My living room gets plenty of sun from big, west facing windows. As the cold weather is approaching, I wanted to make a device that leveraged this exposure to sunlight to store some energy and slowly release it through the night.

The initial idea was to have a solar panel, battery for electrical energy buffer, some sort of aquarium heater, and a water tank.

Water would be heated by the aquarium heater, which would be powered by the solar panel and battery through the night (as long as the battery lasts). This setup would slowly release the water's heat to the living room.

Will this be more efficient than having just a water tank painted black next to the window? The sun would heat it pretty well during the day, and the solar panel+battery+heater might not be much more efficient, if at all.

  • 1
    Trying to capture solar energy with photovoltaic panels and storing it in batteries will be so much less efficient than directly storing the energy in water. I'd think more along the lines of a solar water heater and then coupling the tank to a radiator.
    – brhans
    Sep 18, 2017 at 11:55
  • @brhans but would that be reasonably compact and self-contained? Sep 18, 2017 at 12:52
  • The amount of energy you'd store with PV in a small self-contained unit will be negligible. You would need a huge PV array and battery bank to store enough energy to power any sort of heater.
    – Steve
    Sep 18, 2017 at 15:47
  • a tank has little surface area in which to absorb heat, no matter the color. it cannot be aimed to track either. electricity gets converted to heat at 100% efficiency, and PV cells can be easier to optimally place. all in all, it's a close call that depends on specifics instead of generalizations. Solar hot water would be much cheaper, but both could be (in theory) feasible.
    – dandavis
    Sep 18, 2017 at 20:31
  • @Steve: storing the energy thermally, no batteries or lossy DC conversion/controls needed...
    – dandavis
    Sep 18, 2017 at 20:33

1 Answer 1


Any dark colored mass would become a passive solar heater for when the sun goes away and the room begins to cool. The mass could be as simple as concrete blocks, tiles or as costly as eutectic tube solutions. Years ago,I was interested in solar systems (I was years ahead in my imagination and thinking), and these were common ideas that were studied and written about. Before you give your brain an overload go to the local book store or library and read and study books on passive solar ideas. You will be amazed at the ideas people had to gather solar energy and use it at a later time. "Hope this helps".

  • When writing the question I started to think about these solutions..are there any specific web spots you can recommend? Sep 19, 2017 at 16:19
  • "google" passive solar heating; www.WBDG.org; energy.gov; www.yourhome.gov-- just to name a few.
    – d.george
    Sep 19, 2017 at 23:26
  • While you are at it, buy yourself the "Mother Earth News" DVD with their last 40 years of issues. Search for solar projects and read them. To make this worthwhile you will need a lot of panels and a lot of water depending on the size of your house. An aquarium heater is not going to get it. Check the wattage on one and convert that to BTU's and see what a small dent you will make in your heating load. A standard gas furnace puts out about 100,000 BTU's per hour. If you just want to warm some fish that might work.
    – ArchonOSX
    Sep 19, 2017 at 23:56
  • Many years ago, I designed an air to air switching to air to water heat-pump system. The water would be used at night, be allowed to freeze,(144btu/lb. in conversion) and be reheated by solar collectors the next day. Geo-thermal systems shot that idea out the water. Radical thoughts can become the start of a better idea at a future time. The geo system is so much better and much more simple than my idea. Use your brain, try things, fail a few times and you may discover a great way enhance your life.
    – d.george
    Sep 20, 2017 at 11:04
  • You'd want water, not some heavier object. You'd think a hyper-dense slug of metallic uranium would be the most heat storage per cubic foot. Actually, uranium is terrible compared to water. It's because of what heat is. The energetic excitement of atoms. Heat storage is loosely a function of the number of atoms present! So a uranium atom has no particular advantage over a hydrogen atom, and water is the ideal heat storage because it fixes a lot of hydrogen in a dense, pumpable liquid. Sep 11, 2019 at 3:51

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.