Immersion heater cost calculator
Heating a 140 litre tank from cold to 60°C takes about 8.1 kWh, which costs £2.14 at the price cap, or about 57p on a 7p off-peak rate.
- A day
- £2.14
- A month
- £65.16
- A year
- £781.94
- Time to heat
- 2 hrs 43 mins
- Energy per tankful
- 8.14 kWh
140 litre tank heated from 10°C to 60°C, 1 time a day, at 26.32p per kWh.
Heating a full tank from cold. A tank also loses some heat while it sits, so the real cost is a little higher. Excludes the standing charge.
Cost to heat a tank from cold
From 10°C to 60°C with a 3 kW heater.
| Tank | Energy | Time | At 26.32p | At 7p off-peak |
|---|---|---|---|---|
| 90 litres | 5.2 kWh | 1.8 hours | £1.38 | 36.6p |
| 120 litres | 7.0 kWh | 2.3 hours | £1.84 | 48.8p |
| 150 litres | 8.7 kWh | 3 hours | £2.30 | 61p |
| 180 litres | 10.5 kWh | 3.5 hours | £2.75 | 73.3p |
| 210 litres | 12.2 kWh | 4 hours | £3.21 | 85.5p |
How we calculate this
Heating 1 litre of water by 1°C takes 4.186 kJ of energy. We multiply that by the tank size and how much the water is heated, and convert it to kWh. An immersion heater turns almost all its electricity into heat in the water.
The time to heat is the energy needed divided by the heater’s power.
Questions people ask
Is it cheaper to leave the immersion heater on all the time?
No. Leaving it on keeps the tank topped up to temperature all day, so you pay to replace heat that is lost while you’re not using hot water. Use a timer to heat it before you need it.
Is an immersion heater cheaper than gas?
Usually not on a standard tariff, because electricity costs 26.32p per kWh against 7.97p for gas. On a cheap overnight rate it can be closer, especially if you have solar panels that would otherwise export.
How long does an immersion heater take to heat water?
A 3 kW heater takes about 2.8 hours to heat a 140 litre tank from cold. Topping up a part-used tank is quicker.
What temperature should my hot water be?
The Health and Safety Executive advises storing hot water at 60°C or more, to stop Legionella bacteria growing.
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