April 5th, pool water was 53 degrees. I turned on the heater, roughly 4-5 hours a day, had a couple nice 80 sunny degree days, and it was 90 by April 12th. I maintained 90 degrees the entire month, and kept the solar cover on overnight, with the exception of a couple of nights. Pool is ballpark 16k gallons, heater is a Raypak 330k BTU natural gas heater. Gas bill for April 2019 was $250 more than April 2018...so $250 to heat the pool. I used a total of 338 therms (previous monthly averages are low 30s)...so assume about 300 therms for the heater. A "therm" is 100k BTU.
From what I can tell so far in May, the heater isn't really running much. I find numbers helpful, and hope this might help someone trying to decide about a heater. Last summer, when I told people I was putting in a heater I heard everything from "that is going to cost you a fortune" to "you will never use it...". Wrong and wrong for us. $250 for 90 degree water...we have been in the pool daily for 4 weeks, and I anticipate us getting until the end of October without any issues. Warm water is the difference between a chilly dip and hanging out in the pool for a couple hours playing games with my sons.