[pfdietz]

I think unless you're in an area dominated by cooling needs, an optimally sized heat pump system will not cover 100% of heating needs. It would make sense to make it smaller and use a backup resistive heater for rare very cold events.

[bluGill]

cooling is more important but not by much. however the real problem is temperature delta: 100f-70f is 30 degrees, 70f-10f is 60 degrees. If you size a system for cooling it can't make up.

of course things arenot actually linear on temperature but as a rough estimate it gets the point across.