How much does an immersion heater cost to run?

An immersion heater is a 3 kilowatt element in your hot water cylinder, so it uses 3 units for every hour it's on. How long it runs depends on how much hot water you use and how well the cylinder is insulated. On a time-of-use tariff, heating the tank in the cheap hours makes a real difference.

How we work it out

A typical immersion heater uses about 3kWh for 3kw element for an hour, taking around 1 hour. Electricity on a standard variable tariff costs about 26p a unit across Great Britain at the moment (it varies a little by region), so one run comes to about 79p. Your own model may use more or less: the energy label, or a plug-in energy monitor, will tell you.

If you're on a tariff where the price changes through the day, such as Agile Octopus, Economy 7 or an electric car tariff, the same run can cost much less at the right time. The Cheap Watts tool finds that time for you on your own tariff.

How to spend less

• Put it on a timer so it heats the water you need, when it's cheapest, rather than keeping a tank hot all day.
• Check the cylinder has a thick jacket or foam insulation. A poorly insulated tank loses heat all day.
• Set the thermostat to 60°C: hot enough to be safe, not so hot it wastes energy.
• If you have gas central heating, check whether the boiler heats the cylinder more cheaply.