What is the most efficient way to heat water?

Question: I'm in Croatia and I'm having a debate with my friend about water heaters. I have a water heater in my bathroom. I take on shower per day. I set it at just the right level so that it gives me just enough hot water for a 5-minute shower, no more. I turn on the boiler about 1-hour before I want to take a shower and then turn it off right when I get into the shower. I don't turn it on until the next day, where I follow the same ritual. My friend says I'm being inefficient and not saving any energy. He says I should leave the boiler on 24-hours a day, because it takes a minimal amount of energy to keep the water hot once it's hot. If I turn it off, the water cools and then must be re-heated from scratch, requiring far more energy than if it had been on all along. Who's right? Francis Tapon

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 itís 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. Itís the same as when weíre 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 weíre 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 whoís bothered not to over-fill the kettle when making a cup of tea then you should certainly follow our Croatian friendís example and only heat the shower water when itís needed.