How much does it cost to heat water with electricity?

A 150-litre cylinder heated from 10°C to 60°C uses about 8.7 kWh, roughly £2.30 at the 26.32p/kWh price cap. A 10-litre under-sink heater warmed from 10°C to 55°C uses about 0.5 kWh (14p).

Can an electric water heater plug into a normal socket?

Only units of 3 kW or less, such as small under-sink heaters. Larger instantaneous heaters and electric showers need their own circuit installed by an electrician.

Is an immersion heater expensive to run?

It costs the same per kWh as any electric heater. It becomes expensive when left on all day on a single-rate tariff. A timer and a well-insulated cylinder keep the cost down.

What size electric water heater do I need?

For one sink, a 5–15 litre under-sink unit. For a whole home, a cylinder sized to the household: larger families need a bigger cylinder or faster recovery.

Electric water heaters for UK homes: types and running costs

By Dale @ ukaircontracker · facts checked 7 November 2026

Quick answer

Electric water heaters come in two kinds: storage water heaters that keep a tank hot (immersion cylinders and under-sink units) and instantaneous heaters that heat water as it flows. Heating water takes 0.00116 kWh per litre per °C, so filling a 150-litre cylinder from 10°C to 60°C uses about 8.7 kWh, roughly £2.30 at the 26.32p/kWh price cap.

Electric water heater types
Type	Typical power	Storage	Best for
Immersion heater in a cylinder	3 kW	Usually 90 – 210 litres	Whole-house hot water, often on an off-peak tariff
Unvented (mains-pressure) cylinder	3 kW element(s)	Usually 120 – 300 litres	Strong showers without a cold tank in the loft
Under-sink storage	Up to 3 kW	5 – 15 litres	One sink far from the boiler
Instantaneous (inline)	3 – 11 kW	None	A sink or shower that needs hot water on demand
Electric shower	7.5 – 10.8 kW	None	Showers without a hot water tank

Only units of 3 kW or less can run from a 13 A plug; they need a dedicated circuit from the consumer unit.

What it costs to heat the water

The energy needed is set by physics, not by the heater: about 0.00116 kWh per litre for each degree of rise. A 10-litre under-sink heater warmed from 10°C to 55°C uses 0.5 kWh (14p); a full 150-litre cylinder from 10°C to 60°C uses 8.7 kWh (£2.30). Every electric water heater turns electricity into heat at close to 100% efficiency, so the bill depends on how much hot water you use and how much heat the tank loses while it stands.

On a time-of-use tariff such as Economy 7, heating the cylinder overnight at the cheaper rate lowers the cost per litre. A heat pump water heater uses much less electricity for the same hot water, but costs more to buy.

Storage or instantaneous?

Storage heaters give a strong flow to several taps at once and can use cheap night electricity, but a tank loses some heat all day. Instantaneous heaters lose nothing standing, but their flow is limited by power: a heater can only raise so many litres a minute by so many degrees.

FAQ

Common questions

How much does it cost to heat water with electricity?: A 150-litre cylinder heated from 10°C to 60°C uses about 8.7 kWh, roughly £2.30 at the 26.32p/kWh price cap. A 10-litre under-sink heater warmed from 10°C to 55°C uses about 0.5 kWh (14p).
Can an electric water heater plug into a normal socket?: Only units of 3 kW or less, such as small under-sink heaters. Larger instantaneous heaters and electric showers need their own circuit installed by an electrician.
Is an immersion heater expensive to run?: It costs the same per kWh as any electric heater. It becomes expensive when left on all day on a single-rate tariff. A timer and a well-insulated cylinder keep the cost down.
What size electric water heater do I need?: For one sink, a 5–15 litre under-sink unit. For a whole home, a cylinder sized to the household: larger families need a bigger cylinder or faster recovery.