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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

Posted on Dec 26 2014 by Martin Holladay

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(PV) Generation of electricity directly from sunlight. A photovoltaic cell has no moving parts; electrons are energized by sunlight and result in current flow. 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 (PVPhotovoltaics. Generation of electricity directly from sunlight. A photovoltaic (PV) cell has no moving parts; electrons are energized by sunlight and result in current flow.) 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 pumpHeating and cooling system in which specialized refrigerant fluid in a sealed system is alternately evaporated and condensed, changing its state from liquid to vapor by altering its pressure; this phase change allows heat to be transferred into or out of the house. See air-source heat pump and ground-source 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.

How can I determine the annual electricity production of a proposed PV system?

The easiest way to figure out how many kWh will be produced each year by a PV system is to use a free online calculator called PVwatts. The calculator allows users to change a number of parameters, including geographical location.

How much does a water heater cost?

Installation costs vary from region to region. This article assumes that the installed cost for an electric-resistance water heater is $1,200, and the installed cost of a heat-pump water heater is $3,000.

Of course, your local costs may be higher or lower than these figures.

How many gallons of hot water per day does the average family use?

According to a Canadian study, the average Canadian family uses 44 gallons of hot water per day. The Canadian researchers’ findings mirror those of several U.S. researchers; there is growing evidence from monitoring studies that the assumption used in the DOE’s Energy Factor test for water heaters — namely, that an American family uses 64 gallons of hot water per day — is unjustifiably high.

This article assumes that the average North American family uses 44 gallons of domestic hot water per day.

In homes with a solar thermal system, what percentage of the home’s domestic hot water needs are met by the solar equipment?

One of the best studies on the “solar fraction” question was performed in 2006 by researchers from Steven Winter Associates. The researchers monitored two residential solar thermal systems for a year, one in Wisconsin and one in Massachusetts. Each house had two solar collectors. According to the researchers’ report, Cost, Design and Performance of Solar Hot Water in Cold Climate Homes, the solar fractions of these two carefully monitored systems were 63% and 61%, respectively.

I’m going to assume that a two-collector solar thermal system supplies 63% (on an annual basis) of a family’s domestic hot water needs. (The energy used to heat the remaining 37% of a family’s hot-water needs is provided by a backup water heater — for example, an electric resistance water heater).

Some solar thermal enthusiasts may argue that there are locations in the U.S. where the solar fraction for a solar thermal system is likely to be higher than 63%. They're right. However, it's important to remember that in locations with lots of sunshine, the annual output of a PV system will also be higher than it would be in a location like Wisconsin or Massachusetts.

How much electricity is needed to make domestic hot water?

The electrical energy use assumptions in this article are based on data provided by Marc Rosenbaum, who has monitored the energy use of several Massachusetts families for years.

According to Rosenbaum’s monitoring data, a typical electric resistance water heater uses 0.21 kWh/gallon of hot water (3,373 kWh/year to make 44 gallons of hot water per day), while a typical heat-pump water heater uses 0.07 kWh/gallon of hot water (1,124 kWh/year to make 44 gallons of hot water per day).

Comparing three systems

My latest approach to comparing the cost of solar equipment used to make domestic hot water starts with the assumption that the typical solar fraction of a cold-climate solar thermal system is 63%.

Keeping that solar fraction in mind, I have calculated the cost of equipment for three scenarios:

  • House A has a solar thermal system (two rooftop collectors and a solar storage tank in the 80 to 120 gallon range) and an electric-resistance water heater for backup.
  • House B has an electric-resistance water heater and a PV system sized to provide enough electricity on an annual basis to meet 63% of the family’s hot water needs.
  • House C has a heat-pump water heater and a PV system sized to provide enough electricity on an annual basis to meet 63% of the family’s hot water needs.

The table below compares three homes in Boston, each of which uses 44 gallons of domestic hot water per day.

According to this analysis, the PV plus electric-resistance approach is about 25% cheaper than the solar thermal route, and the PV plus heat-pump approach is about 50% cheaper than the solar thermal route.

Everyone's numbers are going to be different

What if you are a solar thermal buff who thinks that my assumptions are unfair to solar thermal? Well, let’s change a few numbers. We’ll assume that a solar thermal system costs only $6,000 to install and that the solar fraction is 75%.

I think that the assumptions made in the above table are unrealistic, since it's hard to find a contractor willing to install a good two-collector solar thermal system for $6,000, and because in a location with a solar fraction of 75% a PV system is likely to produce more electricity than this table shows. But according to this analysis, the PV plus heat-pump approach is still about 23% cheaper than the solar thermal route.


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. But most results will be similar to the results shown above.

One more point to consider: if your numbers result in a tie — if your calculations show that solar thermal equipment and PV equipment cost exactly the same — remember that the maintenance costs for a solar thermal system will be higher than the maintenance costs for a PV system.

Some people love their solar thermal systems

Solar thermal technology attracts a loyal group of fans. Solar hot water systems have been around for more than a century, and for renewable energy enthusiasts of my generation, these systems evoke fond associations. I have two solar thermal collectors on my roof, and I enjoy listening to the quiet hum of my system's small pump when the sun is shining.

Solar thermal buffs make the argument that solar hot water systems are simple, elegant, affordable, and delightful. I sympathize with their emotional attachment to these systems, and I wish them all the luck in the world. However, most people I know who have installed solar thermal equipment have a tale or two involving maintenance headaches. (If you are a hobbyist, these technical glitches are sometimes exciting to solve. If you are an average homeowner, however, these glitches are just ordinary headaches.) Owners of PV systems are much less likely to have as many stories of maintenance problems.

Martin Holladay’s previous blog: “Martin’s 2014 Christmas Parody.”

Click here to follow Martin Holladay on Twitter.

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Dec 26, 2014 12:45 PM ET

by Richard McGrath

There is room and reason for both technologies Martin .

Dec 26, 2014 1:55 PM ET

Response to Richard McGrath
by Martin Holladay

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.

Dec 26, 2014 4:24 PM ET

Edited Dec 30, 2014 7:17 AM ET.

Close to making sense and also not IMO
by aj builder, Upstate NY Zone 6a

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

Solar plane clip.JPG water heater jpg.PNG

Dec 26, 2014 6:10 PM ET

Response to AJ
by Martin Holladay

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.

Dec 26, 2014 6:28 PM ET

Edited Dec 26, 2014 6:50 PM ET. the HP heat free?
by flitch plate

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.

Dec 26, 2014 7:00 PM ET

Response to Flitch Plate
by Martin Holladay

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.

Dec 26, 2014 7:53 PM ET

A revolution has occurred...thanks
by ven sonata

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.

Dec 26, 2014 7:55 PM ET

Edited Dec 26, 2014 8:21 PM ET.

Pretty decisive
by Nate G

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.

Dec 26, 2014 8:44 PM ET

Nice job Martin. I think
by Ted Kidd

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.

Dec 27, 2014 12:09 AM ET

... best cost not yet posted
by flitch plate

If you make it a DIY solar thermal + resistance HW heater, its costs even less: $3500. and Closed Loop

Dec 27, 2014 12:49 AM ET

by Jin Kazama

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.

Dec 27, 2014 7:33 AM ET

Response to Flitch Plate (Comment #10)
by Martin Holladay

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.

Dec 27, 2014 11:26 AM ET

Edited Dec 27, 2014 11:29 AM ET.

Response to Ven Sonata (Comment #7)
by Martin Holladay

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.

Dec 27, 2014 12:43 PM ET

Has the revolution come to off-grid too?
by ven sonata

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!

Dec 27, 2014 1:13 PM ET

Edited Dec 27, 2014 1:13 PM ET.

DIY and Costs
by Malcolm Taylor

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.

Dec 27, 2014 5:32 PM ET

Edited Dec 27, 2014 5:33 PM ET.

Are there good controls for PV hot water?
by Eric Sandeen

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?

Dec 28, 2014 5:17 AM ET

Edited Dec 28, 2014 6:43 AM ET.

Response to Eric Sandeen
by Martin Holladay

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.

Dec 28, 2014 10:30 AM ET

Direct connect MPPT solar power
by richard turner

May be worth taking a look at.
No battery. No charge controller. No inverter........

Dec 28, 2014 11:04 AM ET

Response to Richard Turner
by Martin Holladay

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.

Dec 28, 2014 12:17 PM ET

What about space heating?
by Marcus de la fleur

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?

Dec 28, 2014 12:26 PM ET

Edited Sep 11, 2015 9:58 AM ET.

Response to Marcus de la fleur
by Martin Holladay

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.

Dec 28, 2014 1:40 PM ET

Response to Martin
by richard turner

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.

Dec 28, 2014 7:07 PM ET

Edited Dec 28, 2014 7:11 PM ET.

Solar Heat Pump - More Performance at Lower Cost
by Tania G

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.

Dec 28, 2014 9:59 PM ET

nothin' there
by flitch plate

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

Dec 29, 2014 12:08 AM ET

How well developed is the "solar pump" technology
by flitch plate

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.

Dec 29, 2014 1:33 AM ET

never heard of ...
by Jin Kazama

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 )

Dec 29, 2014 4:00 AM ET

Zombie Technology
by Karl Overn

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.

Dec 29, 2014 6:57 AM ET

Response to Tania G (Comment #23)
by Martin Holladay

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.

Dec 29, 2014 11:00 AM ET

Responce to comments #3,#4
by Daniel Young

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)

Dec 29, 2014 11:44 AM ET

Edited Dec 29, 2014 11:55 AM ET.

Response to Daniel Young
by Martin Holladay

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?"

Dec 29, 2014 11:51 AM ET

Solar Thermal is indeed badly
by Mark Klein

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.

Dec 29, 2014 12:37 PM ET

Reply to Mark Klein
by ven sonata

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.

Dec 29, 2014 12:57 PM ET

Response to Ven Sonata
by Martin Holladay

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

Dec 29, 2014 3:00 PM ET

My post, is being misread. I
by aj builder, Upstate NY Zone 6a

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.

Dec 29, 2014 3:15 PM ET

Response to AJ Builder
by Martin Holladay

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.

Dec 30, 2014 1:36 PM ET

Edited Dec 30, 2014 1:38 PM ET.

Thanks once again, Martin,
by brian carter

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.

Dec 30, 2014 7:36 PM ET

Big PV simply does not
by flitch plate

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.

Dec 31, 2014 7:47 AM ET

Edited Dec 31, 2014 7:48 AM ET.

Response to Flitch Plate
by Martin Holladay

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.

Dec 31, 2014 1:23 PM ET

off grid solar
by David Hicks

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.]

Dec 31, 2014 1:41 PM ET

Response to David Hicks
by Martin Holladay

Great idea.

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

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

Dec 31, 2014 5:35 PM ET

The End user who gets hosed by Con Ed (Conned is right)
by Darren Finch

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

Dec 31, 2014 5:38 PM ET

Edited Dec 31, 2014 5:39 PM ET.

Premature death
by Tom Gocze

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

Dec 31, 2014 6:07 PM ET

Thermal shared pain
by kim shanahan

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!

Dec 31, 2014 7:27 PM ET

More accurately, SHW is dead for small households
by Fortunat Mueller


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

Dec 31, 2014 7:41 PM ET

California Questins
by jim soules

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.

Dec 31, 2014 9:47 PM ET

Not quite dead yet
by Vaughan Woodruff

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
    Insource Renewables

Dec 31, 2014 11:08 PM ET

X prize
by Tom Gocze

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.

Jan 1, 2015 12:11 AM ET

by Jin Kazama

- 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.

Jan 1, 2015 1:16 AM ET

Solar Thermal died a long time ago
by Ed Dunn

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.

Jan 1, 2015 2:41 AM ET

Guest blog request for vensonata
by ven sonata

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!

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