The British thermal unit (Btu) is a measure of heat, which is a form of energy. It was originally defined as the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit. It is also part of the United States customary units.[1] The SI unit for energy is the joule (J); one Btu equals about 1,055 J (varying within the range of 1,054–1,060 J depending on the specific definition of BTU; see below).
While units of heat are often supplanted by energy units in scientific work, they are still used in some fields. For example, in the United States the price of natural gas is quoted in dollars per the amount of natural gas that would give 1 million Btu (1 "MMBtu") of heat energy if burned.[2][3]
A Btu was originally defined as the amount of heat required to raise the temperature of 1 avoirdupois pound of liquid water by 1 degree Fahrenheit at a constant pressure of one atmospheric unit.[4] There are several different definitions of the Btu that differ slightly. This reflects the fact that the temperature change of a mass of water due to the addition of a specific amount of heat (calculated in energy units, usually joules) depends slightly upon the water's initial temperature. As seen in the table below, definitions of the Btu based on different water temperatures vary by up to 0.5%.
| Variant | Energy (J) | Notes |
|---|---|---|
| Thermochemical | ≈1,054.35[a] | Originally, the thermochemical Btu was defined as the heat required to raise the temperature of one pound of water from its freezing point to its boiling point, divided by 180 (the temperature change being 180 °F). The basis for its modern definition in terms of SI units is the conceptually similar thermochemical calorie, originally defined as the heat required to raise the temperature of one gram of water from freezing to boiling divided by 100 (the temperature change being 100 °C). The thermochemical calorie is exactly 4.184 J by definition of the International Organization for Standardization (ISO).[4] The thermochemical Btu is calculated by converting from grams to pounds and from Celsius to Fahrenheit.[5] |
| 59 °F (15.0 °C) | ≈1,054.80[6] | Used for American natural gas pricing.[3] |
| 60 °F (15.6 °C) | ≈1,054.68[7] | Mainly Canadian.[citation needed] |
| 39 °F (3.9 °C) | ≈1,059.67[7] | Uses the calorie value of water at its maximum density (4 °C, 39.2 °F).[citation needed] |
| IT | ≈1,055.06[b] | An early effort to define heat units directly in terms of energy units, and hence to remove the direct association with the properties of water, was taken by the International Steam Table Conferences. These conferences originally adopted the simplified definition that 860 "IT" calories corresponded to exactly 1 international watt-hour (not the same as a modern watt-hour). This definition ultimately became the statement that 1 IT calorie is exactly 4.1868 J.[4][8] The Btu is then calculated from the calorie as is done for the thermochemical definitions of the BTU and the calorie, as in International standard ISO 31-4 Quantities and units—Part 4: Heat and British Standard BS 350:Part 1:1974 Conversion factors and tables.[5][9][10] |
Units of kBtu are used in building energy use tracking and heating system sizing. Energy Use Index (EUI) represents kBtu per square foot of conditioned floor area. "k" stands for 1,000.[11]
The unit Mbtu is used in natural gas and other industries to indicate 1,000 Btu.[12][13] However, there is an ambiguity in that the metric system (SI) uses the prefix "M" to indicate 'Mega-', one million (1,000,000). Even so, "MMbtu" is often used to indicate one million Btu particularly in the oil and gas industry.[14]
Energy analysts accustomed to the metric "k" ('kilo-') for 1,000 are more likely to use MBtu to represent one million, especially in documents where M represents one million in other energy or cost units, such as MW, MWh and $.[15]
The unit 'therm' is used to represent 100,000 Btu.[12] A decatherm is 10 therms or one MMBtu (million Btu). The unit quad is commonly used to represent one quadrillion (1015) Btu.[14]
One Btu is approximately:
A Btu can be approximated as the heat produced by burning a single wooden kitchen match or as the amount of energy it takes to lift a one-pound (0.45 kg) weight 778 feet (237 m).[16]
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Main article: Natural gas prices |
The SI unit of power for heating and cooling systems is the watt. Btu per hour (Btu/h) is sometimes used in North America and the United Kingdom - the latter for air conditioning mainly, though "Btu/h" is sometimes abbreviated to just "Btu".[18] MBH—thousands of Btu per hour—is also common.[19]
The Btu should not be confused with the Board of Trade Unit (BTU), an obsolete UK synonym for kilowatt hour (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/kWh (3.1 kWh/kWh), an efficiency of 32–33%.[24]
The centigrade heat unit (CHU) is the amount of heat required to raise the temperature of one pound of water by one Celsius degree. It is equal to 1.8 Btu or 1,899 joules.[25] In 1974, this unit was "still sometimes used" in the United Kingdom as an alternative to Btu.[26]
Another legacy unit for energy in the metric system is the calorie, which is defined as the amount of heat required to raise the temperature of one gram of water by one degree Celsius.[27]