BTU, or British Thermal Unit, is a unit of measurement used in heat load calculations. It is used to measure the amount of energy required to raise the temperature of one pound of water by one degree Fahrenheit. Heat load calculations are used to determine the size of the HVAC system needed to adequately heat or cool a space. The calculation takes into account the size of the space, the number of windows, the insulation of the walls, and other factors. The BTU measurement is then used to determine the size of the HVAC system needed to provide the desired temperature.