Consider a situation in which I live in a fairly cold climate, and I like my house kept at, say, 17 degrees.

I have a heating system in my house that produces more or less heat depending on the current temperature in the house.

In this situation - is the 'waste heat' given off my things like computer monitors, ovens, power supplies and the like actually wasted? Part of me wants to believe that it's all (in a small way) heating the house - or is this a massive oversimplification...

  • Edits to streamline my prose very much welcomed!
    – Joe
    May 6 '13 at 18:39
  • 1
    Please do be careful keeping your house that cool: it can create problems with condensation and mould.
    – 410 gone
    May 6 '13 at 19:29
  • How does keeping a house a few degrees cooler than "room temperature" lead to condensation and mold? Especially in a cold climate where the air will typically be quite dry. I've kept a house that's unoccupied during the week at 13C with no apparent ill effects.
    – Johnny
    May 6 '13 at 20:08
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    @Johnny yes, unoccupied should be lower risk, because the internal humidity comes from activities while the house is occupied: breathing, washing, cooking.
    – 410 gone
    May 15 '13 at 6:51

It's true that all waste heat goes into heating the house, but whether or not waste heat from a lamp or appliance provides efficient and "free" heat depends on what kind of heat source your heat your home with (so turning off the appliance when not in-use or buying a more energy efficient appliance could be better overall). If you have electrical resistance heating, then your computer or a lamp is going to give you nearly identical heating efficiency. A tiny amount of energy will leave your house through magnetic and/or RF fields generated by the appliance as well as light leaving through a window, but this amount is negligible.

But if you have a heat source that is more efficient or cost effective than electric resistance heating, like Natural Gas or an electric heat pump, then your "waste heat" will be less efficient at heating your house than your regular heater.

Also, the placement of the heat source matters too -- while a freestanding light inside your room will deliver nearly all of its energy as heat, a ceiling lamp will conduct some of the heat up out of the room through the ceiling.

  • 4
    While this is all true, appliances that you would use regardless of heat generation provide "free" heat because you're already buying the electricity for a purpose other than heat. May 6 '13 at 19:20
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    Ahh yes, this is true - I was thinking more of the case of someone saying "Well, I don't need to turn off the computer when I'm done with it because it's heating my house"
    – Johnny
    May 6 '13 at 19:23
  • For that case, I fully agree with your answer. May 6 '13 at 19:28
  • @Johnny in fact, if you are running an electric resistive heater, you may as well let it mine cryptocurrency (other than bitcoin)
    – user253751
    Jul 28 '21 at 10:36

No, it is not true if you have a themostat, then there is no such thing as 'waste heat'. If you have a thermostat, there can still be such a thing as waste heat.


The positioning and timing of heaters around the house typically isn't random: heating devices are typically sited in particular places, facing particular directions, for a good reason: we want heat to be some place in particular.

A fridge and freezer will typically make one corner of your kitchen quite warm. Now, depending on air circulation and the surfaces behind it, much of that heat might be going out through an external wall. The same goes for the rest of waste heat: an incandescent lamp in the centre of a ceiling might only result in a warm centre of your ceiling that's no use at all.


Heating systems usually have time controllers on them, because it's rare to want heating on 24 hours a day, every day of the year. Waste heat from other appliances may be there when you don't want it, and may have leaked away by the time you did.

Increased cooling

And anything that dumps heat year-round, in climates and buildings where part of the year is very hot, may be adding to the overheating.


When you're trying to heat your living space, any heat added to that space is useful. If you would be using those appliances anyway, you are essentially getting a free heat boost. You should keep in mind that devices located far from the thermostat will heat rooms above the set temperature, while heat-producing devices near the thermostat will tend to reduce the amount of heat it tells your heating system to generate.

However, I would not recommend using appliances solely for the purpose of generating heat--that would be wasteful.


In Alberta my electricity costs me about 15 c per kWh. That 3.6 megajoules.

A gigajoule of electricity then is 270 kWh or about $40.

Natural gas right now costs $4/gigajoule.

So, no I don't like to use my computer to heat my house.

That said, you are correct in essence. The electrical byproduct heat is not waste. But its an expensive way to heat your house.

  • Wow. You guys do have cheap natural gas. I pay about 10c per kWh and $10/GJ for gas, but from a cost standpoint, that still doesn't make electric heat competitive :)
    – Nate
    May 25 '13 at 9:26

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