Heat Pump Efficiency
As we saw earlier, a heat pump may use only one-third as much energy as electric resistance heat (electric furnace and baseboards, for example) during mild winter weather (outdoor temperature about 45 degrees F). In the heat pump industry, this is described as a COP (Coefficient of Performance) of 3. COP is the ratio of heat output, to electrical energy input. A number of factors prevent air-source heat pumps from maintaining COPs of 3 throughout the heating season:
Air temperature	Heat pumps operate at temperatures colder than 45 degrees F much of the winter. When the temperature is 20 degrees F the COP of the heat, pump will be closer to 2 than 3.
Defrost	Because there is very cold refrigerant flowing through the outdoor heat exchanger, ice can form on the coils, just as it does in freezers. When outdoor temperatures get below 40 degrees F the heat pump may need to defrost periodically. To melt, the ice, the heat pump takes heat from the house to heat the outdoor coils. This reduces average heat pump efficiency.
Supplemental Heat	As it gets colder outside, the heat pump provides less heat. Yet the house needs more heat to keep comfortable. At some outdoor temperature it will be too cold for the heat pump to provide all the heat the house needs. To make up the difference, heat pumps have a supplemental heating system - usually electric resistance coils (basically an electric furnace inside the heat pump indoor cabinet). This part of the system is sometimes called "back-up" or "emergency" heat because the same coils can be used to provide some or all the heat in the event of heat pump failure. Since the supplemental electric heating system doesn't operate with the same efficiency as the heat pump (the COP of electric resistance heat is 1), the total heat pump COP will be much lower when the supplemental heat is on. Gas and oil furnaces provide supplemental heat in some new homes with heat pumps. Existing gas and oil furnaces can also be used as supplemental heat with "add-on" heat pumps that allow a heat pump to be added to an existing system. Controls for these systems are different since the combustion system and the heat pump don't operate at the same time. Special care is required to make sure there are proper air flows for both the heat pump and the furnace. The economics of purchasing and operating this type of system will depend on local energy costs.
Cycling Losses	When heating systems first come on, they need to operate for a while just to get warm enough to heat the house. When they are shut off, there is still heat in the system that doesn't get into the house. The losses associated with stopping and starting the heat pump are referred to as "cycling losses."
HSPF	The industry standard test for overall heating efficiency provides a rating known as HSPF (Heating Season Performance Factor). This laboratory test attempts to take into account the reductions in efficiency caused by defrosting, temperature fluctuations, supplemental heat, fans and on/off cycling. The higher the HSPF, the more efficient the heat pump. A heat pump with an HSPF of 6.8 has an "average COP" of 2 for the heating season. To estimate the average COP, divide the HSPF by 3.4. Don't assume the HSPF will be an accurate predictor of your actual installed performance. HSPF assumes specific conditions that, are unlikely to coincide with your climate. West of the Cascade mountains your actual performance with a well-designed and installed system may approach the HSPF rating. But east of the Cascades your actual performance may be somewhat lower due to colder winter air temperatures. Most utility-sponsored heat pump programs require that heat pumps have an HSPF of at least 6.8. Many heat pumps meet this requirement. Some heat pumps have HSPF ratings above 9. In general, more efficient heat pumps are more expensive. Compare the energy savings to the added cost.
SEER	Cooling performance is rated using the SEER (Seasonal Energy Efficiency Ratio). The higher the SEER the more efficiently the heat pump cools. The SEER is the ratio of heat energy removed from the house 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 expensive supplemental heat during air conditioning weather. Unless you live in an area where cooling is more important than heating the HSPF is a more important measure of efficiency than the SEER.

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