I am forced to reply to myself again before my parent comment gets downvoted to oblivion. YES - Heat pump efficiency drops with external temperature. HOWEVER

I am forced to reply to myself again before my parent comment gets downvoted to oblivion. YES - Heat pump efficiency drops with external temperature. HOWEVER - A typical heat pump will be unable to operate at full power when it approaches it's lower temperature limit. It is unable to move more heat at any efficiency. The compressor will slow down or stop completely while waiting for the liquified refrigerant to vaporize in the evaporator coils. THEREFORE - Regardless of efficiency the compressor will be using less power because it is either not running or running slowly. WITH THE EXCEPTION OF - certain heat pumps with auxiliary electrical heaters that cause energy use to spike at extreme low temperatures.