How many watts does an immersion heater use?

By Tom Calder · Figures checked 4 November 2026 · Costs at 26.32p per kWh (Ofgem cap, 1 October to 31 December 2026)

In short

A standard immersion heater element is 3kW: every hour it is heating uses 3 kWh, about 79p at 26.32p per kWh. Its thermostat switches it off once the cylinder is hot, so what you pay depends on how long it actually heats.

A kettle coming to the boil beside a microwave and slow cooker in a warmly lit kitchen on a rainy evening

The numbers

Running watts: 3,000W; Backer SH311A-AA11A1: 3kW at 240V, with a combined thermostat and safety cut-out.
Amps at 230V (most): 12A; From the 240V rating: at 230V the same appliance draws a little less.
Energy an hour while heating: 3 kWh; 3kW for an hour. The cylinder thermostat switches it off once the water is hot, so the hours it actually heats decide the cost.
Cost an hour while heating: 79p
On a 1kWh power station: Hard-wired to the hot water cylinder, so it can’t run from a power station
On a 2kWh power station: Hard-wired to the hot water cylinder, so it can’t run from a power station

Costs use 26.32p per kWh, the Ofgem price cap's average Direct Debit electricity unit rate for 1 October to 31 December 2026. Your own rate is on your bill; on a fixed or time-of-use tariff it will differ.

How it’s used, and what that costs

Backer’s SH311A-AA11A1, a common 11-inch replacement element, is rated 3kW at 240V with a combined thermostat and safety cut-out.

Leaving an immersion on all the time is a common way to spend more than needed. A timer, and heating once a day when you need it, keeps the hours down.

The honest bit

If your hot water is all electric, heating it overnight on a time-of-use tariff can be cheaper than the cap rate used here; our electricity bill guide covers those tariffs.

In a power cut: how long a power station would run it

Hard-wired to the hot water cylinder, so it can’t run from a power station.

To size one for everything you’d want to keep running, use the sizer on our power stations comparison.

Common questions

How many amps does a 3kW immersion heater draw?

Amps are watts divided by volts. Immersion heater (3kW) ratings are given at 240V; at the UK’s nominal 230 volts the same appliance draws a little less power, about 12 amps at full power. It is wired in at the cylinder, not plugged in, so a power station can’t run it.

Smart thermostats →Power stations compared, with a sizer →Home backup power sizer →Running a fridge-freezer on a power station →How to lower your electricity bill →Electric heater safety and running costs →Every appliance’s running cost →

Compare: Electric shower · Kettle

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Sources (6)

Heater Shop: Backer 3kW 240V 11in Incoloy immersion heater SH311A-AA11A1; combined operating and safety cut-out thermostat
Ofgem: Energy price cap unit rates and standing charges, 1 October to 31 December 2026electricity 26.32p per kWh, Direct Debit; no VAT on electricity from 1 October 2026 to 31 March 2027
BLUETTI UK — Elite 200 V2 (runtime formula: DoD 90%, η 85%, self-consumption about 10W)
BLUETTI UK — Elite 100 V2 (self-consumption about 10W)
Jackery UK — runtime formula (capacity × 0.85 ÷ watts)
EcoFlow — test runtime before an emergency0.80–0.90 for AC; DC ports for routers and phones

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