Site Energy vs. Source Energy

The report cards for individual buildings show information on Site Energy and Source Energy for the most recent 3 years. Site Energy is the energy used at the site which essentially assumes that the energy to make and deliver it to the site is 100% efficient. However, for coal and other forms of carbon based energy that is used to make electricity, the production and delivery of that electricity is not 100% efficient. For example, it takes roughly 3000 BTUs (British Thermal Units – energy in calories to raise 1 gram of water by 1 degree centigrade) of combustion energy in coal to make 1000 BTUs of electricity from coal. The other 2000 BTUs are rejected at the electric power plant in the form of waste heat. Source energy is a measure of the energy value of the coal and other forms of energy that are used to make electricity. For the purposes of reporting realistic energy impact, especially the carbon impact, source energy is clearly the best measure.

Energy Star ratings for buildings are based on Source Energy (though the reported measure is often Site Energy because it is easier to determine). Please visit the EPA’s site for additional information.