Site energy intensity is the amount of energy (electricity and natural gas) consumed per square foot of gross building area. This energy is measured by meters at the site. To show electricity and natural gas together, they are converted to a common unit, British thermal units (BTU). There are 3,412 BTU per kilowatt-hour (kWh) of electricity. There are 100,000 BTU per therm of natural gas.
Source energy intensity takes into account the fuels consumed in the generation, transmission, and distribution of electricity, as well as the energy losses from storing, distributing, and dispensing natural gas. The national average is that for every BTU of useful electricity used at the point of end use, 3.34 BTU of energy were consumed to create and deliver that electricity. For natural gas, the ratio is 1.047, meaning that slightly more than 1 BTU of energy was actually needed for each 1 BTU used by the end customer. Two buildings may have the same site energy intensity but different source energy intensities if they use a different ratio of natural gas and electricity.
For example, a typical small commercial building might be 5,000 sq. ft. and consume 50,000 kWh of electricity and 2,500 therms of natural gas in a year.
The site energy intensity is calculated as:
For the same building, the source energy intensity is calculated as: