Coefficient of Performance (COP)

A heat pump does not make heat, it moves it, which is why a COP above 1 is not a violation of anything — a COP of 4 means every kilowatt of electricity shoves four kilowatts of heat across the wall. Electric resistance sits stubbornly at COP 1.0; a modern air-source heat pump manages 3.5–4.5 at 8 °C outdoors and perhaps 2.0 at −15 °C; a ground-source loop sitting in stable 10 °C earth holds 4–5 all winter, which is exactly why geothermal contractors care so much about loop flow and antifreeze concentration.

The ceiling is Carnot: COP_max = T_hot/(T_hot − T_cold) in kelvin, so a machine lifting heat from 0 °C to 40 °C can never beat 313/40 ≈ 7.8, and real equipment lands at 40–55 % of that. The trap in cold climates is the auxiliary heat strip: a 3 kW resistance element energised alongside the compressor drags the system COP down fast, so the sticker COP and the winter bill tell different stories. Worked example: a unit delivering 12 kW while drawing 3 kW has COP = 12/3 = 4.0, equivalently EER 13.6 or 400 % efficiency in the marketing brochure.