I'm looking to possibly buy an Craigslist water heater($120), replace the heating element with a DC unit(12V), and power it with a 100 watt solar panel (12V). On an 40 to 80 gallon insulated tank should I be able to get the water warmed enough in 4 hours of sunlight, during the day, store until nighttime, to cut the use of draw from an electric on demand system? Thoughts?

  • only 100W for 4 hours , most water heaters have elements that are 3000W plus, I dont think it will raise the temp more than a few degrees.
    – Ed Beal
    Jan 19, 2016 at 20:19
  • 100W for 4 hours is only .4KWh. If the system really produced 100W of usable power for 4 hours each day for a year, you're looking at saving around 150KWh of power, or around $22 worth at 15 cents/KWh, probably not worth the expense and effort to do what you're proposing. Since you're putting in a holding tank anyway, you'll probably get more "bang for the buck" by using a solar thermal collector.
    – Johnny
    Jan 19, 2016 at 22:59
  • Get the water heater, and a 100w 120v heating element, and a common lamp timer, and set it to run 4 hours a day. Now you can see whether that's going to give enough heat to be worth buying a solar panel. I suspect it is not. Jan 20, 2016 at 0:31
  • I have two 100 watt panels, I purchased for camping, so I'll experiment with that. I'm really curious what the 12v DC element will do... I think the heat transfer/recovery method is likely the quickest method. Maybe a mix of the two... I switched to an all electric house after loading up with solar panels on the roof. My one cost I'm struggling with is my hot water heating. 90% of our remaining electric bill, is due to water heating. Outdoor Passive water heating is out in NH due to the extreme cold nights. If I can recover that I will be utility bill free.
    – Peter M
    Jan 21, 2016 at 22:51
  • 1
    If you want to heat water, it might be more efficient to use the sunlight directly to heat water...
    – keshlam
    Jan 25, 2016 at 1:31

1 Answer 1


Sadly not much. 100W would not effectively heat a 40 gallon water heater, let alone an 80 gallon. Also, remember that the panel likely won't really put off 100W unless your element has the exact right resistance for your specific panel.

You can do some simple calculations if you're curious how much heat that will produce... If you use 100 Watts for 4 hours, that is 400 Wh (Watts * hours = Wh) It takes approximately 2.4 Wh to heat 1 gallon of water 1 degree Fahrenheit.

So 400 Wh per day could heat 10 gallons of water 16.67 degrees. (The equation is [(Wh) ÷ 2.4 ÷ (#Gallons) = (change in degrees). So in other words, if your water comes into the tank at 70° (pretty warm) and you use 10 gallons per day, the theoretical temperature it could produce would be 86° (not very impressive is it?). This will somewhat reduce your draw on the on-demand but not by much.

So doing a little math the other way, if your water came into the tank at 50° and you wanted it to heat up to 120°, that is a 70° increase. If I used -say- 30 Gallons per day and I wanted to heat it up 70°... The formula is (Wh)=(Change in Degrees)*(#Gallons)*2.4 So that would be [Wh = 70° * 30 Gal * 2.4] which comes to 5040 Wh. Spread that across 4 hours and you can see that you could do it with 1260 Watts.

I understand that your goal isn't to heat your water exclusively with the solar but the numbers show how to do the basic math. For example, if I really do use 30 Gallons per day and I heat it 70° above its original temperature, my total need would be around 5040 Wh so if I offset it by 400 Wh (100W for 4hrs) then I would be offsetting apporximately 5/63, (or 7.9%) of my usage

Please note that this doesn't account for a myriad of factors that could affect it, such as heat-loss, inefficiency, or the derating of your solar panel... These numbers should some idea but don't expect them to be exact. Please let me know if this was helpfull or if there is anything I could explane better by leaving me a comment.

Thank-you, Maxfield Solar

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