British thermal unit
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The British thermal unit (symbol Btu or sometimes BTU) is a traditional unit of energy equal to about 1055 joules. It is the amount of energy needed to heat one pound of water by one degree Fahrenheit. In scientific contexts the Btu has largely been replaced by the SI unit of energy, the joule.
The unit is most often used as a measure of power (as Btu/h) in the power, steam generation, heating and air conditioning industries, and also as a measure of agricultural energy production (Btu/kg).[verification needed] It is still used in metric English-speaking countries (such as Canada or the United States), and remains the standard unit of classification for air conditioning units manufactured and sold in many non-English-speaking metric countries.[verification needed] In North America, the term "Btu" is used to describe the heat value (energy content) of fuels.
A Btu is defined as amount of heat required to raise the temperature of 1 pound (0.454 kg) of liquid water by 1 °F (0.56 °C) at a constant pressure of one atmosphere. As is the case with the calorie, several different definitions of the Btu exist, which are based on different water temperatures and therefore vary by up to 0.5%. A Btu can be approximated as the heat produced by burning a single wooden match or as the amount of energy it would take to lift a one-pound weight to a height of 778 feet (237 m).
|Nominal temperature||Btu equivalent in joules||Notes|
|39 °F (3.9 °C)||≈ 1059.67||Uses the calorie value of water at its maximum density (4 °C or 39.2 °F)|
|Mean||≈ 1055.87||Uses a calorie averaged over water temperatures 0 to 100 °C (32 to 212 °F)|
|IT||≡ 1055.05585262||The most widespread Btu, uses the International [Steam] Table (IT) calorie, which was defined by the Fifth International Conference on the Properties of Steam (London, July 1956) to be exactly 4.1868 J|
|ISO||≡ 1055.056||International standard ISO 31-4 on Quantities and units—Part 4: Heat, Appendix A. This value uses the IT calorie and is rounded to a realistic accuracy|
|59 °F (15.0 °C)||≡ 1054.804||Chiefly American. Uses the 15 °C calorie, itself now defined as exactly 4.1855 J (Comité international 1950; PV, 1950, 22, 79–80)|
|60 °F (15.6 °C)||≈ 1054.68||Chiefly Canadian|
|63 °F (17.2 °C)||≈ 1054.6|
|Thermochemical||≡ 1054.35026444||Uses the "thermochemical calorie" of exactly 4.184 J|
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 the SI mega (M) prefix, which multiplies by 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 105 Btu, and a quad as 1015 Btu. Some companies also use BtuE6 in order to reduce confusion between a thousand Btu vs. a million Btu.
One Btu is approximately:
- 1.054 to 1.060 kJ (kilojoules)
- 0.293071 W·h (watt hours)
- 252 to 253 cal (calories, or "little calories")
- 0.25 kcal (kilocalories, "large calories," or "food calories")
- 25 031 to 25 160 ft·pdl (foot-poundal)
- 778 to 782 ft·lbf (foot-pounds-force)
- 5.40395 (lbf/in2)·ft3
For natural gas 
- In natural gas, by convention 1 MMBtu (1 million Btu) = 1.054615 GJ. Conversely, 1 gigajoule is the amount of energy in 26.8 m3 of natural gas at defined temperature and pressure. So, 1 MMBtu is the amount of energy in 28.263682 m3 (998.12 ft3) of natural gas at defined temperature and pressure.
- 1 standard cubic foot of natural gas yields ≈ 1030 Btu (between 1010 Btu and 1070 Btu, depending on quality, when burned)
- As a coarse approximation, 1000 ft3 of natural gas yields ≈ 1 MMBtu ≈ 1 GJ
As a unit of power 
When used as a unit of power for heating and cooling systems, Btu per hour (Btu/h) is the correct unit, though this is often abbreviated to just "Btu".[verification needed].
- 1 watt is approximately 3.41214 Btu/h
- 1000 Btu/h is approximately 293.071 W
- 1 horsepower is approximately 2544 Btu/h
Associated units 
- 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 freeze one short 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 Btu59 °F while the EU uses the BtuIT.
- 1 quad (short for quadrillion Btu) is defined as 1015 Btu, which is about one exajoule (1.055×1018 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 in 2005. 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).
The Btu is often used to express the conversion-efficiency of heat into electrical energy in power plants. Figures are quoted in terms of the quantity of heat in Btu required to generate 1 kW·h of electrical energy. A typical coal-fired power plant works at 10,500 Btu/kW·h, an efficiency of 32–33%.
See also 
- "What is British thermal unit (Btu)? definition and meaning". Businessdictionary.com. Retrieved 2011-11-11.
- Energy and the Environment. Ristinen, Robert A. c.2006, pg 13
- Energy and the Environment. Ristinen, Robert A. c.2006, pg14
- International standard ISO 31-4:1992 Quantities and units—Part 4: Heat
- EIA (2012-09-17). "What are Mcf, Btu, and therms? How do I convert prices in Mcf to Btus and therms?". US Government.
- 2009 ASHRAE Handbook – Fundamentals (I-P Edition). (pp: 38.2). American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc
- Husher, John Durbin. Crises of the 21st Century: Start Drilling-The Year 2020 Is Coming Fast, iUniverse, 2009. Page 376.
- Electric Generation Efficiency, NPC Global Oil & Gas Study, 18 July 2007