The Heating Seasonal Performance Factor (HSPF2) rating measures energy efficiency during the heating season. It is calculated by measuring the total heating output of a heat pump over an entire normal heating season, divided by the total electric energy input. A higher HSPF2 indicates a more efficient heat pump. Similar to SEER2 for cooling efficiency, HSPF2 is an important metric for evaluating the efficiency of a heat pump, especially in regions with long and cold winters.