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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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Jan 1, 2015 5:25 AM ET

Edited Jan 1, 2015 5:50 AM ET.

Response to Tom Gocze (Comment #43)
by Martin Holladay

You wrote, "Solar thermal is a simple technology and should be less expensive."

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

But it is what it is.

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

Jan 1, 2015 5:30 AM ET

Response to Kim Shanahan (Comment #44)
by Martin Holladay

Your reference to Chinese solar panels sounded to me as if you hold out hope that Chinese manufacturers might help bring down the cost of solar thermal collectors.

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

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

Jan 1, 2015 5:40 AM ET

Edited Jan 1, 2015 8:05 AM ET.

Response to Fortunat Mueller (Comment #45)
by Martin Holladay

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

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

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

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

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

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

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

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

Jan 1, 2015 5:48 AM ET

Edited Jan 1, 2015 6:03 AM ET.

Response to Vaughan Woodruff (Comment #47)
by Martin Holladay

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

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

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

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

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

Jan 1, 2015 5:57 AM ET

Another response to Vaughan Woodruff
by Martin Holladay

You wrote that my writing is "intentionally being provocative."

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

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

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

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

Jan 1, 2015 6:01 AM ET

Edited Jan 8, 2015 6:41 AM ET.

Response to Ed Dunn
by Martin Holladay

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

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

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

Jan 1, 2015 8:43 AM ET

Response to Martin
by Vaughan Woodruff


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

Here's a real world example:

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

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

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

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

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

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

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

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

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

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

Vaughan Woodruff
Insource Renewables

Jan 1, 2015 9:49 AM ET

Response to Martin
by Tom Gocze

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

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

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

Jan 1, 2015 1:16 PM ET

Edited Jan 1, 2015 1:18 PM ET.

Follow up question for Martin (and everybody else)
by Marcus de la fleur

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

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

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

Jan 1, 2015 5:00 PM ET

Wisconsin Electric
by Vaughan Woodruff

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

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

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

Vaughan Woodruff
Insource Renewables

Jan 1, 2015 8:45 PM ET

by Malcolm Taylor

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

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

Jan 2, 2015 6:34 AM ET

Edited Jan 2, 2015 6:36 AM ET.

Response to Vaughan Woodruff (Comment #58)
by Martin Holladay

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

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

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

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

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

Jan 2, 2015 6:47 AM ET

Edited Jan 2, 2015 6:53 AM ET.

Response to Marcus de la Fleur (Comment #60)
by Martin Holladay

You wrote, "Ductless minisplits ... may work for a pretty good house or passive house. But that is a real small market out there."

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

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

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

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

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

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

Jan 2, 2015 7:02 AM ET

Edited Jan 2, 2015 7:03 AM ET.

Response to Malcolm
by Vaughan Woodruff


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

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

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

Jan 2, 2015 10:37 AM ET

Thanks, Vaughan
by Dan Kolbert

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

Jan 2, 2015 11:43 AM ET

Solar Thermal died a long time ago at my house(s)
by Wes Stewart

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

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

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

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

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

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

Jan 2, 2015 12:31 PM ET

Edited Jan 2, 2015 12:35 PM ET.

Response to Wes Stewart
by Martin Holladay

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

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

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

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

Jan 2, 2015 2:14 PM ET

Response to Martin Holladay
by Wes Stewart

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

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

I enjoy your writings. Happy New Year.

Jan 2, 2015 4:12 PM ET

Some thoughts on this good discussion
by Marc Rosenbaum

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

Jan 2, 2015 11:28 PM ET

Keep it flowing folks!
by Paul Kuenn

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

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

Just thinking out loud!? ...

Jan 3, 2015 5:08 AM ET

Edited Jan 3, 2015 5:28 AM ET.

Response to Paul Kuenn
by Martin Holladay

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

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

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

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

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

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

Jan 3, 2015 8:13 AM ET

Response to Martin Holliday
by Tom Lane

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

Jan 3, 2015 4:59 PM ET

Response to Tom Lane
by Martin Holladay

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

Tom, thanks very much for your comments.

Jan 4, 2015 2:55 PM ET

PV for DHW
by Darryl in Winnipeg

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

Jan 4, 2015 6:49 PM ET

Edited Jan 4, 2015 10:46 PM ET.

solar and future ..
by Jin Kazama

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

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

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

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

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

start from here and navigate

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

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

Jan 4, 2015 8:46 PM ET

Thank you Tom Lane...
by Marcus de la fleur

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

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

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

Jan 4, 2015 9:03 PM ET

To Martin re: Tom Lane Book
by Wes Stewart

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

Jan 5, 2015 5:35 AM ET

Edited Jan 6, 2015 8:13 AM ET.

Response to Darryl in Winnipeg (Comment #75)
by Martin Holladay

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

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

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

Jan 5, 2015 5:44 AM ET

Edited Jan 5, 2015 9:13 AM ET.

Response to Marcus de la Fleur (Comment #77)
by Martin Holladay

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

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

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

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

Jan 5, 2015 5:51 AM ET

Response to Wes Stewart (Comment #78)
by Martin Holladay

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

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

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

Jan 5, 2015 6:41 PM ET

Edited Jan 5, 2015 6:50 PM ET.

RIP ST? Really?
by Ron Theaker

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

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

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

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

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

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

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

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

Jan 5, 2015 9:11 PM ET

Free heat for life
by James Morgan

Omigod. Right up there with the insulating paint.

Jan 5, 2015 9:49 PM ET

Ron Theaker
by Jin Kazama

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

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

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

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

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

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

"C'est un probleme a deux faces."

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

Jan 6, 2015 6:00 AM ET

Response to Ron Theaker
by Martin Holladay

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

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

Jan 6, 2015 7:17 AM ET

Energy too cheap to meter!
by Dan Kolbert

To quote David Byrne:

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

Jan 7, 2015 9:32 PM ET

More David Byrne
by Malcolm Taylor

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

Jan 8, 2015 12:34 AM ET

Edited Jan 8, 2015 12:38 AM ET.

solar thermal is alive and well and growing.
by terrence moag

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

Jan 8, 2015 5:50 AM ET

Edited Jan 8, 2015 6:41 AM ET.

Response to Terrence Moag
by Martin Holladay

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

My article focuses specifically on single-family homes. In my original article (you can find the link in the first sentence on this page), I wrote, "In the northern half of the U.S. — and even much of the South — installing a residential solar hot water system doesn’t make any sense. ... Unless you’re building a laundromat or college dorm, solar thermal is dead."

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

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

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

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

Jan 8, 2015 11:16 AM ET

Edited Jan 8, 2015 11:33 PM ET.

The grid as storage: advantage pv over solar thermal
by ven sonata

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

Jan 8, 2015 2:41 PM ET

Edited Jan 9, 2015 10:25 AM ET.

Post deleted by author
by GBA Team

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

Jan 8, 2015 3:23 PM ET

Edited Jan 8, 2015 3:47 PM ET.

Response to Michael Levitz
by Martin Holladay

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

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

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

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

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

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

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

Jan 8, 2015 11:29 PM ET

Edited Jan 13, 2015 2:48 PM ET.

Michael Levitz redo your pv and inverter math
by ven sonata

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

Jan 12, 2015 1:28 PM ET

There is no way that so little PV can produce the power
by Cor van de Water

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

Jan 12, 2015 7:32 PM ET

Cor van de Water...accurate pv production numbers
by ven sonata

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

Jan 12, 2015 11:46 PM ET

Cor van de Water
by Jin Kazama

I fail to see how you can get less than ~1000Kwh/kw installed in southern Canada usuing NRC maps :,4240&SETS=1707,1708,1709,...

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

Jan 15, 2015 9:03 PM ET

I apologize if I missed this
by Brian Godfrey

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

Jan 17, 2015 9:10 AM ET

Calculating Water Heating Energy
by Trevor Bowden

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

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

Jan 18, 2015 5:21 AM ET

Response to Brian Godfrey (Comment #97)
by Martin Holladay

I agree with you.

Your points were noted in my original article (Solar Thermal is Dead), where I wrote: "But the number of gallons of hot water produced by a solar collector is always less than the number of gallons actually used by the homeowners. After all, if great quantities of hot water are produced on a day when it isn’t needed, you can’t really count the energy production in your annual tally. .... Unlike owners of a grid-connected PV system, who can be credited for their excess electricity production, ... owners of a solar thermal system can't sell the excess production of their hot water systems."

Jan 18, 2015 5:31 AM ET

Response to Trevor Bowden (Comment #98)
by Martin Holladay

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

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

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

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

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

Mar 11, 2015 11:11 PM ET

Interesting analysis, but debatable conclusions
by Graham Irwin

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

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

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

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

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

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

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

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