How much does an immersion heater cost to run?
A 3kW immersion heater costs about 79p an hour while the element is on, at the Ofgem cap rate of 26.32p/kWh. Heating a full 150-litre cylinder from cold (about 15°C) to 60°C takes about 7.8 kWh, so it costs around £2.07. Topping up a partly used tank costs less.
Prices last checked: 26 September 2026 · Source: Ofgem energy price cap, 1 October to 31 December 2026 · Great Britain average, standard variable tariff, paid by Direct Debit
Cost to heat a cylinder
The energy needed depends on how much water you heat and by how much. It is physics, so it can be calculated directly: 4.186 kJ heats one litre by 1°C. The table heats cold water from 15°C to 60°C and ignores heat lost from the cylinder while it waits, which adds more on top.
| Cylinder | Energy to heat from cold | Cost at cap rate | Time with a 3kW element |
|---|---|---|---|
| 90 litres | 4.7 kWh | £1.24 | about 1 h 35 min |
| 120 litres | 6.3 kWh | £1.65 | about 2 h 5 min |
| 150 litres | 7.8 kWh | £2.07 | about 2 h 35 min |
| 210 litres | 11 kWh | £2.89 | about 3 h 40 min |
Should you leave the immersion heater on?
Usually not. A cylinder kept hot all day loses heat continuously, and the immersion pays to replace it. Heating once, shortly before you need the water, and letting the insulated cylinder hold it is cheaper. A plug-in or wired immersion timer is inexpensive and makes this automatic.
How much the standing loss costs depends on the cylinder's insulation. Old cylinders with thin or loose jackets lose much more heat than modern factory-foamed ones. A proper cylinder jacket on an old tank is one of the cheapest hot-water savings there is.
Temperature: don't go too low
It is tempting to turn the thermostat right down to save money. The Health and Safety Executive's guidance for hot water systems is to store hot water at 60°C or higher, to control Legionella bacteria. Saving money by storing lukewarm water is not a good trade. Mix with cold at the tap instead.
Economy 7 and immersion heaters
Many homes with an immersion heater have an Economy 7 meter and a dual immersion: a bottom element on the cheap night rate to heat the whole tank, and a top element for boosting a small amount during the day. The saving depends on the gap between your day and night rates. Enter your own night rate in the calculator to see it. Using the top "boost" element by habit during the day pays the full day rate.
Immersion vs other ways to heat water
- Gas boiler: gas is much cheaper per kWh (7.97p at the cap), so if your boiler heats the cylinder, use that and keep the immersion as a backup.
- Electric shower: heats only the water you use, with no standing loss. See the electric shower page.
- Heat pump or solar diverter: both can heat a cylinder for much less, but need investment.
Common mistakes
- Leaving the immersion on 24/7 "so there's always hot water".
- Using the boost switch and forgetting it's on.
- Multiplying 3kW by the hours it's switched on. The thermostat cycles, so real use is lower. Only a measurement tells you by how much.
Limits of this estimate
The heating-from-cold figures are theoretical minimums. Standing losses and pipe losses come on top and vary a lot by cylinder. The unit rate is the Ofgem cap average (Great Britain, Direct Debit, standard variable tariff).
Frequently asked questions
How much does it cost to run an immersion heater for an hour?
About 79p for a 3kW element that is heating for the whole hour. Once the water is hot, the thermostat switches it off and it costs less.
How long does an immersion heater take to heat water?
For a 150-litre cylinder from cold with a 3kW element, about two and a half hours, plus a little extra for heat losses. A partly used tank is quicker.
Is it cheaper to heat water with gas or an immersion heater?
Gas, in almost all cases, because the unit price is much lower. The immersion is best used as a backup or with cheap off-peak or solar electricity.