Kilowatt-hour (kWh)

The kilowatt-hour is a unit of energy equal to the energy delivered by one kilowatt of power flowing for one hour, or exactly 3.6 million joules. Its symbol is kWh. It is the unit in which virtually all electricity in the world is bought and sold.

Its form explains its usefulness. Power multiplied by time gives energy, so a device rated in watts and a duration in hours produce a figure that can be read straight off a bill. A two-kilowatt heater running for three hours consumes six kilowatt-hours, and no conversion is needed between the rating on the appliance and the number on the meter.

Domestic consumption sits at a comprehensible scale. A typical European household uses somewhere between 2000 and 5000 kilowatt-hours a year, an American one closer to 10,000. A kettle boiling once costs about 0.1, a load of washing about 0.7, and a modern refrigerator around 200 a year. Those figures make the unit an effective tool for reasoning about where energy actually goes in a home.

Electric vehicles brought it to a new audience. Battery capacity is quoted in kilowatt-hours, typically between 40 and 100 for a passenger car, and efficiency is expressed as kilowatt-hours per hundred kilometres, the electrical analogue of litres per hundred kilometres. Charging power in kilowatts then determines how long a given amount of energy takes to deliver.

The unit is not SI and is sometimes criticised for compounding a prefixed unit with a non-SI time unit. The megajoule is the formally correct alternative, and some countries do bill gas in megajoules or gigajoules, but the kilowatt-hour's direct link to appliance ratings has kept it dominant for electricity everywhere.

Larger multiples run the electricity industry. A megawatt-hour is the unit of wholesale trading, a gigawatt-hour describes the output of a power station over a day, and a terawatt-hour is used for national annual consumption.

Tariffs have turned the unit into a signal as much as a measurement. Time-of-use pricing charges different amounts per kilowatt-hour by hour of the day, wholesale markets settle in half-hourly blocks, and in periods of high wind and low demand the price has gone negative, so that generators pay to keep supplying. Households with a battery or an electric car respond by shifting consumption into the cheap hours, which flattens the load on the network. The kilowatt-hour is also the unit in which the carbon intensity of electricity is reported, in grams of carbon dioxide per kilowatt-hour, which lets a country's generation mix be judged on one number.

One kilowatt-hour equals 3.6 million joules, 3600 kilojoules, or about 860 kilocalories.

Kilowatt-hour conversion table