Definitions

COP (Coefficient Of Performance): A heat pump may use only one third as much energy as electric resistance heat. COP is the ratio of heat output to electrical energy input. When the temperature is 40 degrees Fahrenheit, the COP may be 3.0; when the temperature is 20 degrees Fahrenheit, the COP will be closer to 2.0. The higher the COP, the more efficient the system.

HSPF (Heating Season Performance Factor): The test for overall heating efficiency provides a rating known as the HSPF. This laboratory test attempts to take into account the reductions in efficiency caused by defrosting, temperature fluctuation, supplemental heat, fans, and on/off cycling. The higher the tested HSPF, the better the heating efficiency for the heat pump. Most heat pumps now have at least a 6.8 HSPF rating, with some rated over 9.0.

SEER (Seasonal Energy Efficiency Ratio): Cooling performance is rated using the SEER. The higher the SEER the more efficiently the heat pump cools. The SEER is the ratio of heat energy removed from the home compared to the energy used to operate the heat pump (including fans). The SEER is usually noticeably higher than the HSPF since defrosting isn't needed and there is no need for supplemental heat during air conditioning weather.

Supplemental Heat: As it gets colder outside, the heat pump provides less heat, yet the home needs more heat to remain comfortable. To make up the difference, heat pumps have a supplemental heating system, usually electric resistance coils. Since resistance heat doesn't operate with the same efficiency (the COP of electric resistance heat is 1.0), the total heat pump COP will be lower when the supplemental heat is on.

Defrost: When temperatures get below 40 degrees Fahrenheit, ice may form on the coils and air-source heat pumps may need to defrost periodically. To melt the ice, the heat pump automatically takes heat from the home to heat up the outdoor coils.