HSPF, or Heating Seasonal Performance Factor, is the ratio of BTU heat output and the watt-hours of electricity used over the heating season. BTU is a measurement of the energy efficiency of an appliance: how much heat is required to raise the temperature of one pound of water one degree Fahrenheit. So an HSPF rating compares how much heat a heater produces to how much energy it uses. the higher the percentage the more efficient the product. HSPF can range from 6.8 to 10. The lowest HSPF rating for a heating pump to be considered ENERGY STAR rated is 8.2 for single package units and 8.5 for split systems.