AFUE (Annual Fuel Utilization Efficiency) - The minimum efficiency standard for furnaces is 78% AFUE. The minimum for gas-fired water boilers is 80% AFUE and the minimum for gas-fired steam boilers is 78% AFUE. The standard is set by the U.S. Department of Energy under the National Appliance Energy Conservation Act of 1987, which became effective January 1, 1992.

HSPF (Heating Season Performance Factor) - Heating efficiency for air source heat pumps is indicated by the heating season performance factor (HSPF). The HSPF tells you the ratio of the seasonal heating output in Btu's divided by the seasonal power consumption in Watt-hours.

SEER (Seasonal Energy Efficiency Ratio) - This ratio gives an indication of the performance efficiency of the system. The higher the SEER, the more efficient the unit, which in turn lowers operating costs. As of 2009, federal regulations require all new central air conditioners have a SEER of at least 13.