Kilowatt Hour
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A kilowatt-hour (
unit symbol A unit of measurement is a definite magnitude of a quantity, defined and adopted by convention or by law, that is used as a standard for measurement of the same kind of quantity. Any other quantity of that kind can be expressed as a multi ...
: kW⋅h or kW h; commonly written as kWh) is a
unit Unit may refer to: Arts and entertainment * UNIT, a fictional military organization in the science fiction television series ''Doctor Who'' * Unit of action, a discrete piece of action (or beat) in a theatrical presentation Music * ''Unit'' (alb ...
of
energy In physics, energy (from Ancient Greek: ἐνέργεια, ''enérgeia'', “activity”) is the quantitative property that is transferred to a body or to a physical system, recognizable in the performance of work and in the form of heat a ...
: one kilowatt of power for one hour. In terms of
SI derived units with special names SI derived units are units of measurement derived from the seven base units specified by the International System of Units (SI). They can be expressed as a product (or ratio) of one or more of the base units, possibly scaled by an appropriate ...
, it equals 3.6 megajoules (MJ). Kilowatt-hours are a common billing unit for electrical energy delivered to consumers by
electric utilities An electric utility is a company in the electric power industry (often a public utility) that engages in electricity generation and distribution of electricity for sale generally in a regulated market. The electrical utility industry is a major p ...
.


Definition

The kilowatt-hour is a composite unit of energy equal to one kilowatt (kW) sustained for (multiplied by) one hour. Expressed in the standard unit of energy in the
International System of Units The International System of Units, known by the international abbreviation SI in all languages and sometimes pleonastically as the SI system, is the modern form of the metric system and the world's most widely used system of measurement. E ...
(SI), the
joule The joule ( , ; symbol: J) is the unit of energy in the International System of Units (SI). It is equal to the amount of work done when a force of 1 newton displaces a mass through a distance of 1 metre in the direction of the force applie ...
(symbol J), it is equal to 3,600 
kilojoule The joule ( , ; symbol: J) is the unit of energy in the International System of Units (SI). It is equal to the amount of work done when a force of 1 newton displaces a mass through a distance of 1 metre in the direction of the force applied ...
s or 3.6 MJ."Half-high dots or spaces are used to express a derived unit formed from two or more other units by multiplication.", Barry N. Taylor. (2001 ed.
''The International System of Units.''
(Special publication 330). Gaithersburg, MD: National Institute of Standards and Technology. 20.


Unit representations

A widely used representation of the kilowatt-hour is "kWh", derived from its component units, kilowatt and hour. It is commonly used in billing for delivered energy to consumers by
electric utility An electric utility is a company in the electric power industry (often a public utility) that engages in electricity generation and distribution of electricity for sale generally in a regulated market. The electrical utility industry is a major pr ...
companies, and in commercial, educational, and scientific publications, and in the media. It is also the usual unit representation in electrical power engineering. This common representation, however, does not comply with the
style guide A style guide or manual of style is a set of standards for the writing, formatting, and design of documents. It is often called a style sheet, although that term also has multiple other meanings. The standards can be applied either for gene ...
of the
International System of Units The International System of Units, known by the international abbreviation SI in all languages and sometimes pleonastically as the SI system, is the modern form of the metric system and the world's most widely used system of measurement. E ...
(SI). Other representations of the unit may be encountered: * "kW⋅h" and "kW h" are less commonly used, but they are consistent with the SI. The SI brochure states that in forming a compound unit symbol, "Multiplication must be indicated by a space or a half-high (centred) dot (⋅), since otherwise some prefixes could be misinterpreted as a unit symbol." This is supported by a voluntary standard issued jointly by an international (
IEEE The Institute of Electrical and Electronics Engineers (IEEE) is a 501(c)(3) professional association for electronic engineering and electrical engineering (and associated disciplines) with its corporate office in New York City and its operat ...
) and national ( ASTM) organization, and by a major style guide. However, the IEEE/ASTM standard allows "kWh" (but does not mention other multiples of the watt-hour). One guide published by NIST specifically recommends against "kWh" "to avoid possible confusion". * The United States official fuel-economy
window sticker The Monroney sticker or window sticker is a label required in the United States to be displayed in all new automobiles and includes the listing of certain official information about the car. The window sticker was named after Almer Stillwell "M ...
for electric vehicles uses the abbreviation "kW-hrs". * Variations in capitalization are sometimes encountered: KWh, KWH, kwh, etc., which are inconsistent with the International System of Units. * The notation "kW/h" for the kilowatt-hour is incorrect, as it denotes kilowatt per hour. The hour is a unit of time listed among the non-SI units accepted by the
International Bureau of Weights and Measures The International Bureau of Weights and Measures (french: Bureau international des poids et mesures, BIPM) is an intergovernmental organisation, through which its 59 member-states act together on measurement standards in four areas: chemistry ...
for use with the SI. Its combination with the kilowatt, a standard SI unit, is therefore permitted within the standard. An electric heater consuming 1,000 watts (1 kilowatt), and operating for one hour uses one kilowatt-hour of energy. A television consuming 100 watts operating continuously for 10 hours uses one kilowatt-hour. A 40-watt electric appliance operating continuously for 25 hours uses one kilowatt-hour.


Electricity sales

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 A product's average price is the result of dividing the product's total sales revenue by the total units sold. When one product is sold in variants, such as bottle sizes, managers must define "comparable" units. Average prices can be calculated b ...
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. While smaller customer loads are usually billed only for energy, transmission services, and the rated capacity, larger consumers also pay for peak power consumption, the greatest power recorded in a fairly short time, such as 15 minutes. This compensates the power company for maintaining the infrastructure needed to provide peak power. These charges are billed as demand changes. Industrial users may also have extra charges according to the power factor of their load. 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 fiscal year (or financial year, or sometimes budget year) is used in government accounting, which varies between countries, and for budget purposes. It is also used for financial reporting by businesses and other organizations. Laws in many ...
. 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.


Examples

In 2020, the average household in the United States consumed 893 kWh per month. In terms of
human power Human power is work or energy that is produced from the human body. It can also refer to the power (rate of work per time) of a human. Power comes primarily from muscles, but body heat is also used to do work like warming shelters, food, or o ...
, a healthy adult male manual laborer performs work equal to about half a kilowatt-hour over an eight-hour day.


Conversions

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.


Watt-hour multiples

All the SI prefixes are commonly applied to the watt-hour: a kilowatt-hour is 1,000 Wh (kWh); a megawatt-hour is 1 million Wh (MWh); a milliwatt-hour is 1/1,000 Wh (mWh) and so on. The kilowatt-hour is commonly used by electrical energy providers for purposes of billing, since the monthly energy consumption of a typical residential customer ranges from a few hundred to a few thousand kilowatt-hours. Megawatt-hours (MWh), gigawatt-hours (GWh), and terawatt-hours (TWh) are often used for metering larger amounts of electrical energy to industrial customers and in power generation. The terawatt-hour and petawatt-hour (PWh) units are large enough to conveniently express the annual electricity generation for whole countries and the
world energy consumption World energy supply and consumption is global production and preparation of fuel, generation of electricity, energy transport, and energy consumption. It is a basic part of economic activity. It includes heat, but not energy from food. This art ...
.


Distinguishing between kilowatt-hours (energy) and kilowatts (power)

Work Work may refer to: * Work (human activity), intentional activity people perform to support themselves, others, or the community ** Manual labour, physical work done by humans ** House work, housework, or homemaking ** Working animal, an animal t ...
is the amount of energy transferred to a system;
power Power most often refers to: * Power (physics), meaning "rate of doing work" ** Engine power, the power put out by an engine ** Electric power * Power (social and political), the ability to influence people or events ** Abusive power Power may a ...
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.


Annualized power

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 . Thus, 1 GWh/yr = 1 GWh/8766 h ≈ .


Misuse of watts per hour

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 The duck curve is a graph of power production over the course of a day that shows the timing imbalance between peak demand and renewable energy production. Used in utility-scale electricity generation, the term was coined in 2012 by the Cali ...
), or ramp-up behavior of
power plant A power station, also referred to as a power plant and sometimes generating station or generating plant, is an industrial facility for the electricity generation, generation of electric power. Power stations are generally connected to an el ...
s. For example, a power plant that reaches a power output of from in 15 minutes has a ramp-up rate of . Other uses of terms such as ''watts per hour'' are likely to be errors.


Other related energy units

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
milliwatt The watt (symbol: W) is the unit of power or radiant flux in the International System of Units (SI), equal to 1 joule per second or 1 kg⋅m2⋅s−3. It is used to quantify the rate of energy transfer. The watt is named after James Wat ...
s applied constantly during one year. The energy content of a
battery Battery most often refers to: * Electric battery, a device that provides electrical power * Battery (crime), a crime involving unlawful physical contact Battery may also refer to: Energy source *Automotive battery, a device to provide power t ...
is usually expressed indirectly by its capacity in
ampere-hour An ampere hour or amp hour (symbol: A⋅h or A h; often simplified as Ah) is a unit of electric charge, having dimensions of electric current multiplied by time, equal to the charge transferred by a steady current of one ampere flowing for ...
s; 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 Universal Serial Bus (USB) is an industry standard that establishes specifications for cables, connectors and protocols for connection, communication and power supply (interfacing) between computers, peripherals and other computers. A broad ...
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.


See also

* Ampere hour * Electric vehicle battery *
Electric energy consumption Electric energy consumption is the form of energy consumption that uses electrical energy. Electric energy consumption is the actual energy demand made on existing electricity supply for transportation, residential, industrial, commercial, and ot ...
* IEEE Std 260.1-2004 *
Orders of magnitude (energy) This list compares various energies in joules (J), organized by order of magnitude. Below 1 J 1 to 105 J 106 to 1011 J 1012 to 1017 J 1018 to 1023 J Over 1023 J } , - , 1050 , , ≳1050 J , Upper limit of 'apparent'/isotropic energy ...


References

{{DEFAULTSORT:Kilowatt Hour Units of energy Electric power Non-SI metric units