Киловатт-час: единица измерения энергии

Unit of energy, often used for electrical billing

A kilowatt hour (unit symbol: kW⋅h or kW h; commonly written as kWh) is a non SI unit of energy equal to 3.6 megajoules (MJ) in SI units which is the energy delivered by one kilowatt of power for one hour. Kilowatt hours are a common billing unit for electrical energy supplied by electric utilities. Metric prefixes are used for multiples and submultiples of the basic unit, the watt hour (3.6 kJ).

The kilowatt hour is a composite unit of energy equal to one kilowatt (kW) sustained for (multiplied by) one hour. The International System of Units (SI) unit of energy meanwhile is the joule (symbol J). Because a watt is by definition one joule per second, and because there are 3,600 seconds in an hour, one kWh equals 3,600 kilojoules or 3.6 MJ.

In 2020, the average household in the United States consumed 893 kWh per month. In terms of human power, a healthy adult male manual laborer performs work equal to about half a kilowatt hour over an eight hour day.

To convert a quantity measured in a unit in the left column to the units in the top row, multiply by the factor in the cell where the row and column intersect. Joule Watt hour Kilowatt hour Electronvolt Calorie 1 J = 1 kg⋅m2⋅s−2 = 1 2.77778 × 10−4 2.77778 × 10−7 6.241 × 1018 0.239 1 Wh = 3.6 × 103 1 0.001 2.247 × 1022 859.8 1 kWh = 3.6 × 106 1,000 1 2.247 × 1025 8.598 × 105 1 eV = 1.602 × 10−19 4.45 × 10−23 4.45 × 10−26 1 3.827 × 10−20 1 cal = 4.184 1.162 × 10−3 1.162 × 10−6 2.612 × 1019 1

A kilowatt is a unit of power (rate of flow of energy per unit of time). A kilowatt hour is a unit of energy. Kilowatt per hour would be a rate of change of power flow with time. Work is the amount of energy transferred to a system; power is the rate of delivery of energy. Energy is measured in joules, or watt seconds. Power is measured in watts, or joules per second. For example, a battery stores energy. When the battery delivers its energy, it does so at a certain power, that is, the rate of delivery of the energy. The higher the power, the quicker the battery's stored energy is delivered. A higher power output will cause the battery's stored energy to be depleted in a shorter time period.

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 8760. Thus, 1 GWh/yr = 1 GWh/8760 h ≈ 114.12.

Many compound units for various kinds of rates explicitly mention units of time to indicate a change over time. For example: miles per hour, kilometres per hour, dollars per hour. Power units, such as kW, already measure the rate of energy per unit time (kW=kJ/s). Kilowatt hours are a product of power and time, not a rate of change of power with time. 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 from 0 in 15 minutes has a ramp up rate of 4. Other uses of terms such as watts per hour are likely to be errors.

Several other units related to kilowatt hour are commonly used to indicate power or energy capacity or use in specific application areas. Average annual energy production or consumption can be expressed in kilowatt hours per year. This is used with loads or output that vary during the year but whose annual totals are similar from one year to the next. For example, it is useful to compare the energy efficiency of household appliances whose power consumption varies with time or the season of the year. Another use is to measure the energy produced by a distributed power source. One kilowatt hour per year equals about 114.08 milliwatts applied constantly during one year. 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 2,500 mAh battery, not five hours. The Board of Trade unit (B. T. U.) is an obsolete UK synonym for kilowatt hour. The term derives from the name of the Board of Trade which regulated the electricity industry until 1942 when the Ministry of Power took over. This should not be confused with a British Thermal Unit (BTU) which is 1055 J. In India, the kilowatt hour is often simply called a unit of energy. A million units, designated MU, is a gigawatt hour and a BU (billion units) is a terawatt hour.