, but it is already in low single-digits fraction, and doubling every 18-22 months. The share of gas generators is up slightly since then with the closure of Vermont Yankee, but under accelerating erosion by renewables and efficiency. The closing of Vermont Yankee and pending Pilgrim nukes, as well as the large coal burner in Somerset Massachusetts that just closed will be replaced by efficiency, PV, imported hydro from existing resources in Canada, and massive offshore wind development, development that has been mandated by the state. (Bidding is just beginning on the offshore wind- it'll be a much clearer picture by January.)

    By 2030 gas will still be in the game but it won't be anything like 50% market share.

    [edited to add]

    Until recently it was possible to estimate the current and future effective local grid-average heat rate on a state by state basis using the EPA's lbs CO2/kwh estimate under different scenarios presented under the Clean Power Plan, but the ostrich-in-chief has removed that information from the web site (though I'm sure it's available on web archive sites.) I used that source as a napkin-math method of estimating the carbon footprint of mini-splits in a gba blog piece about a year ago ( ) But it's clearly a moving target, and the very local and time of use particulars still matter.

    From a grid carbon reduction point of view residential rooftop PV wins hands-down against residential solar thermal. The lousiest heat rate generators are peakers employed during peak load hours, which in New England is air conditioning load. While not perfectly aligned with peak load, PV output is being delivered during the higher mid-day and PM peak grid loads. Solar thermal is rarely (pretty much never) employed for residential cooling, but PV output can cover the lion's share of residential cooling load with effectively zero transmission grid losses, relieving grid load (and deferring or eliminating the need to upgrade grid capacity).

    Use of solar thermal in hybrid absorption chillers for larger scale cooling plants is sometimes cost-effective, but it still has to be analyzed against using the available real estate for PV instead, and it's not a napkin-math problem.

    As more space heating get's pushed onto heat pumps the wintertime peak grid loads will eventually exceed the summertime loads, but the grid will have evolved quite a bit by the time that happens. In New England offshore and near-short wind output is be higher in this region in winter than in summer, capable of offsetting the reduced PV output regionally, relieving the reliance on fossil-burners for grid balancing. The ISO-NE demand response market set to go online in June next year will be able to manage a good fraction of balancing and peak shaving as well. As more electric vehicles plug into the grid, smart car-chargers will also be participating in grid balancing and other ancillary grid services markets. The whole 20th century model of large scale centralized power and one way power flows is going away sooner than most people are aware of, and it will be within the lifecycle of new solar (PV or thermal) that gets installed in 2017.

  36. user-1006269 | | #36

    Never end
    Hi Martin,

    A couple quick parting comments and I will stand down!
    We work with DIY customers who do solar thermal and PV's so I tend to lapse into that realm.
    I have no doubt that PVs will be a DIY project as the market matures. Micro inverters lend themselves to inherently safe systems (with the aid of a licensed electrician to do the tie in.) Most of PV cost is the act of physically mounting hardware.

    I have had more than one solar thermal/PV contractor tell me that PVs are so much easier because there is nothing mechanical. And usually no call backs for a bad (fill in the blank).
    We cannot overlook that aspect.

    But, DIY solar thermal is relatively simple and is likely the way to keep the cost of solar thermal palatable.
    Maybe DIY does not exactly fit this conversation, but it will not go away as the technology gets simpler.

  37. rockies63 | | #37

    Martin's July 7th Blog Post
    I do hope that it will be available to all people, not just the Prime members of this site. I don't use credit cards and can't become a Prime member so I would hate to miss reading the blog.

  38. GBA Editor
    Martin Holladay | | #38

    Response to Scott Wilson
    I'm glad to hear of your interest in my upcoming blog. Our usual advice to anyone interested in reading my Friday blogs is to sign up for a free trial membership in GBA Pro, or to pay $15 a month when the trial period expires.

    If you don't use credit cards, it's time to borrow one from your mom, your dad, or a friend.

  39. GBA Editor
    Martin Holladay | | #39

    Response to Tom Gocze (Comment #36)
    I appreciate your comments, which are based on long experience. And I like to end any discussion by emphasizing our points of agreement.

    We agree on an important point: "PVs are so much easier [than solar thermal systems] because there is nothing mechanical. And usually no callbacks for a bad (fill in the blank)."

    Easier! No callbacks! We agree on that.

  40. user-6605237 | | #40

    Minisplit Water Heater
    Robert mentions that the problem with heat pump water heaters is they rob Peter to pay Paul. But there is nothing inherent in water heating that requires this integration of hot water tank and heat pump compresser/exchanger. The real problem with heat pump water heaters has traditionally been that, unlike other heat pump systems, they were not split.

    The Sanden CO2 finally gets things right, putting the compresser outside the building envelope, where it belongs.

    The Sanden design also makes installation possible for DIYers, since the connection between the compresser and the water tank is just plumbing, not refrigerant. (Of course, we can argue about the pros and cons of connecting the two units with water instead of refrigerant, but that is a separate argument.)

    Unfortunately, the price of the Sanden units is currently breathtaking.

  41. Jon_R | | #41

    The Chiltrix air->water heat
    The Chiltrix air->water heat pump can also be used for DHW. But if you have one, you might as well use it for space heating/cooling too.

  42. GBA Editor
    Martin Holladay | | #42

    Response to Comments #40 and #41
    GBA readers who are interested in split-system air-to-water heat pumps should read this article on the topic: Air-to-Water Heat Pumps.

  43. Expert Member
    Dana Dorsett | | #43

    Robbing Peter to pay Paul is rarely a zero sum game.
    Between half to to two thirds of the energy going into an indoor heat pump water heater does indeed come from in side the house. But that doesn't mean that 1/2 - 2/3 fraction is adding directly proportional cost, energy use, or carbon footprint to the water heating:

    1. Even in VT removing heat & moisture from the room air is an immediate benefit for 3-4 months out of the year, reducing the indoor humidity and the cooling load. The water heater offsets the power otherwise used by mechanical dehumidifiers, and puts that heat into the water rather than adding to the sensible cooling load.

    2. When located in a semi-conditioned basement that is not normally heated directly, it lowers the average temperature of the basement a few degrees, which lowers the heat loss of the basement, and the heating energy use of the house during the heating season. (This benefit is not nearly as large as #1, but it's easily measurable..)

    3: In houses heated with heat pumps, the heat for the 1/2 - 2/3 taking from the room is also leveraged by the space heating heat pump by a comparable amount. In houses served by oversized heat pumps the additional marginal load increases the as-used HSPF efficiency. (This is more complicated to measure, but not impossible.)

  44. rockies63 | | #44

    Response to Martin Holladay
    Unfortunately, I can't take advantage of the 15 day free trial membership without using a credit card. I choose not to use credit cards. I'm also not planning on borrowing one "from my mom, dad or a friend".

    How incredibly patronizing.

  45. GBA Editor
    Martin Holladay | | #45

    Response to Scott Wilson
    When I lived without a credit card, I borrowed one when I needed one, paying the credit card owner in cash for the favor. The advice wasn't patronizing; it was practical, based on life experience.

    If you want to buy something online, and you choose to live without a credit card, your options are limited. I'm sorry that I wasn't able to come up with a solution that worked for you.

  46. kevin_in_denver | | #46

    Collector efficiency vs System efficiency
    "solar thermal collectors are about five times more efficient than PV panels, "

    Big deal. It's the annual system efficiency that counts.

    Here is a report from NREL that highlights just how a bad real life system efficiency tends to be:

    To quote: "We calculated a simple overall system efficiency for the solar water heater by dividing the thermal energy delivered from the solar tank to the backup water heater by the total solar radiation on the plane of the collectors times the area of the collectors. The monthly average efficiency varied from 2.8% in August to 7.4% in December. The annual average efficiency was 4.8%."

    3+ times WORSE than PV. Pathetic.

    And this was a very well designed drainback system that was vetted and approved by experienced engineers at NREL. In a VERY sunny climate.

    Robert has always been good at stirring the pot:

  47. GBA Editor
    Martin Holladay | | #47

    Response to Kevin Dickson
    Thanks for a spot-on useful comment backed up by data from an impeccable source. I have a feeling that I will be quoting from that NREL paper in the future! I appreciate it very much.

  48. tommay | | #48

    the real reason
    The real reason PV is pushed is that when using SHW, the utilities do not make money, they lose it. With grid tied PV, they pay the supplier a fee then the power goes out to the grid to their neighbors who pay full price, along with all the delivery and other fees at no cost to the utility. At one time they tried to meter hot water systems but that didn't go over so well.
    Another thing you never hear about for the same reason is passive solar for lighting, heating and cooling. Why is that?
    Energy is free throughout the universe! People only pay to have it change form and delivered. If one can harness their own power, on site, then the big guys are out of business. This is the real discussion. If I dig up a piece of coal, free out of the earth, then drop it off at your house, does that give me the right to charge you multiple fees? One may say, well I took the time to dig it up so I should be compensated, but does the guy who made the shovel get his cut? We all work together to supply all that is needed for the world to go round and that should be enough. So arguing over whether one form of energy is better or more economical is moot. All forms of energy should be considered and used for the benefit of all.

  49. comet48 | | #49

    Solar thermal is not dead
    The above article misses one very important point. It compares PV efficiency (15%) to solar thermal (75%), but fails to include the COP factor of using heat pumps etc.
    A modern electric water heat has a COP approaching 4, and modern PV is 18%+, so the efficiency of PV is really closer to 72%. I'm looking at installing a 30 SEER split hear pump for air/heating. That's a COP of 9 to go with the 18% PV for a total of 162% efficiency.

  50. comet48 | | #50

    Solar thermal is not dead
    You can now purchase PV components for around $1.50 a watt ( Commercial installs about $3.50 a watt. Install it yourself, claim 30% tax deduction on the commercial price and you a 50 cents a watt system!

    [Editor's note: To read responses to Comet48's comments, as well as other comments, click the number 2 in the box below to advance to Page 2.]

  51. GBA Editor
    Martin Holladay | | #51

    Response to Comet48
    Although I agree with you that the comparison between solar thermal collector efficiency and PV module efficiency isn't particularly helpful -- it makes more sense to compare the useful energy produced per $1,000 invested -- your figures are exaggerated.

    On an annual basis, a heat-pump water heater is unlikely to have a COP of 4; a COP of 2 is more realistic.

    And even if you could install a PV system for $1.50 a watt by providing your own labor, the 30% federal tax credit brings the cost of the installation down to $1.05 per watt, not 50 cents per watt.

  52. comet48 | | #52

    Solar thermal is not dead
    The other big plus for electric is that you can treat the utility as a giant battery. Overproduce in the summer, draw down in the winter. True up at the end of the year (California here).

  53. comet48 | | #53

    Response to Martin
    Notice I said claim 30% on the commercial price - $3.50. IRS does not check.

    Currently I heat my hot water natural gas. $1.20 a therm (30kWH). I have a 320 ft Fafco array for the pool - does about 750K btu's a day, using a 90 watt pump.

    Not sure which way to go next with hot water. I'd love to see some test data on heat pump water heaters. I have an attic (unused) that runs over 100 deg F for much of the year. I figure a heat pump could make good use of that.

  54. GBA Editor
    Martin Holladay | | #54

    Test data on heat-pump water heaters
    For a thorough discussion of heat-pump water heaters, including a discussion of tests and research performed by Steven Winter Associates, see the GBA article "Heat-Pump Water Heaters Come of Age."

  55. comet48 | | #55

    Response to Martin
    Thanks Martin. 2-2.5 not as interesting. I already have 3x20 ft panels ready to install on the roof, but reading about enhanced electric heating slowed me down. I figure the tank and other paraphernalia will run me another $2K. That's about four years of natural gas costs. So if I displace 2/3 with solar, that's about 6 years to recover. How long do these solar tanks last? Recommendation?

    1. severaltypesofnerd | | #59

      Solar only tanks with no gas heat, or no electric element, seem to last decades. And, you can change the anode rode every decade, and probably go 30, 40 50 or more years.

  56. Expert Member
    MALCOLM TAYLOR | | #56

    You wrote:
    "Notice I said claim 30% on the commercial price - $3.50. IRS does not check."

    I'm not sure it's a good approach to base your economic comparison on what is essentially tax fraud.

  57. severaltypesofnerd | | #58

    Not every home is a single family home. The economics for solar pre-heat on a commercial apartment complex are far more favorable. First, the heaters themselves are still gas fired, second the usage profile is far more spread out over the day as tenants take showers at different times.

    Let's not confuse the two in terms of economics.

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