A kWh, or kilowatt hour, is the standard unit of energy used to measure gas and electricity consumption on a bill. It represents the energy used by a one-kilowatt appliance running for one hour, and is the quantity suppliers charge the unit rate against.
In one line: A kWh is the unit of energy, equal to one kilowatt drawn for one hour, that energy bills are measured and priced in.
How a kWh works
Meters record consumption in kWh, and the supplier multiplies that figure by the unit rate to calculate the usage charge. Both gas and electricity bills convert to kWh so they can be priced consistently.
A 2,000-watt fan heater is a 2 kW appliance, so running it for three hours uses 6 kWh. At a unit rate of 25p per kWh that costs 1.50 GBP for those three hours of use.
Gas meters that read in cubic metres or hundreds of cubic feet are converted to kWh using a calorific value, so the gas unit rate is also applied per kWh.
kWh vs kW
A kW (kilowatt) measures power, the rate at which energy is drawn at an instant, while a kWh measures energy, the amount used over time. Bills charge for kWh, not kW.
This distinction matters because a high-power appliance used briefly can consume fewer kWh than a low-power one left on for hours.
Primary source: Ofgem: Get help with energy