350w solar panel, a 12v 150w element inside a galvanized 35 gallon drum used to heat a 10x16 cabin?

Hi, I am wondering how well this will work:

I thought I could use my solar panel to heat a drum full of water. I would put a hole near the bottom for the element and another at the top for a pressure release valve. My cabin is off grid and fully insulated with thermal windows, so a small, constant supply of heat might do the trick. I have all the components to set this up for the winter, and I like the passive approach - no pump.

So i wouldn't completely rely on this system. I know where i am some times we do not get any usable sun for days. Sometimes we can get a few hours of sun per week.

I also am somewhat worried about the galvanized tank off gassing the galvanization process from being heated. Maybe with the water it will not reach the temperature for that however.


In all honesty if it is easy and does not require retrofitting your house. I would give it a go.


How are you going to control the extra power from the 350w panel? Maybe you are going to use a battery and charge controller?

I believe if you put a 350w solar panel to a 150w element it wouldn't be good. Why not get a 300w element?


Where we are. We have a residential water tank for our hot water. Its most likely 30 gallons. When the sun is out we can heat it with 1 of the elements in the tank. It takes about 3 hours to get the water to 140* Maybe longer if the water tank hasn't been heated at all. The element draws about 1000 watts.


I suspect if you are heating the tank up and the tank is than radiating out to the room 150w might not be enough due to the tank giving off most of its heat.


Are you going to have a water line plumbed to the tank? or just take the top off and fill it up weekly?

If you could use all 350 watts, and you have a lot of sunny weather, then you might be able to get 35 gallons of water warm. You would get a lot more gain by just letting some of the sunlight inside - could you add an extra window to your cabin?

If the climate is often cloudy, then forget it. Your your water will not heat. If you wire a 150w rated load to a 350w panel, you will simply burn it out. I suspect that the idea with the water is to create some sort of thermal storage? In that case, it is likely much too small to make a meaningful contribution to heating overnight, unless you sleep next to it. You can get wire wound resistors that can handle up to 1000 watts which will heat the air in the cabin, but again, if the sun is shining, you are going to be much better off using the sunlight directly - solar panels are only about 15 - 20% efficient.

It would be helpful to know the general climate you're operating in.

I don't think your PV solar panel will generate anywhere near enough heat to make a difference.

You may have better luck with a big thermal solar panel, using the PV panel to pump water/glycol through it.

Thanks for all the replies. The cabin is well insulated, and I have a charge controller and a battery, but I agree I should try a 300w element.

I'm in zone 4b

Assuming your panel and battery can power the heating element 24 hours per day: 300W = 1023 BTU. That's not a lot.

A Mr. Heater Little Buddy puts out 3800 BTU. A plug-in 110V 1500W heater puts out just over 5000 BTU. A Portable Buddy puts out up to 9000 BTU. Even my antique Coleman backpacking stove puts out 8000 BTU.

It's a neat idea, but I don't think you have nearly enough energy input to heat your space.

I have an insulated 5 gallon bucket with 3 x 25 watt elements in it.  It will keep 3 gallons of water from freezing down to about 20 deg f.  In the winter 40 deg f water feels mighty cold.  From my tinkering it would take a couple thousand watts to keep an uninsulated drum in a house above a temperature that feels warm.  To me it only feels warm 85 deg f and up.  I like your idea, and I keep trying different ways to do this too.  I like to do this sort of thing when I have a surplus of sunshine.

A human body releases between 300-400 BTUs an hour.  It changes dependent on calories and activity of course.

I think I would just get a couple of dogs.  They still create heat on cloudy days and they are great company :)

I think there are some good reasons to get a little heat out of a PV panel. Sure a person could hook up a mini split heat pump and make four times as much heat, but then it would take quite a bit more money than some solar panels and a space heater.  

One 35o watt panel is just not much heat, but maybe four panels and 15 dollar space heater from walmart. Start the system up late in the summer when there is more sunshine and blow the heat into a rock pile under the cabin.  Slip a cover off the rock pile in October to let the heat rise.  3.41 BTUs a Watt times maybe 1000 watts times four hours a day times seven days starts to add up to some heat(95,000BTUs)

Maybe don't think of it as a reliable heat source though but that many less logs to split and that much less smoke to breath. Might as well just store the heat in living space objects if its not too much heat.

Solar electric panels typically have an efficiency of well under 50%, meaning that you can get a lot more heat from the sun if you use the sun's light and heat directly rather than through a PV panel. https://en.wikipedia.org/wiki/Solar_cell_efficiency

Thanks for the helpful comments.  My experiment will be to heat my 3" (insulated with recycled styrofoam and vapor barrier) 12x16 cabin with different kinds of  thermal mass. I am seeing more potential with filling the crawl space with rocks and ducting and connecting a rocket stove to it. Because my cabin is between 1' and 3' above grade I have realized i have to close up and insulate underneath and around the periphery if I want to have a radiant floor above, which would be half a yard or 32 bags of concrete to make a 1" floor. And if that crawl space was filled with thermal mass waiting to be heated by fire during the day it would accumulate enough heat to get through the night with the assistance of the floor which could be heated by different methods. My solar panel dump would keep the floor above freezing while I was away and then I would be able to switch it over to a copper coil on a rocket stove when there.

The amount of power a solar panel generates usually depends on the amount of sunshine absorbed. For example, if two houses situated in different regions require the same amount of power, the house with the sunniest weather will require fewer solar panels.

This is a neat idea.
Others have covered the potential issues better than I.
Since you are planning to circulate water anyway, consider a solar thermal collector.
Maybe store the heated water in old water heaters, since they are already plumbed and insulated.
I live surrounded by trees and buildings,  but if I did not, solar thermal collectors would be my go to.

Check out builditsolar.com for some amazing solar power and conservation ideas.

One more thing.
Since you have a complete cabin(congratulations by the way, a great accomplishment! ), you could surround/cover it in a hoop house.
This could increase your living and working space, and act as a sunspace for capturing heat.
During summer, you could switch to shade cloth and/or rely on plants to create shade.
There are a lot of potential advantages, even if the hoophouse was utterly disconnected from the cabin, it could store firewood quite well.