The energy efficiency of an air conditioner is measured by the energy efficiency ratio, which ranges from 8.0 to 11.5. This ratio determines efficiency by indicating the amount of cooling capacity for each watt used. The higher the efficiency rating, the more efficient the unit.
Air conditioner units with an EER of 10.0 or greater have the ability to cut energy costs considerably. Although national appliance standards set the EER range between 8 and 11.5, window air conditioners must have a high EER rating to be officially ENERGY STAR-qualified. The U.S. Environmental Protection Agency's ENERGY STAR qualification confirms an above-average EER.