kWh

A kilowatt-hour is a unit of energy, representing the total amount of energy used or delivered when a one-kilowatt load runs for one hour. Unlike the kilowatt, which expresses a rate, the kilowatt-hour expresses an accumulated quantity, and it is the standard yardstick for both the size of an electric vehicle's battery and the electricity a household buys. One kilowatt-hour equals 3.6 megajoules in the formal SI system, but the kilowatt-hour persists because it maps so cleanly onto how energy is metered and billed.

The defining relationship is simple: energy in kilowatt-hours equals power in kilowatts multiplied by time in hours. A 2 kW heater left on for three hours consumes 6 kWh; a 50 kW charger running for half an hour delivers about 25 kWh. This formula lets drivers move freely between the rate at which energy flows and the quantity that results, and it underlies almost every practical calculation involving electricity use.

For electric cars the kilowatt-hour is first and foremost the measure of battery capacity. A car described as having a 60 kWh battery can, in principle, store sixty kilowatt-hours of usable energy, which is the electric equivalent of fuel-tank size. A larger figure means more energy on board and, all else being equal, greater range, though the manufacturer's quoted total capacity is usually a little larger than the usable capacity actually available to the driver, since a buffer is held back to protect the cells.

The same unit also expresses efficiency and cost. Consumption is commonly quoted as kilowatt-hours per 100 kilometres, the electric counterpart to litres per 100 km; a car using 16 kWh/100 km is more frugal than one using 22 kWh/100 km. Dividing battery capacity by this consumption figure gives a rough range estimate. On the billing side, electricity tariffs are priced per kilowatt-hour, so the same number that sizes the battery also lets an owner work out the cost of a charge by multiplying the energy added by the price per unit.

The crucial nuance, as with its partner unit, is to separate energy from power. Two cars may share an identical 60 kWh battery yet charge at very different speeds because one accepts 50 kW and the other 150 kW; the energy stored is the same, but the rate of filling is not. Likewise, range depends on how much energy is stored and how efficiently it is used, not on how powerful the motor or charger is. Treating the kilowatt-hour as a measure of quantity, distinct from the kilowatt as a measure of rate, is the key to interpreting battery specifications, efficiency figures and energy bills correctly.