Relevant Efficiency Metrics

Proper selection of heat pump systems requires a thorough understanding of their performance at a range of outdoor temperatures. Heat pump efficiency is based on the heating or cooling delivered, compared to the electrical energy required to operate the system. Separate ratios and different terminology are used to characterize heat pump heating and cooling efficiency, both in terms of seasonal efficiency and efficiency at particular operating conditions. The following values are found on the energy rating label or manufacturer's information:

HEATING

Coefficient of performance (COP): The ratio of heating delivered, compared to the electrical energy input. COP varies based on operating conditions (e.g. temperature).

Heating seasonal performance factor (HSPF): Characterizes the average efficiency of an ASHP over a typical heating season. This calculation assumes standard operation, which includes defrost cycles.

Cooling

Energy efficiency ratio (EER): The ratio of cooling delivered, compared to the electrical energy input. EER varies based on operating conditions (e.g. temperature).

Seasonal energy efficiency ratio (SEER): Characterizes the average efficiency of an ASHP over a typical cooling season. This calculation includes the heat generated by the system components while in operation.

For a variety of reasons, the actual performance of heat pumps differs from their rated efficiency. Factors that influence the actual efficiency of a heat pump include but are not limited to the following:

• Local climate, heating and cooling loads, and source and supply temperatures, which likely differ from the testing conditions
• Local exposure to solar radiation and wind
• Auxiliary energy consumption (e.g. pumps and
  fans)
• Heat pump sizing relative to heating and
  cooling loads
• Operating characteristics, including occupancy
  and setback schedules
• Installation issues and/or poor compatibility with existing systems