SEER refers to an air conditioning system's seasonal efficiency. As applied to a heat pump, it refers to it's efficiency during cooling season, and can only be used to compare to a conventional A/C system.


When quoting seasonal efficiency in heating mode (which is what we should be thinking of when converting to a heat pump), the measure is HSPF. This will give you something to start with when comparing a heat pump to your present heating appliance. But I think the more important thing to consider is the COP curve, which will tell you how it would perform during abnormally cold weather, hence whether or not you will require supplemental heat. What you want to do is begin with a design temperature, the lowest temperature you expect to encounter, then determine the COP at that temperature and size accordingly. Averages only matter in baseball: when it comes to designing an HVAC system, you have to design for the extremes. If the system performs at the design temperature for heating, you may find that the efficiency in cooling season suffers, because the system will be oversize for cooling. This can lead to complicated and costly design choices as you consider supplemental heat in winter vs. summer cooling performance. These things are tricky to engineer, and engineering will ensure your misery or comfort.

Hell is isothermal :D