The kilowatt hour (symbol kWh) is a derived 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.
The kilowatt hour is a unit of energy equivalent to one kilowatt (1 kW) of power sustained for one hour.
One watt second is equal to one joule. 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 non-SI units accepted by the BIPM for use with the SI, although the hour, from which the kilowatt hour is derived, is.
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 40-watt light bulb operating continuously for 25 hours uses one kilowatt hour.
Electrical energy is sold in kilowatt hours; cost of running equipment is the product of power in kilowatts multiplied by running time in hours and 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.
Symbol and abbreviations for kilowatt hour
Other abbreviations and symbols may be encountered:
- "kW h" is less commonly used. It is consistent with SI standards (but note that the kilowatt-hour is a non-SI unit). The international standard for SI states that in forming a compound unit symbol, "Multiplication must be indicated by a space or a half-high (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 with SI standards, but it is very rarely used in practice.
- The US official fuel-economy window sticker for electric vehicles uses the abbreviation "kW-hrs".
- Variations in capitalization are sometimes seen: KWh, KWH, kwh, etc. these are inconsistent with International System of Units.
- The notation "kW/h", as a symbol for kilowatt-hour, is not correct.
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 =||4.1868||1.163 × 10−3||1.163 × 10−6||2.613 × 1019||1|
Watt hour multiples and billing units
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.
|10−3||mWh||milliwatt hour||103||kWh||kilowatt hour|
|10−6||µWh||microwatt hour||106||MWh||megawatt hour|
Confusion of kilowatt hours (energy) and kilowatts (power)
The terms power and energy are frequently confused. Power is defined as the rate of delivery of energy, the work performed per unit time, measured in units joules per second or watts. To produce power over any period of time requires energy. Either higher levels of power for a given period or longer periods of run time at a given power level require more energy.
An electrical load, e.g. a lamp, TV-set, electric motor, has a rated size in watts. This is its running power level, which equates to the instantaneous rate at which energy must be generated and consumed to run the device. The amount of energy that is consumed at that rate depends on how long the device is operated. However, its power requirements are constant while running. The unit of energy for residential electrical billing, kilowatt hours, integrates changing power levels in use at the residence over the past billing period, which is nominally 720 hours for a 30-day month, thus showing cumulative electrical energy use for the month.
For another example, when a light bulb with a power rating of 100 watts is turned on for one hour, the energy used is 100 watt hours (Wh), 0.1 kilowatt hour, or 360 kilojoules. This same amount of energy would light a 40-watt bulb for 2.5 hours, or a 10-watt low-energy bulb for 10 hours. A power station electricity output at any particular moment would be measured in multiples of watts, but its annual energy sales would be in multiples of watt hours. A kilowatt hour is the amount of energy equivalent to a steady power of 1 kilowatt running for 1 hour, or 3.6 megajoules.
Energy cost can be illustrated in a similar example: a 100-watt (i.e. 0.1 kilowatt) light bulb turned on for 8 hours uses 800 Wh or 0.8 kWh. If the energy cost is 25 cents per kilowatt hour, it would cost 0.1 kW × 8 h × $0.25/kWh = $0.20.
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 (TWh) for a given period that is often a calendar year or financial year. One terawatt hour is equal to a sustained power of approximately 114 megawatts for a period of one year.
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-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. It might be used to characterize the 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.
Other energy-related 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 ampere-hours; to convert watt hours (Wh) to ampere hour (Ah), the watt-hour value must be divided by the voltage of the power source. This value is approximate, since the voltage is not constant during discharge of a battery, and because higher discharge rates reduce the total amount of energy 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 for Li-ion) that must be used to calculate rather than the device output (usually 5.0 for USB portable chargers), since use of the lower figure allows manufacturers of such devices to overstate the capacity or run-time, inflating it by 35% ( 5⁄3.7=1.35). This results in a 500mA USB device running for only about 3.7 hours on a 2500mAh battery, rather than the expected five hours.
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 B.O.T.U. should not be confused with the British thermal unit or BTU, which is a much smaller quantity of thermal energy.
Burnup of nuclear fuel is normally quoted in megawatt days per tonne (MWd/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.
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- ASTM SI10-10, IEEE/ASTM SI 10 American National Standard for Metric Practice, ASTM International, West Conshohocken, PA, 2010, [www.astm.org] "The symbols for certain compound units of electrical power engineering are usually written without separation, thus: watthour (Wh), kilowatthour (kWh), voltampere (VA), and kilovoltampere (kVA)"
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