British thermal unit

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 by one degree Fahrenheit (Physical analogue; one four inch, wooden kitchen match consumed completely generates 1 BTU). 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 or BTUH) in the power, steam generation, heating, and air conditioning industries, and also as a measure of agricultural energy production (BTU/kg). 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. In North America, the heat value (energy content) of fuels is expressed in BTUs.

Definitions
A BTU is 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 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 kitchen match or as the amount of energy it takes to lift a one-pound weight 778 ft.

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.

Conversions
One BTU is approximately:
 * 1.054 to 1.060 kJ (kilojoules)
 * 0.293071 W·h (watt hours)
 * 252 to 253 cal (calories, or "little calories")
 * 0.25 kcal (kilocalories, "large Calories," or "food Calories")
 * 25,031 to 25,160 ft·pdl (foot-poundal)
 * 778 to 782 ft·lbf (foot-pounds-force)
 * 5.40395 (lbf/in2)·ft3

For natural gas

 * In natural gas, by convention 1 MMBtu (1 million BTU) = 1.054615 GJ.
 * 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 the number of BTU to volume. 1 standard cubic foot of average 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 yields ≈ 1 MMBTU ≈ 1 GJ
 * 1 ft3 of natural gas yields ≈ 1000 BTU
 * For natural gas price conversion 1000 m3 ≈ 36.906 MMBTU and 1 MMBTU ≈ 27.096 m3

As a unit of power
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"..
 * 1 watt is approximately 3.41214 BTU/h
 * 1000 BTU/h is approximately 293.071 W
 * 1 horsepower is approximately 2544 BTU/h

Associated units

 * 1 ton 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 freeze 1 ST of water into ice in 24 hours.
 * 1 therm 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. The therm is used to price natural gas.
 * 1 quad (short for quadrillion BTU) is 1015 BTU, which is about one exajoule ($1.055 J$). 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).

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%.