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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/h.
Definition
The SI (International System of Units) unit of energy is the joule (J), equal to one watt - second; one kilowatt hour is exactly 3.6 megajoules, which is the amount of energy transferred if work is done at a 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; when multiplied by the average battery voltage over the discharge cycle this becomes watt-hours.
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.
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.
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 30 days X 100/1000 = 3 kilowatt hours.
[3]
Multiples
SI multiples for watt hour (W·h)
| Submultiples |
|
Multiples |
| Value |
Symbol |
Name |
|
Value |
Symbol |
Name |
| 10–1 W·h |
dW·h |
deciwatt hour |
|
101 W·h |
daW·h |
decawatt hour |
| 10–2 W·h |
cW·h |
centiwatt hour |
102 W·h |
hW·h |
hectowatt hour |
| 10–3 W·h |
mW·h |
milliwatt hour |
103 W·h |
kW·h |
kilowatt hour |
| 10–6 W·h |
µW·h |
microwatt hour |
106 W·h |
MW·h |
megawatt hour |
| 10–9 W·h |
nW·h |
nanowatt hour |
109 W·h |
GW·h |
gigawatt hour |
| 10–12 W·h |
pW·h |
picowatt hour |
1012 W·h |
TW·h |
terawatt hour |
| 10–15 W·h |
fW·h |
femtowatt hour |
1015 W·h |
PW·h |
petawatt hour |
| 10–18 W·h |
aW·h |
attowatt hour |
1018 W·h |
EW·h |
exawatt hour |
| 10–21 W·h |
zW·h |
zeptowatt hour |
1021 W·h |
ZW·h |
zettawatt hour |
| 10–24 W·h |
yW·h |
yoctowatt hour |
1024 W·h |
YW·h |
yottawatt hour |
| Common multiples are in bold face |
Symbol and abbreviation for kilowatt hour
The brochure for SI[4] and a voluntary standard[5] 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[6] 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".[7] Nonetheless, it is commonly used in commercial, educational, scientific and media publications.[8]
Conversions
| from / to |
joule |
watt hour |
electronvolt |
calorie |
| 1 J = 1 kg m2 s-2 = |
1 |
0.278 × 10-3 |
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
External links
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