British Thermal Unit
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The British thermal unit (BTU or Btu) is a unit of
heat In thermodynamics, heat is defined as the form of energy crossing the boundary of a thermodynamic system by virtue of a temperature difference across the boundary. A thermodynamic system does not ''contain'' heat. Nevertheless, the term is al ...
; it is defined as the amount of heat required to raise the temperature of one pound of water by one degree
Fahrenheit The Fahrenheit scale () is a temperature scale based on one proposed in 1724 by the physicist Daniel Gabriel Fahrenheit (1686–1736). It uses the degree Fahrenheit (symbol: °F) as the unit. Several accounts of how he originally defined hi ...
. It is also part of the United States customary units. The modern SI unit for heat energy is the
joule (J) The joule ( , ; symbol: J) is the unit of energy in the International System of Units (SI). It is equal to the amount of work done when a force of 1 newton displaces a mass through a distance of 1 metre in the direction of the force applied. ...
; one BTU equals about 1055  J (varying within the range 1054–1060 J depending on the specific definition; 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 BTUs (1 "MMBtu") of heat energy if burned.


Definitions

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 The Fahrenheit scale () is a temperature scale based on one proposed in 1724 by the physicist Daniel Gabriel Fahrenheit (1686–1736). It uses the degree Fahrenheit (symbol: °F) as the unit. Several accounts of how he originally defined hi ...
at a constant pressure of one atmospheric unit. 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%.


Prefixes

Units 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. The unit Mbtu is used in natural gas and other industries to indicate 1,000 BTUs. However, there is an ambiguity in that the metric system (SI) uses the prefix "M" to indicate '
Mega- Mega is a metric prefix, unit prefix in metric systems of units denoting a factor of one million (106 or 1000000 (number), ). It has the unit symbol M. It was confirmed for use in the International System of Units (SI) in 1960. ''Mega'' comes fro ...
', one million (1,000,000). Even so, "MMbtu" is often used to indicate one million BTUs particularly in the oil and gas industry. Energy analysts accustomed to the metric "k" ('
kilo- Kilo is a decimal unit prefix in the metric system denoting multiplication by one thousand (103). It is used in the International System of Units, where it has the symbol k, in lowercase. The prefix ''kilo'' is derived from the Greek word () ...
') 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 $. The unit '
therm The therm (symbol, thm) is a non- SI unit of heat energy equal to 100,000 British thermal units (BTU), and approximately megajoules, kilowatt-hours, kilocalories and thermies. One therm is the energy content of approximately of natural gas a ...
' is used to represent 100,000 BTUs. A decatherm is 10 therms or one MMBtu (million Btu). The unit '' quad'' is commonly used to represent one quadrillion (1015) BTUs.


Conversions

One Btu is approximately: * (kilo
joule The joule ( , ; symbol: J) is the unit of energy in the International System of Units (SI). It is equal to the amount of work done when a force of 1 newton displaces a mass through a distance of 1 metre in the direction of the force applied ...
s) * ( watt hours) * ( calories) * (kilocalories) * 25,031 to 25,160 ft⋅pdl (
foot-poundal The foot-poundal (symbol: ft-pdl) is a unit of energy, introduced in 1879, that is part of the Absolute English system of units, which itself is a coherent subsystem of the foot–pound–second system. Edward F. Obert, Thermodynamics, McGraw-Hi ...
) * ( foot-pounds-force) * 5.40395 (lbf/in2)⋅ft3 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 weight .


For natural gas

* In
natural gas Natural gas (also called fossil gas or simply gas) is a naturally occurring mixture of gaseous hydrocarbons consisting primarily of methane in addition to various smaller amounts of other higher alkanes. Low levels of trace gases like carbo ...
pricing, the Canadian definition is that ≡ . * The energy content (high or low heating value) of a volume of natural gas varies with the composition of the natural gas, which means there is no universal conversion factor for energy to volume. of average natural gas yields ≈ 1030 Btu (between 1010 Btu and 1070 Btu, depending on quality, when burned) * As a coarse approximation, of natural gas yields ≈ ≈ . * For natural gas price conversion ≈ 36.9 million Btu and ≈


BTU/h

The SI unit of
power Power most often refers to: * Power (physics), meaning "rate of doing work" ** Engine power, the power put out by an engine ** Electric power * Power (social and political), the ability to influence people or events ** Abusive power Power may a ...
for heating and cooling systems is the
watt The watt (symbol: W) is the unit of power or radiant flux in the International System of Units (SI), equal to 1 joule per second or 1 kg⋅m2⋅s−3. It is used to quantify the rate of energy transfer. The watt is named after James Wa ...
. 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". ''MBH''—thousands of Btus per hour—is also common. * 1 W is approximately * 1,000 Btu/h is approximately * 1 hp is approximately


Associated units

* 1 '' ton of cooling'', a common unit in North American refrigeration and air conditioning applications, is . It is the rate of heat transfer needed to freeze of water into ice in 24 hours. * In the United States and Canada, the R-value that describes the performance of thermal insulation is typically quoted in square foot degree Fahrenheit hours per British thermal unit (ft2⋅°F⋅h/Btu). For one square foot of the insulation, one BTU per hour of heat flows across the insulator for each degree of temperature difference across it. * 1 ''
therm The therm (symbol, thm) is a non- SI unit of heat energy equal to 100,000 British thermal units (BTU), and approximately megajoules, kilowatt-hours, kilocalories and thermies. One therm is the energy content of approximately of natural gas a ...
'' 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. United Kingdom regulations were amended to replace therms with joules with effect from 1 January 2000. the therm is still used in natural gas pricing in the United Kingdom. * 1 '' quad'' (short for
quadrillion Two naming scales for large numbers have been used in English and other European languages since the early modern era: the long and short scales. Most English variants use the short scale today, but the long scale remains dominant in many non-E ...
 Btu) is 1015 Btu, which is about 1 exajoule (). 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 (BTU), an obsolete UK synonym for kilowatt hour (). 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 , an efficiency of 32–33%. The centigrade heat unit (CHU) is the amount of heat required to raise the temperature of one pound of water by one
Celsius The degree Celsius is the unit of temperature on the Celsius scale (originally known as the centigrade scale outside Sweden), one of two temperature scales used in the International System of Units (SI), the other being the Kelvin scale. The ...
degree. It is equal to 1.8 BTU or 1,899 joules. In 1974, this unit was "still sometimes used" in the United Kingdom as an alternative to BTU. Another legacy unit for energy in the
metric system The metric system is a system of measurement that succeeded the Decimal, decimalised system based on the metre that had been introduced in French Revolution, France in the 1790s. The historical development of these systems culminated in the d ...
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 The degree Celsius is the unit of temperature on the Celsius scale (originally known as the centigrade scale outside Sweden), one of two temperature scales used in the International System of Units (SI), the other being the Kelvin scale. The ...
.


See also

*
Conversion of units Conversion of units is the conversion between different units of measurement for the same quantity, typically through multiplicative conversion factors which change the measured quantity value without changing its effects. Overview The process ...
*
Latent heat Latent heat (also known as latent energy or heat of transformation) is energy released or absorbed, by a body or a thermodynamic system, during a constant-temperature process — usually a first-order phase transition. Latent heat can be underst ...
*
Metrication Metrication or metrification is the act or process of converting to the metric system of measurement. All over the world, countries have transitioned from local and traditional units of measurement to the metric system. This process began in F ...
*
Ton of refrigeration A ton of refrigeration (TR or TOR), also called a refrigeration ton (RT), is a unit of power used in some countries (especially in North America) to describe the heat-extraction capacity of refrigeration and air conditioning equipment. It was o ...


Notes


References


External links

* * {{United States Customary Units Units of energy Imperial units Customary units of measurement in the United States