The British thermal unit (symbol Btu or sometimes BTU) is a traditional unit of energy equal to about 1055 joules. It is approximately the amount of energy needed to heat 1 pound (0.454 kg) of water, which is exactly one tenth of a UK gallon or about 0.1198 US gallons, from 39°F to 40°F (3.8°C to 4.4°C).[1] The unit is most often used in the power, steam generation, heating and air conditioning industries. In scientific contexts the Btu has largely been replaced by the SI unit of energy, the joule, though it may be used as a measure of agricultural energy production (Btu/kg). It is still used unofficially 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.
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) is the correct unit, though this is often 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 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 thousands of Btu's vs. millions of Btu's.
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A Btu is defined as amount of heat required to raise the temperature of one 1 pound (0.454 kg) of liquid water by 1 °F (0.556 °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[2] or as the amount of energy it would take to lift a one-pound weight to a height of 778 feet (237 m).[3]
| 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,[4] 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 |
One Btu is approximately:
Other conversions:
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.
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 kWh of electrical energy. A typical coal-fired power plant works at 10,500 Btu/kWh, an efficiency of 32-33%.[6]