I had a small Intex metal frame pool that had 1/2 inch of blue foam board on the walls and 1 inch hard foam (stuff with aluminum on one side) for the floor and had two layers of 1/2 inch blue basically floating on the top underneath the stock Intex cheap cover and with an 11KW electric heater. Once it was heated (I was iin the 89 to 91 degree point) it took nothing to keep it heated because I had insulation saving my butt.
Using non contact thermometer, the 1/2 wall insulation seemed adequate. Very little heat loss from walls once insulated. An overwhelming amount of heat was comming off the top of the pool when heated. 1/2 made a huge difference, and another 1/2 inch on top of that made heat loss a non issue. I cut my 4' x 8' sheets into sizes the wife could easily lift out and either lay on the patio or put into a holder I built specifically to hold them.
I would like to address size. If it was cool to cold outside, my little 11KW heater would not keep up with the heat loss on a little 12' Intex pool once uncovered. But it was only noticable with temps below 50 degress or if there was a good wind, pretty much anything below 60, may 65 it could become noticeable if you stayed outside for say over an hour in the pool. I would recommend getting as big a heater as you can afford ESPECIALLY if you think you are going to get a bigger pool in the future. Keep the chemistry in the pool as in the middle as you can so you can get as much life as you can out of heater components. And yes, I use salt too.....testing is cheap......repairing can be expensive. I also insulated almost all surfaces of my plumbing too to keep heat loss to the minimum possible. Other than the initial heat up, since the wife didn't yell at me for electricity used, I cost me maybe 20-40 dollars a month to heat my pool. The key point is here, I was keeping eye on rates and usage and other than the first month that we turned the thing on, I could for sure see any "real" difference in our electricity bill. I think my heater was under $400 and was good option for me as I may only have two more years at the most of pools in the backyard as kids get older.
Bob