How much does it cost to run an immersion heater per hour in the UK?
A 3 kilowatt immersion heater uses 3 kilowatt hours in an hour, which costs 78.3p at the current Ofgem cap rate of 26.11p per kWh. Heating a 120 litre cylinder from fifteen degrees to sixty takes 6.28 kilowatt hours, so a full tank from cold costs £1.64 and takes a little over two hours.
What an immersion heater actually draws
An immersion heater is a straight resistive element sitting in the water inside your cylinder, and it is one of the few appliances where the plate rating is the whole truth. Nearly every domestic immersion sold in the UK is rated at 3 kilowatts, which the IET confirms in its own guidance on the wiring of these circuits, and the element draws that whole rating for as long as the thermostat is calling for heat.
There is no cycling to allow for and no efficiency loss worth arguing about, since all the energy ends up in the water surrounding the element. What varies is how long it runs, and that comes down to the size of your cylinder, the temperature the water starts at and how well the tank is insulated.
The sum, worked in full
Three kilowatts for one hour is 3 kilowatt hours. Multiply by the cap rate of 26.11 pence and you get 78.3p.
Working out a full tank takes one more step. Lifting 120 litres of water from fifteen degrees to sixty takes 6.28 kilowatt hours of heat, which is £1.64 and a shade over two hours of element time. A 150 litre cylinder takes 7.85 kilowatt hours and costs £2.05, and a big 210 litre tank takes 10.99 kilowatt hours and costs £2.87.
Those tank figures assume the water starts cold at fifteen degrees, which is the worst case. Switch the immersion on again a few hours after a full heat and the water is still warm, so the element only has to make up the difference and the cost falls in proportion. That is the whole reason timers exist on these things.
Work it out for your own immersion heater
Type in the wattage from the plate on the back, then your own unit rate. The numbers update as you type.
How long to leave it on
The honest answer is that an hour is enough for most households and nobody should be running one for four. A single shower takes somewhere around two kilowatt hours of heat out of the tank, a bath at eighty litres takes 2.33 kilowatt hours and costs 61p on the immersion, and a washing up bowl barely registers next to either.
Work backwards from that. Two showers and the washing up is roughly an hour of element time, which is 78.3p. The tank then sits hot and slowly loses heat to the airing cupboard, which is why leaving the switch on all day costs far more than heating twice.
Cost by time and by tank size
| What you are heating | Energy used | Cost | Time at 3 kW |
|---|---|---|---|
| Half an hour of element time | 1.50 kWh | 39.2p | 30 minutes |
| One hour of element time | 3.00 kWh | 78.3p | 60 minutes |
| 120 litre cylinder from cold | 6.28 kWh | £1.64 | 2 hours 6 minutes |
| 150 litre cylinder from cold | 7.85 kWh | £2.05 | 2 hours 37 minutes |
| 210 litre cylinder from cold | 10.99 kWh | £2.87 | 3 hours 40 minutes |
| Appliance | Typical draw | Usage we assume | Per hour (UK) | Per usage (UK) | Per month (UK) | Per hour (US) |
|---|---|---|---|---|---|---|
| Immersion heater | 3000 W | per hour of heating | 78.3p | 78.3p | £23.76 | 55.0 cents |
Rates: Ofgem cap 1 July to 30 September 2026 at 26.11p per kWh (source), EIA residential average June 2026 at 18.34 cents per kWh (source). Checked 2026-09-22.
In the United States a 3 kilowatt element costs 55.0 cents an hour at the EIA residential average of 18.34 cents per kilowatt hour, and $1.15 to heat a 120 litre tank from cold. Most American cylinders are larger and many run on 4.5 kilowatt elements, so the hourly figure goes up while the time to heat comes down.
Five ways to spend less on it
- Heat once and use it, rather than switching on and off through the day. Every reheat pays for the heat the tank lost since the last one.
- Use the boost timer rather than the permanent on switch. An hour of element time will cover showers for a couple of people, and the permanent switch quietly holds a full tank hot all night.
- Check the cylinder jacket. An old tank with a thin jacket loses heat continuously, and the immersion pays for every unit of it.
- Set the cylinder thermostat to sixty degrees. Lower risks legionella in a stored tank, higher costs more and scalds.
- Move to a time of use tariff if the immersion is your main water heating. The element does not care what time it runs, and an overnight rate can cut the figures above sharply.
Immersion heater against the gas boiler
On a straight comparison the boiler wins every time. Gas costs 7.33 pence per kilowatt hour under the same price cap against 26.11 pence for electricity. Heating that 120 litre cylinder needs 6.28 kilowatt hours of heat either way, and even after allowing for a boiler losing roughly fifteen per cent of what it burns, gas does it for about 54p against £1.64 on the immersion.
That is a difference of £1.10 for one tank of water. Heat a tank a day and the immersion costs about £400 a year more than the boiler, which is the real reason the switch on the airing cupboard wall carries so much folklore.
Where the immersion earns its place is as a backup and as a top up. When the boiler fails it keeps the house in hot water, and in a home with solar panels it is the cheapest way to turn spare generation into something useful. In a flat with no gas supply it is not a choice at all, and the sensible response is to heat the tank once on a cheap overnight rate and treat the boost switch as an emergency measure rather than a daily habit.
Common questions
How much does it cost to run a 3kW immersion heater for an hour?
78.3p at the current cap rate of 26.11p per kWh, because a 3 kilowatt element uses exactly 3 kilowatt hours in an hour. Half an hour is 39.2p and two hours is £1.57.
How much does it cost to run an immersion heater per day?
It depends on how much hot water you draw rather than on the clock. An hour a day is £23.84 a month and £285.90 a year. Most households on a working boiler use the immersion for far less than that, and households using it as the only water heating should expect the higher figure.
Is it cheaper to use an immersion heater or a gas boiler?
The boiler, by roughly three to one. Gas sits at 7.33p per kilowatt hour against 26.11p for electricity, so a 120 litre cylinder heated from cold costs about 54p on gas and £1.64 on the immersion, even after allowing for the boiler losing some of the heat up the flue.
Does an immersion heater use a lot of electricity?
Yes, it is one of the largest single loads in a house. Three kilowatts is the same draw as a fan heater on full, and unlike a fan heater it often runs for two or three hours at a time.