The kilowatt-hour (SI symbol: kW-h or kW h; commonly written as kWh) is a unit of energy equal to 3600 kilojoules (3.6 megajoules). The kilowatt-hour is commonly used as a billing unit for energy delivered to consumers by electric utilities.
The kilowatt-hour is a composite unit of energy equal to one kilowatt (kW) of power sustained for one hour. Expressed in the standard unit of energy in the International System of Units (SI), the joule (symbol J), it is equal to 3600 kilojoules (3.6 MJ).
The hour is a unit of time listed among the non-SI units accepted by the International Bureau of Weights and Measures for use with the SI. Its combination with the kilowatt, a standard SI unit, is therefore permitted within the standard.
A widely used symbolic representation of the kilowatt-hour is "kWh", from the unit symbols of its component units, kilowatt and hour. It is commonly used in commercial, educational, and scientific publications and in the media. It is also the usual unit representation in electrical power engineering. This common representation does not comply with the style guide of the International System of Units (SI).
Electrical energy is typically sold to consumers in kilowatt-hours. The cost of running an electrical device is calculated by multiplying the device's power consumption in kilowatts by the operating time in hours, and by the price per kilowatt-hour. The unit price of electricity charged by utility companies may depend on the customer's consumption profile over time. Prices vary considerably by locality. In the United States prices in different states can vary by a factor of three.
Major energy production or consumption is often expressed as terawatt-hours (TWh) for a given period that is often a calendar year or financial year. A 365-day year equals 8,760 hours, so over a period of one year, power of one gigawatt equates to 8.76 terawatt-hours of energy. Conversely, one terawatt-hour is equal to a sustained power of about 114 megawatts for a period of one year.
An electric heater consuming 1000 watts (1 kilowatt), and operating for one hour uses one kilowatt-hour of energy. A television consuming 100 watts operating for 10 hours continuously uses one kilowatt-hour. A 40-watt electric appliance operating continuously for 25 hours uses one kilowatt-hour. In terms of human power, a healthy adult male manual laborer performs work equal to about one half of one kilowatt-hour over an eight-hour day.
Electric energy production and consumption are sometimes reported on a yearly basis, in units such as megawatt-hours per year (MWh/yr) gigawatt-hours/year (GWh/yr) or terawatt-hours per year (TWh/yr). These units have dimensions of energy divided by time and thus are units of power. They can be converted to SI power units by dividing by the number of hours in a year, about 8766 h/yr.
Watts per hour (W/h) is a unit of a change of power per hour, i.e. an acceleration in the delivery of energy. It is used to measure the daily variation of demand (e.g. the slope of the duck curve), or ramp-up behavior of power plants. For example, a power plant that reaches a power output of 1 MW from 0 MW in 15 minutes has a ramp-up rate of 4 MW/h. Hydroelectric power plants have a very high ramp-up rate, which makes them particularly useful in peak load and emergency situations.
The energy content of a battery is usually expressed indirectly by its capacity in ampere-hours; to convert ampere-hour (Ah) to watt-hours (Wh), the ampere-hour value must be multiplied by the voltage of the power source. This value is approximate, since the battery voltage is not constant during its discharge, and because higher discharge rates reduce the total amount of energy that the battery can provide. In the case of devices that output a different voltage than the battery, it is the battery voltage (typically 3.7 V for Li-ion) that must be used to calculate rather than the device output (for example, usually 5.0 V for USB portable chargers). This results in a 500 mA USB device running for about 3.7 hours on a 2500 mAh battery, not five hours.