b.t.u.

(noun) a unit of energy equal to the work done by a power of 1000 watts operating for one hour. Synonyms: kilowatt hour, kW-hr, Board of Trade unit, B.T.U..

(noun) a unit of heat equal to the amount of heat required to raise one pound of water one degree Fahrenheit at one atmosphere pressure; equivalent to 251.997 calories. Synonyms: British thermal unit, BTU, B.Th.U..