# What's the most efficient way to heat water?

How did you go about heating the water for this morning's shower? Did you leave the boiler on all night or just heat what you needed? Find out which is the most energy efficient in this Question of the Week! Plus, we ask why people get dark circles around their eyes.

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### 00:00 - What is the most efficient way to heat water?

### What is the most efficient way to heat water?

We put this to Hugh Hunt, from Cambridge University's Engineering Department...

Hugh - This is a very interesting and topical question. If the hot water is stored in a perfectly insulated tank, then it makes no difference whether the water is heated one hour before it's needed or one day before, or even one year before. There's nothing at all to be gained from heating the water up 22 hours in advance, but that requires that the insulation is perfect.

In reality, insulation is not perfect and the water heater will lose some heat in advance and this heat cannot be recovered. It's the same as when we're boiling water for a cup of tea. If we boil the water when we need it, that makes sense because the kettle is not that well-insulated. If you boil the water an hour in advance, the water would be cold when we needed it.

How much energy wastage are we talking about? Suppose a 5-minute shower uses 60 litres of water, which is about right; and suppose the water is delivered at about 40 degrees, which is about right; and in the winter the water is cold, somewhere around 0, which is about right. So we're talking about 4 kilojoules/degree/litre of water.

Now suppose the tank loses 10% of its heat in 24 hours. Well then that's a loss of getting up to 1 Mega Joule of heat. That's equivalent to running an electric kettle for 6 minutes, enough to boil the water for several cups of tea. So if you're the kind of person who's bothered not to over-fill the kettle when making a cup of tea then you should certainly follow our Croatian friend's example and only heat the shower water when it's needed.

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