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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 In scientific contexts the Btu has largely been replaced by the SI unit of energy, the joule.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), 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:
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].
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%.