We are considering building a pool and having the solar system as the main source of heat for a 20x40 pool. I know it will shorten the swimming season but we are considering installing propane or a heat pump the following year. What would be the perfect square footage for a solar system? I just don't want to cut myself short, that being said, can you really have too much? The pool builder wants to use Enersol. Is this a good brand? Seems pricey!