The KILOWATT HOUR (symbol KWH, KW⋅H or KW H) is a unit of energy equal to 3.6 megajoules . If the energy is being transmitted or used at a constant rate (power) over a period of time, the total energy in kilowatt hours is the power in kilowatts multiplied by the time in hours. The kilowatt hour is commonly used as a billing unit for energy delivered to consumers by electric utilities . CONTENTS * 1 Definition * 2 Examples * 3 Symbol and abbreviations for kilowatt hour * 4 Conversions * 5 Watt hour multiples and billing units * 6 Confusion of kilowatt hours (energy) and kilowatts (power) * 7 Misuse of watts per hour * 8 Other energyrelated units * 9 See also * 10 References * 11 External links DEFINITION The kilowatt hour (symbolized kW⋅h as per SI ) is a composite unit of energy equivalent to one kilowatt (1 kW) of power sustained for one hour. One watt is equal to 1 J/s. One kilowatt hour is 3.6 megajoules , which is the amount of energy converted if work is done at an average rate of one thousand watts for one hour. The base unit of energy within the International System of Units (SI) is the joule . The hour is a unit of time "outside the SI", making the kilowatt hour a non SI unit of energy. The kilowatt hour is not listed among the nonSI units accepted by the BIPM for use with the SI, although the hour, from which the kilowatt hour is derived, is. EXAMPLES An electric heater rated at 1000 watts (1 kilowatt), operating for one hour uses one kilowatt hour (equivalent to 3.6 megajoules) of energy. A television rated at 100 watts operating for 10 hours continuously uses one kilowatt hour. A 40watt electric appliance operating continuously for 25 hours uses one kilowatt hour. In terms of human power , a healthy adult male manual laborer will perform work equal to about half a kilowatt hour over an eight hour day. Electrical energy is often sold in kilowatt hours. The cost of running an electric device is calculated by multiplying the device's power in kilowatts, by the running time in hours, by the price per kilowatt hour. The unit price of electricity may depend upon the rate of consumption and the time of day. Industrial users may also have extra charges according to their peak usage and the power factor . Whereas individual homes only pay for the kilowatt hours consumed, commercial buildings and institutions 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 charges. Major energy production or consumption is often expressed as terawatt hours (TW⋅h) for a given period that is often a calendar year or financial year . A 365day year equals to 8,760 hours, therefore one gigawatt equals to 8.76 terawatt hours per year. Conversely, one terawatt hour is equal to a sustained power of approximately 114 megawatts for a period of one year. SYMBOL AND ABBREVIATIONS FOR KILOWATT HOUR The symbol "kWh" is commonly used in commercial, educational, scientific and media publications, and is the usual practice in electrical power engineering. Other abbreviations and symbols may be encountered: * "kW h" is less commonly used. It is consistent with SI standards. The international standard for SI states that in forming a compound unit symbol, "Multiplication must be indicated by a space or a halfhigh (centered) dot (⋅), since otherwise some prefixes could be misinterpreted as a unit symbol" (i.e., kW h or kW⋅h). This is supported by a voluntary standard issued jointly by an international (IEEE ) and national ( ASTM ) organization. However, at least one major usage guide and the IEEE/ ASTM standard allow "kWh" (but do not mention other multiples of the watt hour). One guide published by NIST specifically recommends avoiding "kWh" "to avoid possible confusion". * "kW⋅h" is, like "kW h", preferred by SI standards, but it is very rarely used in practice. * The US official fueleconomy window sticker for electric vehicles uses the abbreviation "kWhrs". * Variations in capitalization are sometimes seen: KWh, KWH, kwh, etc.; these are inconsistent with International System of Units. * The notation "kW/h" is not a correct symbol for kilowatt hour, as it denotes kilowatt per hour instead. CONVERSIONS Further information: Conversion of units of energy 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 W⋅H = 3.6 × 103 1 0.001 2.247 × 1022 859.8 1 KW⋅H = 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 = 4.2 1.163 × 10−3 1.163 × 10−6 2.613 × 1019 1 WATT HOUR MULTIPLES AND BILLING UNITS Further information: Metric prefixes All the SI prefixes are commonly applied to the watt hour: a kilowatt hour is 1,000 W⋅h (symbols kW⋅h, kWh or kW h; a megawatt hour is 1 million W⋅h, (symbols MW⋅h, MWh or MW h); a milliwatt hour is 1/1000 W⋅h (symbols mW⋅h, mWh or mW h) and so on. The kilowatt hour is commonly used by electrical distribution 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 . SI multiples for watt hour (W⋅h) SUBMULTIPLES MULTIPLES VALUE SYMBOL NAME VALUE SYMBOL NAME 10−3 mW⋅h milliwatt hour 103 kW⋅h kilowatt hour 10−6 µW⋅h microwatt hour 106 MW⋅h megawatt hour 109 GW⋅h gigawatt hour 1012 TW⋅h terawatt hour 1015 PW⋅h petawatt hour CONFUSION OF KILOWATT HOURS (ENERGY) AND KILOWATTS (POWER) The terms power and energy are frequently confused. Power is the rate
of delivery of energy. Power is work performed per unit of time.
Energy
Power is measured using the unit watts, or joules per second. Energy is measured using the unit watt hours, or joules. A common household battery contains energy. When the battery delivers its energy, it does so at a certain power level, that is, the rate of delivery of the energy. The higher the power level, the quicker the battery's stored energy is delivered. If the power is higher, the battery's stored energy will be depleted in a shorter time period. For a given period of time, a higher level of power causes more energy to be used. For a given power level, a longer run period causes more energy to be used. For a given amount of energy, a higher level of power causes that energy to be used in less time. MISUSE OF WATTS PER HOUR Power units measure the rate of energy per unit time. Many compound
units for rates explicitly mention units of time, for example, miles
per hour, kilometers per hour, dollars per hour.
Kilowatt
The proper use of terms such as watts per hour is uncommon, whereas misuse may be widespread. OTHER ENERGYRELATED UNITS Several other units 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; for example, when comparing the energy efficiency of household appliances whose power consumption varies with time or the season of the year, or 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 amperehours ; to convert amperehour (A⋅h) to watt
hours (W⋅h), the amperehour 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
Liion )
that must be used to calculate rather than the device output (for
example, usually 5.0 V for
USB
The Board of Trade unit (BOTU) 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. The British thermal unit or BTU (not to be confused with BOTU), is a unit of thermal energy with several definitions, all about 1055 Joule or 0.293 watt hour. The quad , short for quadrillion BTU, or 1015 BTU, is sometimes used in nationalscale energy discussions in the United States. One quad is approximately 293 TWh or 1.055 exajoule (EJ). A TNT equivalent is a measure of energy released in the detonation of trinitrotoluene . A tonne of TNT equivalent is approximately 4.184 gigajoules or 1,163 kilowatt hours. A tonne of oil equivalent is the amount of energy released by burning one tonne of crude oil . It is approximately 41.84 gigajoules or 11,630 kilowatt hours. 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. Burnup of nuclear fuel is normally quoted in megawatt days per tonne (MW⋅d/MTU), where tonne refers to a metric ton of uranium metal or its equivalent, and megawatt refers to the entire thermal output, not the fraction which is converted to electricity. SEE ALSO *
Energy
*
Amperehour
*
Watt
*
Joule
REFERENCES * ^ Thompson, Ambler and Taylor, Barry N. (2008). Guide for the Use
of the
International System of Units (SI) Archived June 3, 2016, at
the
Wayback Machine . (
Special
EXTERNAL LINKS * Prices per kilowatt hour in the USA,
Energy
