Final answer:
A heat pump system at 45°F ambient is more efficient than electric resistance heating because it can move multiple units of heat for each unit of electricity, despite its efficiency decreasing in lower temperatures.
Step-by-step explanation:
A heat pump system, operating at 45°F ambient, may be more efficient than electric resistance heat. The reason for this is because a heat pump uses work to transfer energy by heat from a warmer environment to a colder environment, which is generally more energy-efficient than generating heat through electric resistance. Electric resistance heating converts electricity directly into heat with a 1:1 efficiency ratio. On the other hand, heat pumps can move multiple units of heat for each unit of electricity used to operate them. However, as the ambient temperature drops, the efficiency of a heat pump can decrease. Nonetheless, at 45°F, a heat pump would typically still be more efficient than an electric resistance heater.
It is important to note that while heat pumps are efficient, they have certain limits. As the outside temperature decreases, particularly below about -10°C (14°F), the energy used to operate them can become greater than the heat furnished. Consequently, they are more suitable for mild to moderate climates.