We are having an 18x36 rectangular pool installed Spring 15. We 95% sure we are getting a gas heater. We have propane heat and will hook right up to our existing tank. We are rated for a Hayward H250 heater (250,000 BTU). But I am starting to think of getting the 400,000 BTU unit. We plan to use the heat ONLY for quick heat ups. We truly plan to use our solar cover most of the time to heat the pool and keep heat in. But we want the flexibility to heat the pool quickly when needed. I would think the 400,000 would heat the fastest therefore be more efficient? (cost saving)?