I was wondering as it's getting cold : is it better for my electricity bill to shut down completely my (electric) heater during day, and to turn it on again when I come home (then it will have to heat the room from something like 5°C to around 20°C), or should I keep the temperature around, say, 15°C while I'm away ?
I would naively guess it's better off, but I'm also wondering about inertia of the walls for instance...
Answer
This problem is very simple, but it's easy to overcomplicate by looking at too small a scale. At every second – no matter what the heater does – you waste money by heating the outside of your house. The rate of heating – and thus the rate at which you waste money – is given by Newton's Law of Cooling. So
$$ \text{Wasted money} \propto \int (T_\text{in}-T_\text{out})\;dt $$
The lower your house's temperature, the less money you waste – no matter what. So set the thermostat to the lowest practical temperature when you're away.
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