At what temperature does a heat pump become inefficient?

A standard air-source heat pump loses capacity and COP steadily as it gets colder, and it becomes markedly less efficient once the outdoor temperature drops below its balance point — typically 25–40°F — because auxiliary electric-resistance heat (COP of 1.0) starts filling the gap. Cold-climate variable-speed units hold efficiency much lower, often near 0°F. The balance-point calculator pinpoints where your specific unit crosses over.