Air Source Units - SEER (Seasonal Energy Efficiency Ratio) - The total cooling of a central unitary air conditioner or unitary heat pump in BTU's during its normal annual usage period for cooling by the total electric energy input in watt-hours during the same period.

Geothermal Units - EER (Energy Efficiency Ration) - A ratio calculated by dividing the cooling capacity in BTU's per hour (Btuh) by the power input in watts at any given set of rating conditions, expressed in Btuh per watt (Btuh/watt).

EER and SEER can not be compared equally. Air source equipment is rated by SEER and geothermal equipment is rated by EER. EER changes with the inside and outside conditions, falling as the temperature difference between inside and outside gets greater.

Air Source Only - HSPF (Heating Seasonal Performance Factor) - is specifically used to measure the efficiency of air source heat pumps. The efficiency of air conditioners are often rated by the HSPF as defined by the Air Conditioning, Heating.

The higher the HSPF rating of a unit, the more energy efficient it is. HSPF is a ratio of BTU heat output over the heating season to watt-hours of electricity used. It has units of BTU/watt-hr.

Depending on the system, an HSPF ≥ 8 can be considered high efficiency and worthy of a US Energy Tax Credit.

The HSPF is related to the non-dimensional Coefficient of Performance for a heat pump, which measures the ratio of heat energy delivered to electrical energy supplied, independently of the units used to measure energy. The HSPF can be converted to a seasonally-averaged COP by converting both the BTU heat output and the electrical input to a common energy unit (e.g. joules). Since 1 BTU = 1055 J, and 1 watt-hour = 3600 J, the seasonally-averaged COP is given by:

Avg COP = Heat transferred / electrical energy supplied = HSPF * 1055 J/BTU / 3600 J/watt-hour = 0.293 HSPF.

Thus, a system which delivers an HSPF of 7.7 will transfer 2.25 times as much heat as electricity consumed over a season.