You lock up the house, head off for a fortnight in the sun, and one appliance carries on exactly as if you were still at home: the hot water tank. Even with nobody turning a tap, an immersion heater keeps a full cylinder hot around the clock, paying to reheat water that no one will use.
Why it keeps heating an empty house
A hot water cylinder holds a tank of water at whatever temperature its thermostat is set to. As heat slowly escapes through the cylinder walls, the element switches back on to bring it up to temperature again. It is quiet, but it never stops, and standing losses like these account for a large share of what water heating costs over a year. In an empty home, every one of those kilowatt-hours does nothing but replace heat leaking out of a tank no one is drawing from.
What two or three weeks away really costs
The numbers make the point. A 200-litre cylinder uses roughly 1.8 kWh a day just standing there; a smaller 100-litre tank sits closer to 1.3 kWh, and a large 300-litre one nearer 2.4 kWh. Over three weeks, that 200-litre tank quietly gets through almost 38 kWh. At around 26p per kWh under the current electricity price cap, that is close to £10 spent heating water you never touched. It is not a fortune on its own, but it is money wasted on every single trip.
How to switch it off before you go
Turning it off takes moments. If your hot water runs on an immersion heater, simply switch it off at the wall or at its dedicated fuse spur, and leave it off for the whole trip. If your cylinder is heated by a timer or programmer, set the hot water to "off" rather than to a schedule. Homes with a combi boiler have nothing to do here, as water is heated on demand, but it is still worth turning the heating off at the programmer. While you are at it, switching off other devices left on standby is an easy win, and our guide to saving electricity at home covers the rest of the house.
The one thing to do before your first shower back
Switching it back on comes with a simple health rule. Warm, still water can let legionella bacteria grow, and they thrive between about 25 and 45 °C. That is why the cylinder thermostat should sit at a safe 60 °C, and why you should turn the hot water back on several hours before you need it and let the tank reach full temperature, which kills off any bacteria that may have built up. Run the hot taps for a few minutes afterwards and the system is ready to use.
How many days away make it worthwhile
The rule of thumb is straightforward: from about a week away, switching the hot water off is always worth it. The small amount of energy needed to reheat the tank when you get back is far less than days on end spent keeping an untouched cylinder hot. For a weekend it barely moves the dial, and losing instant hot water is not worth it; but for a summer holiday lasting weeks, it is one of the easiest savings you can make on your way out of the door.