It was originally defined as the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit.
[2][3] A Btu was originally defined as the amount of heat required to raise the temperature of one pound of liquid water by one degree Fahrenheit at a constant pressure of one atmospheric unit.
This reflects the fact that the temperature change of a mass of water due to the addition of a specific amount of heat (calculated in energy units, usually joules) depends slightly upon the water's initial temperature.
Energy Use Index (EUI) represents kBtu per square foot of conditioned floor area.
Figures are quoted in terms of the quantity of heat in Btu required to generate 1 kW⋅h of electrical energy.
[27] Another legacy unit for energy in the metric system is the calorie, which is defined as the amount of heat required to raise the temperature of one gram of water by one degree Celsius.