We are replacing a AguaCal electric pool heater . Looking at the scroll compression heater by Raypak (Rheem) and want to know a formula for BTU's. I live in SW Florida and find the electric heaters do the job since we don't have access to natural gas. Help...Width 13 LN FT Length 30 LN FT perimeter at waterline is 74 LN FT and my water surface area is 318 SQ FT. The last pool heater lasted 13 years...want 13 more so any suggestions would be helpful, thanks!