COP (Coefficient of Performance) expresses a heat pump’s capacity to produce heat against the energy needed to extract it. For example, COP 4 means that 1kW of electricity produces 4kW of heat energy. However, due to differing indoor and outdoor temperatures, you need to know a heat pump’s average efficiency throughout the seasons, night and day. This value is called the Seasonal Performance Factor (SPF). Check the effectiveness of the air heat pump when it is really cold. A good air heat pump will retrieve energy to -20°C.