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The British thermal unit (BTU or Btu) is a traditional unit of energy equal to about 1055 joules. It is the amount of energy needed to cool or heat one pound of water by one degree Fahrenheit. In science, the joule, the SI unit of energy, has largely replaced the BTU.
The BTU 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 heat value (energy content) of fuels is expressed in BTUs.
A BTU is the 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 with the calorie, several definitions of the BTU exist, because the temperature response of water to heat energy is non-linear. This means that the change in temperature of a water mass caused by adding a certain amount of heat to it will be a function of the water's initial temperature. Definitions of the BTU based on different water temperatures can 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 takes to lift a one-pound weight 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 (32.0 to 212.0 °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 ("Th")||≡ 1054.35026444||Uses the "thermochemical calorie" of exactly 4.184 J|
The unit MBtu or mBtu was defined as one thousand BTU, presumably from the Roman numeral system where "M" or "m" stands for one thousand (1,000). This notation is easily confused with the SI mega- (M) prefix, which denotes multiplication by a factor of one million (×106), or with the SI milli- (m) prefix, which denotes division by a factor of one thousand (×10−3). To avoid confusion, many companies and engineers use the notation MMBtu or mmBtu to represent one million BTU (although, confusingly, MM in Roman numerals would traditionally represent 2,000) and in many contexts this form of notation is deprecated and discouraged in favour of the more modern SI prefixes. Alternatively, the term therm may be used to represent 100,000 (or 105) BTU, and quad for 1015 BTU. Some companies also use BtuE6 in order to reduce confusion between 103 BTU and 106 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 3,412 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%.