The British thermal unit (BTU or Btu) is a unit of energy used in the power, steam generation, and heating and air conditioning industries. Although it's still used 'unofficially' in metric English-speaking countries (such as Canada, the United Kingdom, and sometimes in New Zealand), it's increasingly an outmoded and outdated unit of measure. Elsewhere (and always in scientific use) the BTU has been replaced by the SI unit of energy, the joule (J). In North America, the term "BTU" is used to d… (More on Btu)