At what temperature does a heat pump become inefficient?

At what temperature a heat pump becomes inefficient depends on type and model, but generally air source heat pump efficiency drops significantly below 30 degrees Fahrenheit and becomes very inefficient below 0 to 10 degrees, requiring backup heat. Standard single stage heat pump like Lennox ML14XP1 14 SEER has balance point around 30 to 35 degrees, where heat pump alone cannot meet heating load and electric heat strips or gas furnace backup must turn on, which costs 2 to 3 times more to operate. At 47 degrees outdoor, heat pump has COP of 3 to 4 meaning 3 to 4 times more efficient than electric heat, at 17 degrees, COP drops to 2 to 2.5, still twice as efficient as electric heat, at 0 degrees, COP drops to 1.5, and below minus 10 degrees, COP approaches 1, similar to electric heat, making it inefficient. High efficiency variable speed cold climate heat pumps like Trane XV20i, Lennox XP20, and Mitsubishi Hyper Heating H2i Plus maintain efficiency down to minus 5 to minus 13 degrees with COP of 2 at 0 degrees, making them efficient in cold climates. Most heat pumps have low ambient cutoff around minus 10 to minus 15 degrees where they shut off completely and rely on backup. DOE recommends heat pump for climates where winter average above 30 degrees, but cold climate models now work efficiently down to 5 degrees, so below 20 degrees, efficiency declines but still better than electric heat until below 0 degrees.