[chocophile]

When people use the term "efficient" for heat pumps, I think they are referring to the lowest temperature at which the heat pump will produce heat. Older technology started to lose efficiency (produce less heat) close to 32 degrees, so you needed another source of heat. Newer technology that is specifically rated for temperatures down to (or below) 0 degrees will continue to provide heat down to those colder temperatures without much additional electricity usage. Most manufacturers show a graph of efficiency vs temperature.