Definition: An immersion heater is an electric element — usually rated 3 kW — fitted inside a hot water cylinder, heating water directly like a kettle. Most homes with a cylinder have one as a backup; some rely on it as their main water heater.
What it costs to run
Heating a 150-litre cylinder from 15°C to 60°C takes about 7.9 kWh. At the October–December 2026 cap rate of 26.32p/kWh that costs around £2.05 — versus roughly 64p for the same hot water from a 90%-efficient gas boiler at 7.97p/kWh. Peak-rate immersion heating is therefore about three times dearer than gas; off-peak tariffs such as Economy 7 narrow the gap considerably.
Two cheap wins if you use one regularly: fit a timer so it heats only what you need, when you need it, and make sure the cylinder wears a proper insulating jacket — heat lost from a bare tank is money down the drain. The immersion’s own thermostat should be set to 60°C: hot enough to kill legionella bacteria without wasting energy.
When an immersion earns its keep
Three cases stand out: as a backup when the boiler fails; charged overnight on a cheap time-of-use tariff; and — best of all — fed free surplus electricity from solar panels via a solar diverter (see our guide to solar diverters and free hot water). Heat pump cylinders also use the immersion for a weekly anti-legionella cycle, briefly heating the stored water to 60°C.
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