The kilowatt hour, or kilowatt-hour, (symbol kW·h, kW h or kWh) is a unit of energy equal to 1000 watt hours or 3.6 megajoules. For constant power, energy in watt hours is the product of power in watts and time in hours. The kilowatt hour is most commonly known as a billing unit for energy delivered to consumers by electric utilities.

The kilowatt-hour (symbolized kWh) is a unit of energy equivalent to one kilowatt (1 kW) of power expended for one hour (1 h) of time.

Inversely, one watt is equal to 1 J/s. One kilowatt hour is 3.6 megajoules, which is the amount of energy converted if work is done at an average rate of one thousand watts for one hour.

A heater rated at 1000 watts (1 kilowatt), operating for one hour uses one kilowatt hour (equivalent to 3.6 megajoules) of energy.

Using a 60-watt light bulb for one hour consumes 0.06 kilowatt hours of energy. Using a 60-watt light bulb for 1000 hours consumes 60 kilowatt hours of energy.

Using a 100-watt light bulb for one hour consumes 0.10 kilowatt hours (100 W / 1000 W/kW) × 1 h = 0.10 kW·h. If the rate being charged for electricity is 10 cents per kilowatt hour, then running the 100-watt bulb for one hour would cost 0.10 kW·h × $0.10/kW·h = $0.01, or one cent. Running a typical space heater rated at 1500 watts would cost 15 cents per hour (1.5 kW × 1 h x $0.10/kW·h = $0.15). Indeed one can determine how much any electrical device will cost to run by multiplying the amount of kilowatts consumed by the device by the rate per kilowatt hour by the time it will be run for (kilowatts × hours × rate-per-kilowatt-hour = cost-to-run), being mindful of the fact that different rates may exist even within one tariff.