At what temperature do heat pumps become inefficient?

At what temperature heat pumps become inefficient depends on type, but standard single stage heat pumps become inefficient below 30 to 35 degrees Fahrenheit, while cold climate variable speed heat pumps remain efficient down to minus 5 to minus 13 degrees. Heat pump efficiency measured by Coefficient of Performance COP, where COP of 3 means 3 units heat for 1 unit electricity, at 47 degrees outdoor, typical heat pump COP is 3.5 to 4, efficient. At 30 degrees, COP drops to 2.5 to 3, still efficient but less heat output, requiring backup heat. At 17 degrees, standard heat pump COP drops to 2 to 2.5, still 200 percent efficient versus electric resistance at 100 percent, but capacity drops to 60 to 70 percent of rated, causing auxiliary electric heat strips to turn on at 3 to 10 kW that cost 3 times more, making overall system less efficient and expensive. At 0 degrees, standard heat pump COP around 1.5 to 2, still better than electric heat but low capacity, so inefficient compared to gas furnace. Cold climate heat pumps like Mitsubishi Hyper Heating H2i Plus, Trane XV20i, Carrier Infinity 20, and Daikin Emura maintain COP of 2 to 2.5 down to 0 degrees and work down to minus 13 degrees without backup, remaining efficient. For gas furnace comparison, heat pump becomes less efficient than 96 percent AFUE gas furnace below about 25 to 30 degrees when gas cheaper than electric. In moderate climate, heat pump efficient year-round, in cold climate need dual fuel with gas backup below 30 degrees.