EER = Energy Efficiency Ratio

Energy Efficiency Ratio (EER). A ratio of the cooling capacity in Btu/h to the power input value in watts at any
given set of Rating Conditions expressed in Btu/(W·h).

The eer rating is a steady state rating, and does not account for the time the unit operates before reaching peak efficiency. This operating time has an unknown efficiency. It also does not account for shutting the system down at the end of a cycle ( when the thermostat satisfies) leaving a cold coil in the duct. The cold coil continues to absorb heat from the surroundings, not the conditioned space. Refrigerant equalizes to the cold coil that must be pumped out at the beginning of the next cycle. This accounts for some inefficiency at the beginning of the cycle.

SEER = Seasonal Energy Efficiency Ratio

The rating is not complete using the system of EER so a rating of SEER has been developed. This rating is tested and verified by a rating agency, and includes the startup and shutdown cycles. The rating agency is the Air Conditioning Refrigeration Institute ( ARI )

SEER is a mathematically determined ratio of the total cooling capacity during normal periods of operation (not to exceed 12 months) divided by the total electric energy input during the same time period. 

SEER ratings for air conditioning and air-source heat pump systems manufactured today range from 13 SEER to 23 SEER, with the highest numbers indicating the most efficient units that offer the most energy savings year after year.

HSPF = Heating Seasonal Performance Factor