British thermal unit

British thermal unit

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The British thermal unit (symbol BTU or Btu) is a traditional unit of energy
Energy
In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...

 equal to about 1055 joule
Joule
The joule ; symbol J) is a derived unit of energy or work in the International System of Units. It is equal to the energy expended in applying a force of one newton through a distance of one metre , or in passing an electric current of one ampere through a resistance of one ohm for one second...

s. It is approximately the amount of energy needed to heat 1 pound (0.45359237 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). The unit is most often used in the power
Electrical power industry
The electric power industry provides the production and delivery of electric energy, often known as power, or electricity, in sufficient quantities to areas that need electricity through a grid connection. The grid distributes electrical energy to customers...

, steam
Steam
Steam is the technical term for water vapor, the gaseous phase of water, which is formed when water boils. In common language it is often used to refer to the visible mist of water droplets formed as this water vapor condenses in the presence of cooler air...

 generation, heating and air conditioning
Air conditioning
An air conditioner is a home appliance, system, or mechanism designed to dehumidify and extract heat from an area. The cooling is done using a simple refrigeration cycle...

 industries. In scientific contexts the BTU has largely been replaced by the SI
International System of Units
The International System of Units is the modern form of the metric system and is generally a system of units of measurement devised around seven base units and the convenience of the number ten. The older metric system included several groups of units...

 unit of energy, the joule
Joule
The joule ; symbol J) is a derived unit of energy or work in the International System of Units. It is equal to the energy expended in applying a force of one newton through a distance of one metre , or in passing an electric current of one ampere through a resistance of one ohm for one second...

, though it may be used as a measure of agricultural energy production (BTU/kg). It is still used unofficially in metric
Metric system
The metric system is an international decimalised system of measurement. France was first to adopt a metric system, in 1799, and a metric system is now the official system of measurement, used in almost every country in the world...

 English-speaking countries (such as Canada
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...

), and remains the standard unit of classification for air conditioning units manufactured and sold in many non-English-speaking metric countries.

In North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...

, the term "BTU" is used to describe the heat value (energy content) of fuels, and also to describe the power
Power (physics)
In physics, power is the rate at which energy is transferred, used, or transformed. For example, the rate at which a light bulb transforms electrical energy into heat and light is measured in watts—the more wattage, the more power, or equivalently the more electrical energy is used per unit...

 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
Roman numerals
The numeral system of ancient Rome, or Roman numerals, uses combinations of letters from the Latin alphabet to signify values. The numbers 1 to 10 can be expressed in Roman numerals as:...

 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
Therm
The therm is a non-SI unit of heat energy equal to 100,000 British thermal units . It is approximately the energy equivalent of burning 100 cubic feet of natural gas....

is used representing 100,000 or 105 BTU, and a quad
Quad (energy)
A quad is a unit of energy equal to 1015 BTU, or 1.055 × 1018 joules in SI units.The unit is used by the U.S. Department of Energy in discussing world and national energy budgets. The global primary energy production in 2004 was 446 quad, equivalent to 471 EJ...

as 1015 BTU. Some companies also use BTUE6 in order to reduce confusion between thousands of BTU's vs. millions of BTU's.

Definitions


A BTU is defined as amount of heat required to raise the temperature of one 1 pound (0.45359237 kg) of liquid water by 1 F-change at a constant pressure of one atmosphere
Atmosphere (unit)
The standard atmosphere is an international reference pressure defined as 101325 Pa and formerly used as unit of pressure. For practical purposes it has been replaced by the bar which is 105 Pa...

. As is the case with the calorie
Calorie
The calorie is a pre-SI metric unit of energy. It was first defined by Nicolas Clément in 1824 as a unit of heat, entering French and English dictionaries between 1841 and 1867. In most fields its use is archaic, having been replaced by the SI unit of energy, the joule...

, 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
Calorie
The calorie is a pre-SI metric unit of energy. It was first defined by Nicolas Clément in 1824 as a unit of heat, entering French and English dictionaries between 1841 and 1867. In most fields its use is archaic, having been replaced by the SI unit of energy, the joule...

 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
London
London is the capital city of :England and the :United Kingdom, the largest metropolitan area in the United Kingdom, and the largest urban zone in the European Union by most measures. Located on the River Thames, London has been a major settlement for two millennia, its history going back to its...

, July 1956) to be exactly 4.1868 J
ISO
International Organization for Standardization
The International Organization for Standardization , widely known as ISO, is an international standard-setting body composed of representatives from various national standards organizations. Founded on February 23, 1947, the organization promulgates worldwide proprietary, industrial and commercial...

 
≡ 1055.056 International standard
International standard
International standards are standards developed by international standards organizations. International standards are available for consideration and use, worldwide...

 ISO 31-4
ISO 31-4
ISO 31-4 is the part of international standard ISO 31 that defines names and symbols for quantities and units related to heat.Its definitions include:...

 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 °C) ≡ 1054.804 Chiefly American
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...

. 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
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...

63 °F (17.2 °C) ≈ 1054.6
Thermochemical ≡ 1054.35026444 Uses the "thermochemical calorie
Calorie
The calorie is a pre-SI metric unit of energy. It was first defined by Nicolas Clément in 1824 as a unit of heat, entering French and English dictionaries between 1841 and 1867. In most fields its use is archaic, having been replaced by the SI unit of energy, the joule...

" of exactly 4.184 J

Conversions


One BTU is approximately:
  • 1.054 to 1.060 kJ (kilojoule
    Joule
    The joule ; symbol J) is a derived unit of energy or work in the International System of Units. It is equal to the energy expended in applying a force of one newton through a distance of one metre , or in passing an electric current of one ampere through a resistance of one ohm for one second...

    s)
  • 0.293071 W·h (watt hours)
  • 252 to 253 cal (calorie
    Calorie
    The calorie is a pre-SI metric unit of energy. It was first defined by Nicolas Clément in 1824 as a unit of heat, entering French and English dictionaries between 1841 and 1867. In most fields its use is archaic, having been replaced by the SI unit of energy, the joule...

    s, or "little calories")
  • 0.25 kcal (kilocalories, "large calories," or "food calories")
  • 25 031 to 25 160 ft·pdl (foot-poundal
    Foot-poundal
    The Foot-poundal is a unit of energy that is part of the foot-pound-second system of units, in Imperial units introduced in 1879, and is from the specialized subsystem of English Absolut ....

    )
  • 778 to 782 ft·lbf (foot-pounds-force)


Other conversions:
  • In natural gas
    Natural gas
    Natural gas is a naturally occurring gas mixture consisting primarily of methane, typically with 0–20% higher hydrocarbons . It is found associated with other hydrocarbon fuel, in coal beds, as methane clathrates, and is an important fuel source and a major feedstock for fertilizers.Most natural...

    , by convention 1 MMBtu (1 million BTU, sometimes written "mmBTU") = 1.054615 GJ. Conversely, 1 gigajoule is equivalent to 26.8 m3 of natural gas at defined temperature and pressure. So, 1 MMBtu = 28.263682 m3 of natural gas at defined temperature and pressure, that is 998.12 ft3 .
  • 1 standard cubic foot of natural gas yields ≈ 1030 BTU (between 1010 BTU and 1070 BTU, depending on quality, when burned)
  • As a coarse approximation, 1000 ft3 of natural gas ≈ 1 MMBtu ≈ 1 GJ

Associated units


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.
  • 1 watt
    Watt
    The watt is a derived unit of power in the International System of Units , named after the Scottish engineer James Watt . The unit, defined as one joule per second, measures the rate of energy conversion.-Definition:...

     is approximately 3.41214 BTU/h
  • 1000 BTU/h is approximately 293.071 W
  • 1 horsepower
    Horsepower
    Horsepower is the name of several units of measurement of power. The most common definitions equal between 735.5 and 750 watts.Horsepower was originally defined to compare the output of steam engines with the power of draft horses in continuous operation. The unit was widely adopted to measure the...

     is approximately 2,544 BTU/h
  • 1 "ton
    Ton
    The ton is a unit of measure. It has a long history and has acquired a number of meanings and uses over the years. It is used principally as a unit of weight, and as a unit of volume. It can also be used as a measure of energy, for truck classification, or as a colloquial term.It is derived from...

     of cooling," a common unit in North American refrigeration and air conditioning applications, is 12,000 BTU/h. It is the amount of power needed to melt one short ton
    Short ton
    The short ton is a unit of mass equal to . In the United States it is often called simply ton without distinguishing it from the metric ton or the long ton ; rather, the other two are specifically noted. There are, however, some U.S...

     of ice in 24 hours, and is approximately 3.51 kW.
  • 1 therm
    Therm
    The therm is a non-SI unit of heat energy equal to 100,000 British thermal units . It is approximately the energy equivalent of burning 100 cubic feet of natural gas....

    is defined in the United States and European Union as 100,000 BTU—but the U.S. uses the BTU59 °F whilst the EU uses the BTUIT.
  • 1 quad (energy)
    Quad (energy)
    A quad is a unit of energy equal to 1015 BTU, or 1.055 × 1018 joules in SI units.The unit is used by the U.S. Department of Energy in discussing world and national energy budgets. The global primary energy production in 2004 was 446 quad, equivalent to 471 EJ...

    (short for quadrillion BTU) is defined as 1015 BTU, which is about one exajoule (1.055 × 1018 J). 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 (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%.

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