When I lived in France and the US, we can an electric boiler for hot water. It was "always on", probably with a thermostat.

Now I live in an area where most people have a power switch that they turn on 30 minutes before taking a shower, and shut down after their shower, to save energy.

At first I thought that was a good idea. But it got me thinking: does it really matter?

I have no test to backup the theory but, heating the entire water tank once a day, how does it compare to KEEPING it hot with a thermostat??

  • 1
    This is what tankless water heaters are for. Only heat the water you use.
    – Tester101
    Apr 16, 2014 at 14:05
  • how big is the tank? if this is something like a 50 or 60 liters, heating only when you need it makes sense. if you're talking of 40 gallons or thereabouts, it may help to keep it hot.
    – alt
    Apr 16, 2014 at 19:56

4 Answers 4


You will always (always!) save energy by turning the heater off when you don't need hot water.

How much energy you save depends on the efficiency of the water heater but you will always save energy - it doesn't matter what size your tank is or how well is is insulated.

The ultimate reason for this is "the rate of heat loss of a body is proportional to the temperature difference between the body and its surroundings." (See http://en.wikipedia.org/wiki/Heat_transfer#Convection-cooling)

  • You are correct of course, but I don't feel you adequately explained the reasoning. This is very similar to confusion over energy saved in heating a house by turning down the thermostat when the house is empty against how much energy is needed to bring the house back up to temperature. That reasoning is fallacious. It doesn't matter what energy is needed to raise the temperature. All that matters is how much energy is needed to replace heat lost, and as you point out, there is less heat lost when the temperature difference is less.
    – bcworkz
    Apr 23, 2014 at 16:27
  • Good point - I tried to come up with a clear, simple explanation but really struggled.
    – Alex
    Apr 24, 2014 at 2:54

This is probably along the same sort of idea as turning off the A/C via the breaker in the winter. A dose of paranoia (what if the heater stays on all day because the thermostat is broken?) and a little bit of sense.

Assuming that your water heater is not a perfect insulator, you are probably losing some money by keeping the water heater at a good 'temperature' between 10pm and 6am, but there's the tradeoff of remembering to turn it on and off afterwards.

I'd love to run the numbers on such an exercise, but I have a feeling that they would be so dependent on individual users - tank insulation, water heater efficiency, etc, that it would be difficult to come up with a for sure answer.


The answer as to how much it costs to keep the water hot 24/7 will depend on a number of factors: size and shape of tank, effectiveness of insulation, temperature of surroundings and of the heated water.

FWIW, my standard 180 litre electric hot water heater has a data plate that claims a standby loss rate of 76 Watts. That doesn't seem to present much opportunity for savings: it's only about 1.8 kW-h a day and you could only save a fraction of that...

  • "only" 1.8 kW per day is a relative measure. in the USA, that's probably around a quarter or less per day, but in lesser-developed countries, that's certainly a fairly decent chunk of change, from a consumption POV.
    – alt
    Apr 16, 2014 at 19:54

When we lived in South Africa, we had a 500Litre boiler that was always on.

But it was insulated by the factory and the theory was that on night tariff 23:00-4:00, at 80% cost, we could heat the water up to 80degrees and before it reaches 22:00 the water is still 50ish degrees celsius. But if we used up the water we would have to manual override it and get it to heat up again, so we just left it on the whole time at 65 degrees. We didn't notice a bit difference in bills since the theory was that the insulations kept the water warm.

In England, they use a tank system with a GAS burner. The water gets heated by pumping water from the tank into the boiler over and over until it gets to a certain temp. We also have this insulated, not as well as the one in South Africa. Its cheaper because its GAS.

New energy efficient, condensing combi boilers, heat water on demand, when you turn the tap on but it takes some time to get to the tap though, so there is some water waste. But you can get inline electric water heaters, like in showers, but now under your sink and heat water right by the tap so you do not have to drain litres before warm water comes down. It think these are the most efficient ways to save on energy, since the water is heated right by the sink, and they are really efficient too!

It all depends where you live. If you live near a Hydro plant and you get electricity for 80% discount, then you can just use electricity to heat everything, day and night, but in England, where energy is the most expensive in the world.. we sometimes prefer a warm Polish Wool blanket!

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