Hot water

Immersion heater overnight: cost per kWh moved and how to time it

Last checked: 15 October 2026 · PlugLater

An immersion heater is an electric element inside a hot water cylinder. Every kWh it uses in a cheap night window instead of at the price cap saves 26.32p − 7.6p = 18.72p.

Unit rates only
Energy	At 7.6p (night)	At 26.32p (cap)	Saved
1 kWh	7.60p	26.32p	18.72p
2.0 kWh (calculator example)	15.2p	52.64p	37.44p
3 kW element for 1 hour (3 kWh)	22.8p	79.0p	56.2p
3 kW element for 2 hours (6 kWh)	45.6p	157.9p	112.3p

The homepage timer-shift calculator uses 2.0 kWh per immersion run. The element’s power rating is usually on a label on the element or its switch; many are 3 kW. The thermostat switches it off once the water is hot, so a 3 kW element running for two hours uses up to 6 kWh, and less if the cylinder is already warm.

On Intelligent Octopus Go the day rate is a regional quote (34.95p–38.82p on the fixed version on sale on 15 October 2026), so moving a kWh from day to night on that tariff saves 27.35p–31.22p.

How to time it

Citizens Advice suggests setting the main immersion’s timer for off-peak hours, for an hour or two before the off-peak period ends, which should give enough hot water for the day.
On a 23:30–05:30 window that means something like 03:30–05:30. On Economy 7 the hours depend on your meter, so check your bill.
Make sure the switch is on the timer setting (often a clock symbol), not permanently on. Citizens Advice notes the separate “boost” element uses peak-rate electricity on a time-of-use tariff.
Set the cylinder thermostat as your installer or manufacturer recommends. Don’t turn it down to save money without checking, because stored hot water needs to be hot enough to control bacteria.

Don’t use a plug-in smart plug

A hard-wired immersion heater has no plug, so a plug-in smart plug is not an option, and the wiring should not be altered to fit one. If the immersion has no timer, or the timer is broken, have a registered electrician fit an immersion timer. This page has no smart plug or timer product links for that reason.

Keep the heat in

The Energy Saving Trust says a cylinder jacket should be at least 80 mm thick, and that a British Standard jacket costs about £30. Insulating the cylinder and nearby pipes means water heated overnight stays hot for longer, so you need less boost in the day.

Shifting other loads: dishwasher and washing machine, smart thermostat schedules. Tariffs with a cheap window: UK EV tariffs compared and Economy 7 vs an EV tariff.

Questions people ask

How much does it cost to run an immersion heater overnight?

Each kWh costs 7.6p on a 7.6p night rate, against 26.32p at the price cap. A 3 kW element running for an hour uses 3 kWh: 22.8p at night or 79.0p at 26.32p.

When should I set my immersion timer?

Citizens Advice suggests an hour or two before the end of your off-peak period. On a 23:30–05:30 window that is around 03:30–05:30.

Can I put my immersion heater on a smart plug?

Not if it is hard-wired, which most are. Have a registered electrician fit an immersion timer instead.

Is this financial advice?

No. It is arithmetic from published rates. Check your element rating and your tariff’s hours.

Not financial advice. These are worked examples from published rates and EV-Database consumption estimates, for planning only. Your own tariff, day rate, standing charge, car, driving and weather will change the numbers. Check your bill and your supplier’s current terms before you switch or buy anything.

Sources

This page has no affiliate links. The calculator on the homepage is free and runs in your browser.