When selecting an air-source heat pump, consider the following three characteristics carefully: the energy efficiency rating, sizing, and the system's components.

In the United States, we rate a heat pump's energy efficiency by how many British thermal units (Btu) of heat it moves for each watt-hour of electrical energy it consumes. Every residential heat pump sold in this country has an EnergyGuide Label, which features the heat pump's heating and cooling efficiency performance rating, comparing it to other available makes and models.

The Heating Seasonal Performance Factor (HSPF) rates both the efficiency of the compressor and the electric-resistance elements. The HSPF gives the number of Btu harvested per watt-hour used. The most efficient heat pumps have an HSPF of between 8 and 10.

The Seasonal Energy Efficiency Ratio (SEER) rates a heat pump's cooling efficiency. In general, the higher the SEER, the higher the cost. However, the energy savings can return the higher initial investment several times during the heat pump's life.

Replacing a 1970s vintage, central heat pump (SEER = 6) with a new unit (SEER = 12) will use half the energy to provide the same amount of cooling, cutting air-conditioning costs in half. The most efficient heat pumps have SEERs of between 14 and 18.

You'll find the Energy Star© label —- sponsored by the U.S Department of Energy (DOE) and the U.S. Environmental Protection Agency (EPA) —- on heat pumps with an HSPF of at least 7 and a SEER of at least 12. Many new heat pumps exceed these ratings, but looking for this label is a good way to start shopping for one.

Click here to get a DOE Fact Sheet