Quick answer: a standard 3kW immersion heater costs about 79p an hour while it is heating, at 26.32p per kWh. Heating a typical 120-litre tank from cold takes about 6.3 kWh, roughly £1.65.
Immersion Heater Running Cost Calculator
Results update as you type. Default price: 26.32p/kWh, the Ofgem cap for October to December 2026.
The immersion heater cost formula
Cost = element kW × hours heating × price per kWh
A 3kW element heating for 2 hours a day: 3 × 2 = 6 kWh, and 6 × 26.32p = £1.58 a day, or about £575 over a year if you do it every day. Prices here use 26.32p per kWh, the Ofgem price cap unit rate for electricity paid by direct debit from 1 October to 31 December 2026. Your own rate is on your bill and may be higher or lower, especially on a fixed or Economy 7 tariff, so change it in the calculator.
How much energy does it take to heat a hot water tank?
You can work this out from the size of the tank. Heating 1 litre of water by 1°C takes about 0.001163 kWh, so heating it from about 15°C to 60°C (a 45°C rise) takes about 0.052 kWh per litre.
| Tank size | Energy from cold | Time with a 3kW element | Cost at 26.32p/kWh |
|---|---|---|---|
| 90 litres | 4.7 kWh | 1 h 34 min | £1.24 |
| 120 litres | 6.3 kWh | 2 h 6 min | £1.65 |
| 150 litres | 7.9 kWh | 2 h 37 min | £2.07 |
| 180 litres | 9.4 kWh | 3 h 8 min | £2.48 |
| 210 litres | 11.0 kWh | 3 h 40 min | £2.89 |
These figures assume a fully cold tank and no heat loss. A tank that is still partly hot from the day before needs less, while a poorly insulated tank needs more.
Economy 7 and off-peak heating
If your tariff has cheaper night-time electricity, set the immersion timer so it heats during those hours. To see the saving, run the calculator twice: once with your day rate and once with your night rate. The difference, multiplied over a year, is what the timer is worth.
Ways to cut immersion heater costs
- Fit an insulating jacket if your cylinder does not have factory foam insulation.
- Set the cylinder thermostat to about 60°C. Lower than that is not recommended because of the risk of bacteria growing in stored water.
- Use a timer instead of leaving it on all day.
- Use the ‘sink’ switch, if you have a dual element, when you only need a small amount of hot water.
Worked examples
- One hour each morning: a 3kW element heating for 1 hour a day costs about 79p a day, £5.53 a week and about £287 a year.
- Sink switch top-up: heating 40 litres by 45°C takes about 2.1 kWh, roughly 55p.
Frequently asked questions
How much does a 3kW immersion heater cost per hour?
A 3kW immersion heater uses 3 kWh per hour while heating, which costs about 79p at 26.32p per kWh.
How long does an immersion heater take to heat a tank?
Heating water by 45°C takes about 0.052 kWh per litre. A 120-litre tank from cold needs about 6.3 kWh, so a 3kW element takes a little over 2 hours. Topping up a partly hot tank takes less.
Is it cheaper to leave the immersion heater on all the time?
Generally no. The thermostat stops it overheating, but a hot tank loses heat through its walls all day. Heating water shortly before you need it, ideally on a cheaper night rate, usually costs less.
Is an immersion heater cheaper on Economy 7?
If you have an Economy 7 or similar time-of-use tariff, heating the tank during the cheap hours can cut the cost a lot. Enter your night rate in the price box to see the difference.