What does hot water cost to heat?

Compare heat sources on the same water and the same temperature rise, not just their power ratings.

Hot water (litres)Default: 100 litres

Warmed by (°C)From about 10°C cold to 50°C

Work it out for one shower

For a mixer shower, the litres are the flow rate times the minutes. Set "warmed by" to about 30°C for a shower at 40°C. An electric shower heats its own water: see the appliance calculator.

Flow (litres a minute)

Minutes

Change assumptions

Cylinder heat loss (kWh a day)From the cylinder's label, if it has a hot water cylinder

Night rate for the immersion (p/kWh)If you heat water on Economy 7 or similar

Gas boiler at 85% efficiency and a heat pump making 2.3 units of heat per unit of electricity for hot water, as in our heat pump calculator.

To heat 100 litres by 40°C (4.65 kWh of heat)

Electric immersion heater4.65 kWh bought: £1.22
Gas boiler5.47 kWh bought: 43.6p
Heat pump2.02 kWh bought: 53.2p

A hot water cylinder also loses some heat while it waits; add the figure from its label under “Change assumptions”.

Ofgem price cap, GB average, Direct Debit, 1 Oct – 31 Dec 2026. Store hot water at 60°C or above, as the Health and Safety Executive advises, to control legionella.