Denise,
With 20,000 gallons, you'd want to look for at least 100,000 BTUs from the heat pump. BIGGER is BETTER.
Titanium is the way Heat Pumps are going because of the chemical resistivity of the titanium over the copper heat exchanger.

A Solar cover will help out immensly!

Solar panels will work also, but you've got to consider where to place them, with a Southeastern exposure.