Thank you for writing to me. You will need more than one heater. I can have the manufacturer do a computer report for you. I need to know what temperature you want to keep the water at. Will you use a blanket at night and what months do you want to use the pool? All year? Is the pool in the sun or maybe on a lake? Remember that gas heaters and heat pumps are sized differently. We use smaller heat pumps and have them run longer to get the most out of the "free" heat on the air. Also a 400,000 BTU gas heater really only puts 320,000 BTU's in the water as the rest goes up the exhaust. The COP is less at 50 degrees. It is linear. so manufacturers rate them there and some don't. It has been my experience that is a heat pump has a 6 COP at 80 degrees it will have about a 4 or 4.5 COP at 50 degrees. That is still so many more times more efficient than gas. I hope that helps and that I hear back from you soon.
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