首页 | 主题 | 图库 | 问答 | 文摘 | 原创 | 百科

历史 | 地理 | 人物 | 艺术 | 体育 | 科学 | 音乐 | 电影 | 信息技术 | 世界遗产

 开放、中立，源自维基百科

Personal tools

Kilowatt hour

From Wikipedia, the free encyclopedia

  (Redirected from Watt-hour)
Jump to: navigation, search
"KWH" redirects here. For other uses, see KWH (disambiguation).
 This article may require cleanup to meet Wikipedia's quality standards.
Please improve this article if you can. (January 2008)

The kilowatt hour, which is also written kilowatt-hour,[1] (symbol kW·h, kW h or kWh) is a unit of energy.[2] It is most commonly used on household electricity meters, and is 1,000 watt hours.

It is not used in the International System of Units (SI), despite being based on the watt, as the hour is not an SI unit. The SI unit of energy is the joule (J), equal to one watt second. The kilowatt hour is, however, a commonly used unit, especially for measuring electric energy.

One watt hour is equivalent to 3,600 joules (1 J/s × 3600 s), the joule being the SI unit of energy.


Failed to parse (Missing texvc executable; please see math/README to configure.): 1 Watt \cdot hour = 1 \frac{Joule}{second} \times 1 hour = (1 \frac{Joule}{second} \times 1 hour) \times \frac{3600 second}{1 hour} = 3600 Joules



Thus a kilowatt hour is 3,600,000 joules or 3.6 megajoules,


Failed to parse (Missing texvc executable; please see math/README to configure.): 1 kW \cdot hour = 1 \frac{kJ}{s} \times 1 hour = (1 \frac{kJ}{s} \times 1 hour) \times \frac{3600 s}{1 hour} = 3600 kJ \times \frac{1000 J}{1 kJ} = 3,600,000 Joules



and a kilowatt refers to the specific rate at which the amount of joules is used in a second (also known as power). As such, a kilowatt equals the production or usage of 1,000 joules of energy per second (that is, a definitive rate), and a kilowatt hour is the specific amount of energy produced, transmitted, distributed, or consumed in a 3,600-second time period, which is 3,600,000 joules as mentioned above.

Contents

Definition

One watt hour is the amount of (usually electrical or natural gas) energy expended by a one-watt load (e.g., light bulb) drawing power for one hour.

Laymen and utilities tend to use watt hours to measure energy for reasons of convenience and intuition, rather than scientists, who use joules (J). For example, a light bulb draws power (units of watts) over a certain amount of time, resulting in a net amount of used energy; a watt has units of energy-per-time, and an hour is a convenient unit for measuring time, so when multiplied together they produce a unit of energy called the watt hour. The watt hour is derived from the multiplication of the SI unit of power (watt) and a non-SI unit of time (hour). In simple terms, it means the amount of power (watts) used for any given number of hours. A lightbulb that needs 50 J of energy per second to light up (50 watts) will consume 500 watt hours of energy if left on for 10 hours.

The kilowatt hour is commonly used for electrical and natural gas energy. Many electric utility companies use the kilowatt hour for billing. This is a convenient unit because the energy usage of a typical home in one month is several hundred kilowatt hours. In addition, the typical consumer can readily conceptualize the notion of "using a kilowatt for one hour”; common appliances that consume approximately 1 kW include hairdryers, microwave ovens, and vacuum cleaners. Megawatt hours are used for metering of larger amounts of electrical energy.

Pricing for kilowatt hours

Power companies sell energy in units of kilowatt hours. In general, energy (E) is equivalent to power (P) multiplied by time (t).


Failed to parse (Missing texvc executable; please see math/README to configure.): Energy [J] = Power \times Time = Power [\frac{J}{s}] \cdot Time [s]



To determine E in kilowatt hours, P must be expressed in kilowatts and t must be expressed in hours.
Suppose a 1.5 kW electric heater runs for 3 h. Then P = 1.5 kW and t = 3 h, so the energy E in kilowatt hours is: E = 4.5 kW·h.

Failed to parse (Missing texvc executable; please see math/README to configure.): E = P \cdot t


Failed to parse (Missing texvc executable; please see math/README to configure.): = (1.5 kW) \times (3 hr)



Failed to parse (Missing texvc executable; please see math/README to configure.): =4.5 kW \cdot hr



If P and t are not specified in kilowatts and hours respectively, then they must be converted to those units before determining E in kilowatt hours.
Consider the use of one 100 W light bulb (0.1 kW) used for 10 hours per day. This will consume 1 kilowatt hour per day.

Failed to parse (Missing texvc executable; please see math/README to configure.): 100 W = 100 W \times \frac{1 kW}{1000 W} = 0.1 kW



Failed to parse (Missing texvc executable; please see math/README to configure.): E = 0.1 kW \times 10 \frac{hours}{day} = 1 \frac{kW \cdot h}{day}



If a power company charges $0.10/kW·h, then this light bulb will cost $0.10 a day and $0.70 a week to operate.


Failed to parse (Missing texvc executable; please see math/README to configure.): Electricity Price for a Day [\frac{$}{day}] = 1 \frac{kW \cdot h}{day} \times \frac{$0.10}{kW \cdot h}=$0.10/day



Failed to parse (Missing texvc executable; please see math/README to configure.): Electricity Price for a Week [\frac{$}{week}] = \frac{$0.10}{day} \times 7 \frac{days}{week}= $0.70/week



(See Units of measurement#Expressing a physical value in terms of another unit for more information).

Other expressions of the watt hour

Another derived unit that is sometimes used for household purposes is the kW·h/yr., usually considered in annual energy consumption calculations, but with the dimensions of power, with 1 kW·h/yr. = 0.114 W. Note that this unit uses three units of time in one unit, namely second, hour and year, of which only the first is an SI unit.

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 recovered as electricity.

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[3] and a voluntary standard[4] 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[5] 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".[6] Nonetheless, it is commonly used in commercial, educational, scientific and media publications.[7]

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

Energy Portal

References

  1. ^ 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.
  2. ^ "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.
  3. ^ The International System of Units (SI). (2006, 8th ed.) Paris: International Bureau of Weights and Measures. 130.
  4. ^ 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 Conshohoken, PA: Institute of Electrical and Electronics Engineers and ASTM. 15.
  5. ^ Chicago Manual of Style. (14th ed., 1993) University of Chicago Press. 482.
  6. ^ Taylor, Barry N. (1995). 13
  7. ^ 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

bg:Киловатчас ca:Quilowatt-hora cs:Watthodina de:Wattstunde el:Κιλοβατώρα es:Kilovatio-hora eo:Kilovathoro fr:Kilowattheure ko:와트시 hr:Vatsat is:Kílóvattstund it:Wattora he:ואט-שעה nl:Kilowattuur ja:キロワット時 no:Kilowattime nn:Kilowattime pl:Kilowatogodzina pt:Quilowatt-hora ro:Watt-oră ru:Киловатт-час sl:Kilovatna ura fi:Wattitunti sv:Wattimme uk:Ват-година

Retrieved from "http://en.wikilib.com/wiki/Kilowatt_hour"
Navigation
Languages
AD Links