Why is hot water less efficient than heating?

Updated 29 September 2026

Short answer: the hot water tank needs much hotter water than the radiators do, and the hotter the heat pump has to go, the lower its COP (heat out divided by electricity in). So hot water usually scores lower than heating.

On a mild day the radiators may only need water in the 30s°C, while the tank is usually heated to around 45 to 50°C. Some systems also heat the tank higher now and then to keep it hygienic, and those heats score lower still.

For hot water, when it heats matters more than how efficiently. Heating the tank in a cheap Cosy period can cost less than a more efficient heat in the day rate or the peak.