Editing Kilowatt-hour (section)

==Other related energy units==
{{main|Units of energy}}
Several other units related to kilowatt-hour are commonly used to indicate power or energy capacity or use in specific application areas.
 
Average annual energy production or consumption can be expressed in kilowatt-hours per year. This is used with loads or output that vary during the year but whose annual totals are similar from one year to the next. For example, it is useful to compare the [[Efficient energy use|energy efficiency]] of household appliances whose power consumption varies with time or the season of the year. Another use is to measure the energy produced by a distributed power source. One kilowatt-hour per year equals about 114.08 [[milliwatt]]s applied constantly during one year.

The energy content of a [[battery (electricity)|battery]] is usually expressed indirectly by its capacity in [[ampere-hour]]s; to convert ampere-hour (Ah) to watt-hours (Wh), the ampere-hour 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&nbsp;V for [[Li-ion]]) that must be used to calculate rather than the device output (for example, usually 5.0&nbsp;V for [[USB]] portable chargers). This results in a 500&nbsp;mA USB device running for about 3.7 hours on a 2,500&nbsp;mAh battery, not five hours.

The ''Board of Trade unit'' (B.T.U.)<ref>{{cite book|editor=E. M. Kirkpatrick|title=[[Chambers 20th Century Dictionary]]|edition=New|year=1983|publisher=Chambers|location=Edinburgh|isbn=0550102345|page=137}}</ref> 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 (United Kingdom)|Ministry of Power]] took over.<ref>{{cite web|title=The Board of Trade 1621-1970|url=http://www.berr.gov.uk/aboutus/corporate/history/outlines/BT-1621-1970/page13919.html |website=Department for Business Innovation and Skills  |archive-url=http://webarchive.nationalarchives.gov.uk/20100304150704/http://www.berr.gov.uk/aboutus/corporate/history/outlines/BT-1621-1970/page13919.html|archive-date=4 March 2010}}</ref> It is distinct from a [[British Thermal Unit]] (BTU) which is 1055 J.

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.<ref>{{cite news|url=http://www.financialexpress.com/printer/news/122151/ |archive-url=https://archive.today/20120908010852/http://www.financialexpress.com/printer/news/122151/ |url-status=dead |archive-date=8 September 2012 |title=Get enlightened about electricity |date=20 December 2004 |work=[[The Financial Express (India)|The Financial Express]] |access-date=29 November 2009 }}</ref><ref>{{cite news | url=http://www.hindu.com/holnus/006200807241521.htm | archive-url= https://web.archive.org/web/20121107155200/http://www.hindu.com/holnus/006200807241521.htm | url-status=dead | archive-date=7 November 2012 | title=BHEL manufactured units generate record power | date=24 July 2008 | work=[[The Hindu]] | agency=[[Press Trust of India]] |access-date=29 November 2009}}</ref>