megawatt-hours
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The kilowatt-hour ( SI symbol: 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 (album), ...
of
energy In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other words, to the regula ...

energy
equal to one
kilowatt The watt (symbol: W) is a unit of power Power typically refers to: * Power (physics) In physics, power is the amount of energy transferred or converted per unit time. In the International System of Units, the unit of power is the watt, equa ...

kilowatt
of
power Power most often refers to: * Power (physics) In physics, power is the amount of energy In , energy is the that must be to a or to perform on the body, or to it. Energy is a ; the law of states that energy can be in form, bu ...
sustained for one hour or 3600 
kilojoule The joule ( ; symbol: J) is a derived unit of energy In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates ...
s (3.6 
megajoule The joule ( ; symbol: J) is a derived unit of energy In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates ...
s). It is commonly used as a billing unit for energy delivered to consumers by
electric utilities An electric utility is a company in the electric power industry The electric power industry covers the generation A generation is "all of the people born and living Living or The Living may refer to: Common meanings *Life, a condition that ...
.


Definition

The kilowatt-hour is a composite unit of energy equal to one kilowatt (kW) times 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 Pleonasm#Acronyms_and_initialisms, pleonastically as the SI system, is the modern form of the metric system and the world's most wi ...
(SI), the
joule The joule ( ; symbol: J) is a derived unit of energy In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates ...

joule
(symbol J), it is equal to 3600 
kilojoule The joule ( ; symbol: J) is a derived unit of energy In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates ...
s (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.
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 An intergovernmental organization (IGO) is an organization composed primarily of sovereign state ...
for use with the SI. Its combination with the kilowatt, a standard SI unit, is therefore permitted within the standard.


Unit representations

A widely used short form 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 A style guide or manual of style is a set of standards for the writing, formatting, and design of documents A document is a writing, written, drawing, drawn, presented, or memorialized representation of thought, often the manifestation of ...
of the
International System of Units The International System of Units, known by the international abbreviation SI in all languages and sometimes Pleonasm#Acronyms_and_initialisms, pleonastically as the SI system, is the modern form of the metric system and the world's most wi ...
(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 (⋅), 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 professional association A professional association (also called a professional body, professional organization, or professional society) usually seeks to further Further or ...
) and national (
ASTM ASTM International, formerly known as American Society for Testing and Materials, is an international standards organization A standards organization, standards body, standards developing organization (SDO), or standards setting organization ( ...
) 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 The National Institute of Standards and Technology (NIST) is a physical sciences Physical science is a branch of natural science that studies abiotic component, non-living systems, in contrast to life science. It in turn has many branches, e ...
specifically recommends against "kWh" "to avoid possible confusion". * The United States official fuel-economy window sticker 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.


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 by ...
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 In electrical engineering Electrical engineering is an engineering discipline concerned with the study, design, and application of equipment, devices, and systems which use electricity, electronics The field of electronics is a branch of ph ...
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 A calendar is a system of organizing days. This is done by giving names to periods of time Time is the indefinite continued sequence, progress of existence and event (philosophy), events that occur in an apparently irreversible process, ...
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 report Financial statements (or financial reports) are forma ...
. 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

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 continuously for 10 hours uses one kilowatt-hour. A 40-watt electric appliance operating continuously for 25 hours uses one kilowatt-hour. In terms of
human power Human power is 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 * Work (ph ...
, 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 The International System of Units, known by the international abbreviation SI in all languages and sometimes Pleonasm#Acronyms_and_initialisms, pleonastically as the SI system, is the modern form of the metric system and the world's most wi ...
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 1/1000 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 A fuel is any material that can be made to react with other substances so that it releases energy as thermal energy Thermal radiation in visible light can be ...
.


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

Energy is the
work Work may refer to: * Work (human activity) Work or labor is intentional activity people perform to support themselves, others, or the needs and wants of a wider community. Alternatively, work can be viewed as the human activity that cont ...

work
performed; and
power Power most often refers to: * Power (physics) In physics, power is the amount of energy In , energy is the that must be to a or to perform on the body, or to it. Energy is a ; the law of states that energy can be in form, bu ...
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 Karen Edson ...
), 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 generation A generation is "all of the people born and living Living or The Living may refer to: ...

power plant
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 power production or consumption can be expressed in kilowatt-hours per year. This is used with loads or output that vary during the year but that are similar in each year. 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 case 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 a unit of power Power most often refers to: * Power (physics) In physics, power is the amount of energy In , energy is the that must be to a or to perform on the body, or to it. Energy is a ; the la ...
s applied constantly during one year. The energy content of a
battery Battery may refer to: Energy source * Electric battery A battery is a source of electric power consisting of one or more electrochemical cell An electrochemical cell is a device capable of either generating electrical energy Electric ...
is usually expressed indirectly by its capacity in
ampere-hour An ampere hour or amp hour (symbol: A⋅h or A h; often also unofficially denoted as Ah) is a unit Unit may refer to: Arts and entertainment * UNIT Unit may refer to: Arts and entertainment * UNIT, a fictional military organization in the ...
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 A lithium-ion battery or Li-ion battery is a type of rechargeable battery A rechargeable battery, storage battery, or secondary cell, (or archaically accumulator) is a type of electrical battery A battery is a device consisting of o ...
) 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 ''Industry Standard'' is a 1982 album by Dixie Dregs, The Dregs. It is their only album featuring vocals (by Alex Ligertwood of Santana (band), Santana and Patrick Simmons of The Doobie Bro ...

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. 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 The Board of Trade is a British government body concerned with commerce and industry, currently within the Department for International Trade The Department for International Trade (DIT) is a United Kingdom government department responsible f ...
which regulated the electricity industry until 1942 when the Ministry of Power took over. This should not be confused with a
British Thermal Unit The British thermal unit (BTU or Btu) is a unit of heat In thermodynamics Thermodynamics is a branch of physics that deals with heat, Work (thermodynamics), work, and temperature, and their relation to energy, entropy, and the physica ...
(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 An ampere hour or amp hour (symbol: A⋅h or A h; often also unofficially denoted as Ah) is a unit Unit may refer to: Arts and entertainment * UNIT Unit may refer to: Arts and entertainment * UNIT, a fictional military organization in the ...
*
Electric energy consumption Electric energy consumption is the form of energy consumption Energy consumption is the amount of energy In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical ...
*
IEEE Std 260.1-2004 IEEE The Institute of Electrical and Electronics Engineers (IEEE) is a professional association A professional association (also called a professional body, professional organization, or professional society) seeks to advocacy, further a pa ...
*
Orders of magnitude (energy) This list compares various energy, 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 SI multiples See also *Conversion of units of energy *Energ ...


References

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