British Thermal Unit (Btu)

       The British thermal unit (symbol BTU or Btu) is a traditional unit of energy equal to about 1,055 joules. It is approximately the amount of energy needed to heat 1 pound (0.454kg) of water (exactly one tenth of a UK gallon or around 0.1198 US gallons) from 39 to 40°F (3.8 to 4.4°C). The unit is most often used in the power , steam generation , heating and air conditioning industries. In scientific context the BTU has largely been replaced by the SI unit of energy , the joule , though it may be used as a measure of agricultural energy production (BTU/kg).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.