What factor is taken into account by HSPF when measuring efficiency?

HSPF, or Heating Season Performance Factor, specifically measures the efficiency of air-source heat pumps during the heating season by taking into account the variations in temperature throughout that season. This measure is crucial because heat pumps operate differently under varying environmental conditions; their effectiveness can significantly diminish in colder temperatures. By considering the temperature fluctuations, HSPF provides a more accurate representation of a heat pump's performance and energy efficiency over the course of an entire heating season, allowing consumers and professionals to make better-informed decisions regarding heating systems.