Hi, I live in Phoenix and have a Pent air heat pump, the pool is an in ground 12,000 gallon and heat the pool all winter with no problem, I keep the pool 90/91 degrees with a cover. WE run the pool from 7am to 12pm and again from 7pm to 12 am and this maintains the temps fine , we do have have a few weeks out of the season that it has a hard time keeping up, the temps will drop into the low 80s and I have to adjust the run times. the worse bill we have runs around $300.00 Dec,Jan , my neighbor has the same results with his pool. we just installed a 20,000 kw solar system so that will help with the bill hope down to zero, we arent in az during the summer so we will bank a lot of credits at the higher rate. Some of my other neighbors have gas heaters and dont even use them in the winter because of the cost .......