You will have to get a hot gas defrost model (also called heat and cool modes). If you maintain your pool heat in the high 70's to mid 80's, you'll not have too much trouble (or take that much time) to get it up to 100 or so degrees.
The Heat Pump makes it cost effective to maintain your pool and spa. Do you use a solar blanket? It will help your heat loss tremendously if you do.
Just keep it covered overnight when it gets cool out.