- British thermal unit
The

**British thermal unit**(BTU or Btu) is a unit ofenergy used in the power, steam generation, heating and air conditioning industries. Although it is still used 'unofficially' in metric English-speaking countries (such as Canada, the United Kingdom, and sometimes in New Zealand), its use has declined or has been replaced in other parts of the world. In scientific contexts the BTU has largely been replaced by theSI unit of energy, thejoule (J), though it may be used as a measure of agricultural energy production (BTU/kg).In

North America , the term "BTU" is used to describe the heat value (energy content) of fuels, and also to describe the power of heating and cooling systems, such as furnaces, stoves, barbecue grills, and air conditioners. When used as a unit of power, BTU 'per hour' (BTU/h, that is, BTU divided by hour) is understood, though this is often confusingly abbreviated to just "BTU".The unit

**MBTU**was defined as one thousand BTU presumably from the Roman numeral system where "M" stands for one thousand (1,000). This is easily confused with theSI mega (M) prefix, which adds a factor of one million (1,000,000). To avoid confusion many companies and engineers use**MMBTU**to represent one million BTU. Alternatively a "therm " is used representing 100,000 or 10^{5}BTU, and a "quad" as 10^{15}BTU.**Definitions**A BTU is defined as the amount of heat required to raise the temperature of one pound of liquid

water by one degree from 60° to 61°Fahrenheit at a constant pressure of one atmosphere. As is the case with thecalorie , several different definitions of the BTU exist, which are based on different water temperatures and therefore vary by up to 0.5%:**Conversions**One BTU is approximately:

* 1 054 – 1 060 J (

joule s)

* 2.931 ×10^{-4}kWh (kilowatt hour s)

* 252 – 253 cal (calorie s, or "little calories")

* 0.25 kcal (kilocalories, "large calories", or "food calories")

* 25 031 – 25 160 ft·pdl (foot-poundal )

* 778 – 782 ft·lbf (foot-pounds-force)Other conversions:

* In

natural gas , by convention 1 MMBtu (1 million BTU, sometimes written "mmBTU") = 1.054615 GJ. Conversely, 1 gigajoule is equivalent to 26.8 m^{3}of natural gas at defined temperature and pressure. So, 1 MMBtu = 28.263682 m^{3}of natural gas at defined temperature and pressure.

* 1 standard cubic foot ofnatural gas yields ≈ 1030 BTU (between 1010 BTU and 1070 BTU, depending on quality, when burned)**Associated units**The BTU per hour (BTU/h) is the unit of power most commonly associated with the BTU. The term is sometimes shortened to BTU hour (BTU.h) but both have the same meaning.

* 1

watt is approximately 3.413 BTU/h

* 1000 BTU/h is approximately 293 W

* 1horsepower is approximately 2,544 BTU/h

* 1 "ton of cooling", a common unit in North American refrigeration and air conditioning applications, is 12,000 BTU/h. It is the amount of power needed to melt oneshort ton of ice in 24 hours, and is approximately 3.51 kW.

* 1 "therm " is defined in the United States and European Union as 100,000 BTU—but the U.S. uses the BTU_{59 °F}whilst the EU uses the BTU_{IT}.

* 1 "quad (energy) " (short forquadrillion BTU) is defined as 10^{15}BTU, which is about one exajoule (1.055 × 10^{18}J). Quads are used in the United States for representing the annual energy consumption of large economies: for example, the U.S. economy used 99.75 quads/year in 2005. [*http://wilcoxen.cp.maxwell.syr.edu/pages/804.html*] . One quad/year is about 33.43 gigawatts.The BTU should not be confused with the

Board of Trade Unit (B.O.T.U.), which is a much larger quantity of energy (1 kW·h, or about 3412 BTU).**See also***

Conversion of units

*Latent heat

*Metrication **External links*** [

*http://www.opsi.gov.uk/si/si1995/Uksi_19951804_en_2.htm The Units of Measurement Regulations 1995*]

* [*http://www2.nrcan.gc.ca/es/erb/CMFiles/Natural_Gas_Facts209JLL-06032006-3651.pdf Natural Gas: A Primer*]

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