We are planning on putting a heat pump on our 24k gallon pool because we have an oversized PV solar on our roof to cover it's operating costs. We are in Southern California and we want to know how big of a natural gas heater we would need if we only needed to heat an 8x5 integrated spa. Do I need a 400,000 BTU heater to effectively heat the spa if it will already be at 85 degrees? I'd love to heat it within about a half hour or so to the upper 90s. Is this possible with a lower BTU heater and if so, what kind is best? And if we want to heat the spa in the winter, how long would it take when the ambient temperature is 53 degrees with the highs being in the high 60s and the lows being in the low 40s (I don't really know what the water temp would be)? The pool is going to be 38x16 and goes from 3.5 to 7 feet. Any advise or suggestions?