Thank you for writing back to me. I hope that you and your family are well. We like happy endings. That is a great temperature raise in a spa. I usually figure about 18 to 20 degrees an hour for a 500 gallon spa depending on outside conditions. Sorry about the unintended water fountain. You are right that the heat put is affected by the outside temperature. So on a 80 degree day you might have 100,000 BTU out of the heat pump and on a 50 degrees day it might drop to 70,000 BTU's. Even at th lower BTU it is still so much more efficient than gas or oil and it actually uses less electricity at the lower temperatures. Please get a solar blanket for the pool. It is so hard to keep heat in Arizona pools at night without something ion the water. The low humidity causes warm water to evaporate fast and it sucks so much heat out of the pool. Thank you for the feedback.