The energy used in an electrical system is measured at the retail level in units of kilowatt-hours or kWh. At the wholesale market level or for very large amounts of energy, it is common to measure in megawatt-hours or MWh. Historically, most utilities have priced their service to electric customers in cents per kilowatt-hour. 

Energy, as measured in kilowatt-hours or megawatt-hours, is equal to the power being used multiplied by the length of time that it is used. For example, an electric heater that is rated at 1500 watts and is used continuously over a two-hour time period will use 3,000 watt-hours or 3 kWh.

1.5 kW x 2 hours = 3 kWh

To calculate this, take the 1500 watts, or 1.5 kW, and multiply by the number of hours the heater will be used. It is important to understand that the heater uses 1.5 kW of power every instant it is operated. If the heater is used for 10 hours, it would still use 1.5 kW of power every instant it was operated but 15 kWh of energy over the 10-hour period.