A very basic measure of a facility’s energy performance is called the Energy Utilization Index (EUI). This is a statement of the number of Btu’s of energy used annually per square foot of conditioned space. To compute the EUI, all of the energy used in the facility must be identified, the total Btu content tabulated, and the total number of square feet of conditioned space determined. The EUI is then found as the ratio of the total Btu consumed to the total number of square feet of conditioned space.