The kilowatt hour, also written kilowatt-hour,[1] (symbol kW·h, kW h or kWh) is a unit of energy.[2]
Energy delivered by electric utilities is usually expressed and charged for in kWh.
Note that the kWh is the product of power in kilowatts multiplied by time in hours; it is not kW per h.
Definition
The SI (International System of Units) unit of energy is the joule (J), equal to one watt-second (one watt is equal to one joule per second); one kilowatt-hour is exactly 3.6 megajoules, which is the amount of energy expended (or dissipated) if work is done at a constant rate of one thousand watts for one hour.
Other energy-related units
The kilowatt-hour is a convenient unit for electrical bills because the energy usage of a typical electrical customer in one month is several hundred kilowatt-hours. Megawatt-hours and terawatt-hours are used for metering larger amounts of electrical energy.
The energy delivered by batteries is usually expressed indirectly in ampere-hours; to convert watt-hour (Wh) into ampere-hour (Ah), the watt-hour value must be divided by the voltage of the power source.[3]
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.
The Board of Trade unit or B.O.T.U. is an obsolete UK synonym for kilowatt-hour. The term derives from the name of the Board of Trade that regulated the electricity industry. 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. To further the confusion, at least as late as 1937, Board of Trade unit was simply abbreviated ″B.T.U.″ or ″BTU.″
Burnup of nuclear fuel is normally quoted in megawatt-days per ton (MWd/MTU), where ton 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.
Examples
If a heater is rated at 1000 watts (1 kilowatt) and that heater is on for one hour then one kilowatt hour is used (equivalent to 3600 kilojoules).
Using a 60 watt light bulb for one hour consumes 0.06 kilowatt hours of electricity. Using a 60 watt light bulb for one thousand hours consumes 60 kilowatt hours of electricity.
If a 100 watt light bulb is on for one hour per day for 30 days that is 100/1000 (kilowatt) X 30 (hours) = 3 kilowatt hours.
Multiples
Symbol and abbreviation for kilowatt hour
The brochure for SI[5] and a voluntary standard[6] issued jointly by an international (IEEE) and national (ASTM) organization state that when compound unit symbols are formed by multiplication, the individual symbols should be separated by a half-high dot or a space (for example, "kW·h" or "kW h"). However, at least one major usage guide[7] 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".[8] Nonetheless, it is commonly used in commercial, educational, scientific and media publications.[9]
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.
| joule | watt hour | electronvolt | calorie | |
|---|---|---|---|---|
| 1 J = 1 kg m2 s-2 = | 1 | 2.778 × 10−4 | 6.241 × 1018 | 0.239 |
| 1 W·h = | 3600 | 1 | 2.247 × 1022 | 859.8 |
| 1 eV = | 1.602 × 10−19 | 4.45 × 10−23 | 1 | 3.827 × 10−20 |
| 1 cal = | 4.1868 | 1.163 × 10−3 | 2.613 × 1019 | 1 |
See also
References
- ^ Taylor, Barry N. (1995). Guide for the Use of the International System of Units (SI) (Special publication 811). Gaithersburg, MD: National Institute of Standards and Technology. 31.
- ^ "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.
- ^ "Ah (ampere-hour) to Wh (watt-hour) conversion" BatteryHippo.com, 2008, webpage: BatteryHippo.com.
- ^ Frequently Asked Questions. (2008). QueryCAT. Retrieved 20 November 2008.
- ^ The International System of Units (SI). (2006, 8th ed.) Paris: International Bureau of Weights and Measures. 130.
- ^ Standard for the Use of the International System of Units (SI): The Modern Metric System. (1997). (IEEE/ASTM SI 10-1997). New York and West Conshohocken, PA: Institute of Electrical and Electronics Engineers and ASTM. 15.
- ^ Chicago Manual of Style. (14th ed., 1993) University of Chicago Press. 482.
- ^ Taylor, Barry N. (1995). 13
- ^ See for example: Wind Energy Reference Manual Part 2: Energy and Power Definitions Danish Wind Energy Association. Retrieved 9 January 2008; "Kilowatt-Hour (kWh)" BusinessDictionary.com. Retrieved 9 January 2008; "US Nuclear Power Industry" www.world-nuclear.org. Retrieved 9 January 2008; "Energy. A Beginners Guide: Making Sense of Units" Renew On Line (UK). The Open University. Retrieved 9 January 2008.
External links
- Power and Energy in the Home - The Trustworthy Cyber Infrastructure for the Power Grid (TCIP) group at the University of Illinois at Urbana-Champaign has developed an applet which illustrates the consumption and cost of energy in the home, and allows the user to see the effects of manipulating the flow of electricity to various household appliances.
- Battery Capacity: ampere-hour to watt-hour conversion