Both solar thermal water heating systems and photovoltaic systems already have a good commercial background. Efficiency of energy gain of a solar thermal system is about 80%, while photovoltaics is up to 20% nowadays. Producing electricity is a different story than heat so by here it's ok.

But sometimes photovoltaic systems are advertised also as a water heating solution. The electricity produced is fed into an electric water tank heater (no DC/AC conversion needed). This specific usage has many disadvantages and seems as wasting:

  • To have a 2 kWp (peak), we need about 2 m2 of solar panels but more than 10 m2 of photovoltaic panels.
  • The expenses of those two systems might be 1:3.
  • The amount and kind of materials needed to produce a solar vs. photovoltaic panel are in favor of solar panel: glass + copper + heat exchange medium (glycol) vs. fine precessed silicone + wiring.
  • Recycling of glass and copper is already a commonplace, the silicone recycling is only in the beginnings.

But still, are there any advantages when heating water via a photovoltaic system over solar thermal system?

  • 4
    Some photovoltaic systems also run water past the panels, which not only collects extra energy as heat but also would potentially increases the efficiency of the photovoltaic cells by cooling them. More info in Wikipedia. Are you sure you're not looking at a reference to those? Feb 19, 2013 at 23:18
  • 2
    PV panels can have 20 or 25 year underwritten warranties. NO water weight on roof etc, NO plumbing, pipes joining lagging liquids thermosyphons or lack thereof. With proper design, efficiency of energy use in water heater can be close to 100% . Electricity can be swung between heating and other uses instantaneously as desired. Any grid replaced kWh is worth the cost of power from the grid at the moment of use. Overall the system can be very competitive with thermal solar in some cases. May 22, 2014 at 7:14

8 Answers 8



PV is generally superior to solar thermal because it produces a higher-quality energy: electricity rather than low-grade heat: 1kWh of electricity has more exergy than 1kWh of low-grade heat. As you say, PV's efficiency tends to be significantly lower than solar thermal, in straight watt-for-watt terms, which means less power per unit roof area.

And you're right that just using the electricity generated to create low-grade heat could be wasteful.


But exergy and economics are different: PV is becoming cheaper and cheaper. So maybe there are circumstances where PV + resistance heating is cheaper than solar thermal. Misaligned incentives between PV and solar-thermal could make that happen, as could extremely cheap PV systems.

Deemed exports within Feed-in Tariffs

And there are cases, for example in the UK, where the payment to the PV system owner is based on the amount of electricity generated, and where there is a payment for electricity exported to the grid, but this is not measured - it's just deemed to be 50% of the total generated. Yes, that is a silly way to do things, given how cheap and easy it is to meter the exports, but that's the system some utilities are operating.

Under such a regime, the PV is then built with a resistance heater in a hot-water storage-tank, and all the electricity gets dumped as low-grade heat into that tank. Dumping it as heat doesn't change the revenue generated for by the PV system, and does reduce the heating costs of the homeowner, as well as given them very-low-carbon hot water.

Hybrid photovoltaics + solar thermal

Finally, as Highly Irregular notes in the comments, there are various PV-thermal hybrid systems, which provide both electricity and low-grade heat: in those cases, some of the PV electricity is used to pump water, but the water is heated by the sun directly, rather than by resistance heater - and this cools the PV panel, raising its electricity output:

enter image description here source

There are people trying to combine a PV-thermal hybrid with a heat pump, but the economics really don't favour it at the moment, and current heat pumps can be pretty noisy, particularly in their defrost cycle. A ground-source heat pump + PV-thermal hybrid might be superior to just a heat pump on its own.

  • 1
    +1 for hybrid photovoltaics.
    – Peter Ivan
    Feb 21, 2013 at 7:38
  • Suspect this practice has a lot to do with economic incentives.
    – Flyto
    Mar 11, 2014 at 15:53

I will answer your question from a different approach, which is practicality rather than raw efficiency.

You asked if there are advantages to heating water using solar PV instead of direct solar hot water. The first potential advantage is how the variation throughout the year can be handled. In a modern grid-tied solar PV system, you can bank electrical energy credits across many months, effectively generating power in July for use in January. Thus the PV system can be sized based on the needed year-round average production instead of a guaranteed minimum production. Looking at direct solar hot water, energy banking can usually only be done for a matter of days. This means that a system with sufficient output in winter is likely to overproduce in summer. If you heat with the solar hot water this is even more the case, since the system will be larger but heating is not needed during the maximum production months. So the first potential advantage of PV over solar thermal is the percentage of generated energy that actually gets used, on a year-long basis.

A second potential advantage is simplicity. A solar thermal system cannot offset the total energy use of a home, since much of this is used as electricity. Therefore the homeowner desiring to offset the entire household usage would end up buying and maintaining two different types of systems. If the homeowner expects to buy a solar PV system anyway, they may simply find it easier to buy a single system rather than determine the balance between two systems.

If trying to heat water most efficiently with electricity, a heat pump water heater will raise the effective ratio of energy used to heat gained from .9 (for direct heating) to about 2.4. Thus, a 20% efficient solar panel (a high number, but taking it from the question) would effectively be 48% efficient at heating water.

  • Overproduction of a small PV system (my case) is considered as an energy for network loses by the distribution company. So generally it's not used directly, but as a support to run the system. Still, it's some kind of useful usage.
    – Peter Ivan
    Feb 21, 2013 at 7:36
  • I have heard this in the early stages of a utility's experience with small generation. I believe that most give up this practice as they gain more experience with net metering and the number of customer-suppliers goes up. If the energy is being dumped then the economics will be different than the actual generation saved at the power plant. Feb 24, 2013 at 20:34
  • In a modern grid-tied solar PV system, you can bank electrical energy credits across many months, effectively generating power in July for use in January — if only! Storage is expensive.
    – gerrit
    Jul 12, 2023 at 8:16

I can think of one big advantage, in certain instances, to using PV to drive a resistance heating element: ease of transmission. I can run a cable up, down, sideways, etc. with little difficulty, provided that it is large enough for the anticipated current. To do the same thing with pipe can be tricky, especially where you have to deal with extreme temperatures. Also, if the panel is on the roof, and the water is needed on the first floor of an 8-story building, you need quite a pump to get your heat-transfer fluid up there, and it's under quite a bit of pressure near the bottom, and the plumbing has to be capable of handling that. Fortunately, electrons aren't very heavy, so you don't have similar issues with electric cables; it's a matter of length only, not length and orientation.

Of course, I'd consider something more efficient than resistance heating, too, like an air-to-water heat pump. Generally better to use less than to generate more!


Cost is a big advantage; if you already have a hot water tank with space for an immersion heater then it is a lot cheaper to add a small photovoltaic panel then a full solar thermal system. The payback time can be very short for this. No control system is needed, as the immersion heater will just cut out when the tank gets too hot – the GCH can be set to heat to tank to say 60c and the immersion heater to 80c.

It is hard to make a solar thermal systems work well without having a big risk of legionnaires disease, as you need the bottom of the tank cold for the solar thermal to work well, but you must heat the COMPLETE tank to very hot to kill the legionnaires disease.

  • 2
    Solar thermal systems I've met have a built-in protection against Legionnaires disease. They monitor the tank temperature and if it hasn't reached certain limit during a certain period (a week), they command the not-solar-heater (gas or electricity) to heat the whole tank to kill the bacteria.
    – Peter Ivan
    Dec 11, 2013 at 7:29

My 40 gal electric HW heater was dying (18 years old - time to go, even for a Sears Kenmore). I had EIGHT 220 watt PV panels left from an installation (at my office), and I wanted to use them to heat water (much more efficient use than sitting in my shed for a year! Right? ). 220 x 8 is only 1760 watts. I found a new "Brand X" 50 Gal Electric HW heater locally for $315 (no delivery - had to fetch it - UGH!). Was 1760 watts enough to heat my tank to 130 or preferably even higher (say 170F), in 4 hours of full sun??? That was the MAIN question - and would the thermostats work right? (they ARE mechanical and heat is the key, so it should be OK).
I get a little over 4 hours of sun in winter where I wanted to put the panels - almost 5 hrs in summer (flat on porch roof - can't be seen except by Google Sats or planes from the air). They say that normal tank recovery to 140F was 1.25 hours at 3300 watts from 68 degree cold water (it seems too long a time - I didn't do the calculations, but I will - eventually) - takes longer for colder water - luckily I'm in Fla. Cold water temp is fairly stable 65 to 70. A 50 gal tank will last my small family (3 of us) 3 to 4 days (I HOPE), but only if I set it 165 or 170 and use a cold water mixer also called an "infuser" I think (it mixes cold with hot on the output and keeps water from being dangerously hot in the shower, or for kids at a faucet, etc.). The infuser is a trick I learned some years ago with a smaller 30 gal tank from my brother-in-law (thanks RICK!).
I had just enough 8 AWG wire - 10 AWG would have worked, but I had 8 so that's OK - another volt or two not dropped on the way in, I guess. After 7 days and 4 problems (all not directly related to the project itself, but a PAIN nonetheless), I got it installed (the tank was last).

And the answer turns out to be YES - and NO.
Yes it works. But on the NO side,

1) I am only getting 221 VDC at 7.4 amps - should be 230 or so under full sun (29VDC at max output x 8 panels in series). What happened to my 10 volts or so? Not that big a deal, I suppose. The coils are so low a resistance they are MAXING out the panels as I thought they would. Top coil comes on first, and an hour and a half to 2 hours later, CLICK - the lower one is on.
2) It does not always get hot enough - not even a full hour of clouds half-way through the heating cycle messes it up (pretty badly) - lots of warm water! (Note to self: Wife says "warm" is 100% FAILURE. Ooooo-Kay. HOT it must become... somehow ).
3) I only get two days of hot water after 1 full day of sun (4.5 to 5 hours), instead of a minimum of 3 as I want (and need), maybe because I only have it set to 135-140. 170 was WAY too hot since the infuser does not come on right away and with the first burst of hot water you can get scalded pretty badly if you are not careful (thankfully - he have no small kids). I need a faster acting infuser - this one is some type of bi-metal switch that mounts around/against the hot-out-bound copper pipe right out of the tank and it reacts too slow (looks like a WW-II or maybe a WW-I surplus thing). I need an Electric sensor/switch of some kind, or maybe adjustable-pressure based unit of some kind. I am searching for one. Something MUCH faster, anyway.
4) If they were not spare panels - cost might be be prohibitive. We paid 68 cents/watt (per whole palette of 25) - 220 x 8 was $1760 plus $315+tax for the 50 gal "el-cheapo" electric tank, $45 for small breaker box and DC beaker for PV panel supply). No mounting costs - they just lay flat on the porch roof with one bolt as a tie down (protected from wind, great drainage, slightly tilted up toward west - better for the afternoon sun, too).
5) Payback right now is 3.5 years + possible alimony for life if not made HOT-HOT-HOT (about 20% to 22% of electric bill - $550/year). Too long to suit me (but not too bad, really - panels will surely outlast the new tank unless we get a bad hail storm (very rare down here, BUT...I still worry about it). I PRAY a new infuser and going up to 170F will give me the extra day I want (and make my WIFE happy... it HAS to... no choice in the matter.)

NOW for the PROS:
1) Super EASY to install. Bozo the clown could do it, except for "sweating" the copper pipes for the new tank (torch soldering). Adding them to an existing tank installation would be a NO brain-er.
2) No (leaky) pipes to run - no heat exchanger, no "Fred & Ethel Glycol" (old fluid mechanics joke) or other fluid heat carrying medium, or not pushing the cold up, and hot water down from a direct concentrator system.
3) GREAT use for spare PV panels. Actually a good deal if you can get panels for less than about 65 cents/watt any time (without having to buy a train load of them).
4) We have always had AT LEAST a warm shower in 6 months of use - Never cold - (I do warm, OK - My WIFE does NOT - NEVER WILL - EVER - even if H**L freezes over... EVER, NEVER EVER. PERIOD OVER AND OUT! Got it?? I got it.
And did I mention mt WIFE does not like WARM showers. NEVER, EVER? ok? ok.).
5) Also, make sure to check with your wife BEFORE you do this. To be on the safe side - that is. ALWAYS check with your wife! OK? GOT THAT? ok. Me too. I GOT that. And I hear about it often... even 6 months later... once a week at least...

YIKES! I need that NEW, FASTER infuser - (anyone have one?). I wonder who gets the dog in a divorce?
I KNOW for a fact that I will get the NEW hot water heating system. And I may even have to wear it out of the house - around my neck someday.
If you try it out, GOOD LUCK!
SUN POWER! PV direct for hot water - It works!!!
(If warm showers once a month, or so, are OK for you --AND YOUR WIFE... that is).
One other thing - if you try this, for health reasons, make sure your tank switches elements and both the top and bottom get fully heated a minimum of every 96 hours. If you have a 3 to 4 day period of clouds, let the tank get up to full temp before you use it again (make sure both elements come on - get out the Volt meter). I really don't have to worry about this since 4 days of warm water will put me in the hospital where they will be removing shards of glass from all over my body and aluminum solar panel frame parts from various wounds inflicted from flying PV panels... HOT IS GOOD! WARM IS BAD. In my future... I can see a 240 Volt 3 pole double throw relay/switch lifting all DC and applying AC after 2 days of clouds... I'm physic like that. It's a sort of sixth sense - a survival instinct in all of us from thousands of years of marriage evolution - I'm pretty sure that's what causes it. Like a bird's Fright-n-Flight instinct. Yeah... that's it.

  • 3
    Welcome to Sustainable Living! Thank you for your elaborate answer. I do think that your answer would be more valuable to the community here if you shortened it a bit and focused mainly on the pros of using PVs over a solar thermal system to heat water.
    – THelper
    Dec 28, 2013 at 10:08
  • There is some useful information in there +1. And thank you for the funniest answer I have read on Stack Exchange! Dec 3, 2014 at 7:49
  • How has this solution been holding up these past 6 years?
    – w00t
    Aug 31, 2019 at 13:12

This can actually be more advantageous if you have a surplus power diversion manager. During hot seasons or sunny weather conditions, solar PV panels collect more energy nevertheless any surplus to be exported to the grid. The Solar PV surplus diversion manager uses it to heat up water instead of letting it to be exported to the grid and without affecting the Feed-In-Tariff.

  • 2
    Hello there, and welcome. Thanks for this answer. It might be worth emphasising that this solution is specific to places such as Britain where grid exports aren't metered, so there's no economic value to them.
    – 410 gone
    Mar 11, 2014 at 16:15

Regarding directly feeding a tank with a series string of panels. Don't have the figures at hand but basically figured the math on this once (maybe it was 3400 btu's per kwhr?) and remember that 1500 watts of panels will put same amount of btu's in tank as a 22 sguare foot evacuated tube collector which came out to the pv panels requiring about 5 times as much space on your roof or lawn for the same heat output.

Cost was actually less with pv at $1.00/watt) or at least similar with the big plus of the PV setup being much more durable over time, less hassle in the present, plus don't have to hear a pump so if you have the space it may be the smarter choice.

There may be an issue with PV not working as well when cloudy but not sure about that. I.e, if you put a suitably sized pv system next to a solar thermal whereby both perform equally in good sun the pv will not do as well when clouds comes out. Uncertain about this as I mostly just read stuff without doing and not even sure I read that but perhaps someone here can knowledgeably comment. Something to look into for sure.

Two important things come to mind.

  • Use dc rated thermostat, not the stock AC ones on tank, especially if running high volts from series string. Dc is single minded and when a switch is thrown wants to keep jumping gap, producing an arc that will ruin contacts. A real safety issue here.

  • Using AC element with DC voltage is fine but if use the ones tank comes with you will probably lose a lot of power. Are articles available that will show you how to do math on this. listen to the people that discuss using AC elements as diversion or dump loads maybe.

  • 1
    Welcome to the site, and thanks for the answer! Feb 27, 2015 at 9:07

I had to quit my PV water heating experiment since the wife had to take one too many cold showers and I was in fear of waking up dead one morning with a back-brush through my skull... and the solution to critters growing in your "warm" water is a small 45 watt UV sterilizer on the output of the tank. Now that did offset cost savings a wee bit, but not very much (3% or 4% maybe). Amazon had one for $250 with 2 spare UV tubes. This works in Florida (according to me, but did not work according to my wife), but I would already be dead had I tried it in VA where we got married.

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