Heat pumps can be much more efficiency than other systems. Here in the Northwest, a heat pump can have an efficiency starting at 300%, while electric heating systems have an efficiency of 100%. Oil heat systems range in efficiency from 50% to 85%. Natural gas systems range in efficiency from 50% to 95%.
The efficiency of a heat pump is indicated by Coefficient of Performance or COP. The COP is the ratio of what you get in heat energy from the heat pump divided by what you pay for in electric energy to provide that heat. For example, a COP of 3.0 means for every dollar's worth of heat delivered to your home, you only need to buy $0.33 worth of electricity. With standard heat pumps, as the outdoor temperature decreases, the efficiency and COP of a heat pump decreases. When the outdoor air temperature is 47 degrees, many heat pumps work with COPs in the range of 3 to 3.8. At 17 degrees, COPs are typically 2.8 to 3.4. The higher the COP, the more efficient the heat pump.
In the Pacific NW, a properly sized and well-installed heat pump will have an average COP of 2.75 over the course of a heating season. (Based on Base Model Efficiency)
There are other factors which reduce the efficiency of a heat pump throughout the heating season. For instance, a heat pump's outdoor coils periodically need to be defrosted. This is done by reversing the cycle of the heat pump so that the heat from the house thaws the ice accumulated on the coils.
Efficiency is further reduced whenever the back-up heating system is used. This back-up system can be electric, natural gas or oil and is required during times when the outside temperature is so low that the heat pump is not able to provide enough heat for the house. This is called the balance point.
Heat pumps are also rated by a measurement call the Heating Seasonal Performance Factor (HSPF), which estimates the heat output relative to the energy consumed for the entire heating season. The higher the HSPF the less energy you will need to operate the heat pump. An HSPF of 8 corresponds approximately to an average COP of 3.