In the role of a heating system, heat pumps are evaluated based on their Heating Seasonal Performance Factor (HSPF2). Higher HSPF2 values correlate with more efficient heating performance, similar to miles-per-gallon ratings for cars. HSPF2 quantifies the amount of heat provided during the heating season relative to the electricity consumed by the heat pump, factoring in fluctuating outdoor temperatures. The Department of Energy sets the minimum HSPF2 efficiency at 7.5 for residential air-source split-system heat pumps. Models surpassing 9 HSPF2 are generally considered highly efficient.