I am a software engineer and have hobbyist understanding about electronics, so this is something completely out of my scope. Just need to understand if my idea of thermal storage is practical at all.

So, I have a basement in my apartment building and I have renovated and insulated it. But of course it is not heated. Winters here get quite cold. I'v made the basement nice enough to be my home office and workshop. Temperature in there is not comfortable, in winter 10-15°C so of course I need to heat it electrically.

With current electrical prices it is quite expensive. But I was thinking could I use electricity at night when it is 2-3x (tonight it is 30x) cheaper, to store the heat?

I have seen people store solar heat in water barrels so as a layman it makes perfect sense to me. Of course I understand that I probably don't even have the means to understand how wrong I am.

So to get more concrete my question is - would it make economical/practical sense to heat 200-400L of water during the night to redistribute heat back into my 60m3 basement. Instead of heating it with a regular electric heater during the day.

Some downsides I can see:

  1. I can heat the basement more precisely with regular heater, just when I need it and as long as I need it.
  2. Need to insulate the storage device so it won't redistribute heat when I don't need it.
  • 2
    $\begingroup$ You may want to heat stones instead, they dont burst if you freeze suddenly. But really are you allready using a heat pump? $\endgroup$
    – joojaa
    Nov 1, 2021 at 15:30
  • $\begingroup$ There is no chance of basement freezing. The house is heated so it never goes below 10C. $\endgroup$ Nov 1, 2021 at 15:57
  • $\begingroup$ @somerandomusername I'm sure everyone in Texas thought similarly too... Always consider the possibility of being without power for extended periods. $\endgroup$
    – tylisirn
    Nov 2, 2021 at 5:09
  • $\begingroup$ Yes of course it is possible. Thou nothing I would worry about. But I am looking into sand/stone more than water. Will take up less space. $\endgroup$ Nov 2, 2021 at 7:44

4 Answers 4


Although as mentioned from other a heat pump is probably more efficient, the following idea might be compatible with what you are trying to achieve.

In the UK I used to live in an apartment that used an electrical installation that operated in the evenings and heat up stones. Then during the day, you would adjust the rpm setting of fans that were inside the boxes. The forced air increased convection and also increased circulation into the room.

This is probably, something you might still find as a product if you look around.

One more thing that you could do, is that you could improve on insulation in the room you are working. This way any heat will take longer to move out. The only problem with that if you do insulation internally, you might have problems with moisture and mould.

  • $\begingroup$ Thank you for the input. I’m looking for something I could DIY. Both cost and the love for learning and building is a factor here. I still want someone to point out why storing heat in water is a bad idea. I know that people store heat from solar energy in this manner. In theory it seems “simple” - put a heating element in a barrel with a sensor, insulate it and pump the water to radiator when needed. $\endgroup$ Nov 1, 2021 at 16:31
  • $\begingroup$ Stones hold more heat so it helps with form factor. But it feels harder to heat them up in a safe way and “pumping” heat out seems bit more complex $\endgroup$ Nov 1, 2021 at 16:33
  • $\begingroup$ @somerandomusername stone has a much higher density than water an can retain much more heat. They can be hotter too. Takes longer for them to cool. My sauna takes about 4-5 hours to cool down once heated. And it dont have all that good insulation and certainly not 200 liters of stone. Stone fireplaces have been used for ages for this. $\endgroup$
    – joojaa
    Nov 1, 2021 at 16:41
  • $\begingroup$ I certanly will look into that. Need to find how to heat them up. Heating element in water seams easy, not sure how to heat up stones with electricity. $\endgroup$ Nov 1, 2021 at 17:00
  • $\begingroup$ You sometimes see them here still in New Zealand. There are called 'night store heaters' or 'night storage heaters'. If you google that, you'll find more information. Wikipedia has something as well: en.wikipedia.org/wiki/Storage_heater $\endgroup$
    – NZD
    Nov 2, 2021 at 0:43

Heat pumps are more efficient than electrical heaters by a factor of approximately two. That is an alternative.

Regulating the heat flow from a warm water tank and keeping it steady can be a big job of insulation and circulation.

Maybe the optimal solution is to heat up the floor and lower walls during the night by floorboard electrical heaters and during the day use, as needed, a small electrical heater to supplement that.


One way that you could improve your idea is to put the water tank, or several smaller tanks to be easier to place, in a place near the ceiling at the upper level. This will give you a higher ambient temperature to start with.

  • $\begingroup$ Yes I understand that to do it properly is a huge challange. I was thinking more of heating a insulated barrel to 50-60C. And circulating water to a radiator with a small pump during the day. Doesn’t need to be perfect. Just that often electricity is almost free during the night. For example tonight it will be 0.003 EUR per kWh compared to 0.13 EUR during the day $\endgroup$ Nov 1, 2021 at 16:04
  • 1
    $\begingroup$ Then you need to calculate your basement's thermal insulation and that will give you the heat you need during the day. I can suggest a bit larger tank and intelligent use of your desk by placing the radiator under the desk and hanging drapes to channel the direction of warm air to the area you frequent. your PC is a good heat source too. $\endgroup$
    – kamran
    Nov 1, 2021 at 16:51

The problem with using water for thermal storage is that the heat flow is proportional to the difference between water temperature and the room temperature (&Delta T) but this will be decreasing as the water cools down. That makes the heating good initially but poor later.

A better way to do it is to use a "phase-change" material instead. In phase change systems the energy is put in to change the material's phase from solid to liquid and when heat is retrieved the material changes from liquid to solid.

enter image description here

Figure 1. Phase changes of water. Image source: Hyper-Physics.

To understand this better consider how heating ice at a constant rate will raise the temperature until it reaches 0°C. Then the temperature will remain constant until all the ice has changed from solid phase to liquid phase. After that the temperature will start to climb again. A similar process happens in the liquid to gas phase change.

You want a material that has a phase change temperature in the 30 to 50°C range to give a nice steady temperature over a wide range of "charge". If you found one with, for example, a phase-change temperature of 33°C then you would consider it "fully charged" at 34°C and "fully discharged" at 32°C (although you could continue to add or withdraw energy as in the water-tank system but you'd lose the constant temperature advantages.

See if you can make any sense of phase-change-materials-thermal-energy-storage.

  • $\begingroup$ while this is an efficient way to store heat, it certainly isn't the simplest or cheapest. Water and storage for it is as cheap as anything. $\endgroup$
    – Tiger Guy
    Nov 1, 2021 at 21:00
  • $\begingroup$ Thank you. This really makes quite a lot of sense. Thou this really is something out of my scope to create. Now my newest direction is to look into heating stones or it seems that sand is used as well. $\endgroup$ Nov 1, 2021 at 21:13
  • 1
    $\begingroup$ Start with insulation! $\endgroup$
    – Transistor
    Nov 1, 2021 at 21:53

Your idea is sound.

Water is commonly used for such things since it's cheap and has a great heat capacity. But you might find that depending on how fast your room cools down that all that you have to do is warm the room during the night and let it cool down during the day. Furniture, air, walls, etc. all hold heat just like water would, though maybe not as much per kg. Test it out by heating it during the night and turning off the heater during the day.


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