What is a BTU and how is it measured?
A British thermal unit (BTU) is a measure of the heat content of fuels or energy sources. It is the quantity of heat required to raise the temperature of one pound of liquid water by 1 degree Fahrenheit at the temperature that water has its greatest density (approximately 39 degrees Fahrenheit). One British thermal unit (Btu) is approximately equal to the energy released by burning a match.

The easiest way to understand what BTUs are is to to calculate the number of BTUs needed to bring 12 gallons from 150 degrees F to boil – and the calculation is: 1 (BTU) X 12 (gallons) X 8.3 (weight of one gallon) X 62 (degrees difference) = 6175 BTUs.