Heat pumps are nothing more than reversible air conditioners. Their efficiency goes down with the outside temperature. Somewhere around 34 deg F the efficiency drops to 0 - that is, you put one unit of energy into the heat pump and you get one unit out. Below that break even point, (meaning below around 34 deg) you might put in two units of energy (electricity) to get your one unit of heat out. Most heat pumps that were installed for residential heating have auxiliary coils to provide heat when the temp drops low enough. The controls shut off the heat pump and power up the coils. Electric heat is 99% efficient - virtually all the energy you put into the unit comes out as heat. This is in contrast to an old early 1900's steam boiler which might have and efficiency of 30 or 40%, or a modern hot air furnace which can hit efficiencies in the high 90's.