Since I am about the only one stupid enough to heat an AGP with resistive heat, let me add my two cents......insulation.
Everyone is correct that any type of ordinary domestic water heater will not heat any sized AGP worth a Crud unless the sides are insulated and then and only then if foam insulation is floating on top of the water...mainly to prevent heat loss through the top of the pool. How do you say I know this? Well been there done that......started with a small Intex kiddie pool, used separate pump and point of use water heater. As soon as you uncovered the pool, the heat loss would start to show within 10 minutes. Could not and would not keep up with the heat loss. Used non-contact thermometers all around the pool trying to cut heat losses.
I now have a 14 foot Intex Ultra Frame pool with one inch of the rigid blue foam I got from lowes. Bottom, top and sides all have one inch on them. I take the top insulation off and store the pieces in a handly PVC storage rack I built so it won't blow around. Basically the pool is covered unless I am using it. It takes very, very little energy to keep the pool heated this way. I have the Little Giant 11KW pool heater. Yes you can see from the usage diagram the electric company sends out when the pool is open and used, particularly when it's warm to cool outside and heating is needed. This year I went from 24/7 pump operation to around 8 hours a day. Since I am covered, I could do even less than that.
Now that it's very, VERY cold down here I did notice as I finally decided to lower the temp on the pool (no fun to get out of a nice warm pool only to step on a frozen patio) that I lost 16 degrees of temp from last wed night at 7pm to friday night at 7pm. It was below freezing most if not all that time. Insulation works.
And for the record, the regular solar cover or 1/2 inch of blue insulation will still allow a great amount of heat to leave your pool. Only when I went to 1" did the heat loss basically go to nothing. Use a non contact thermometer and you can see what I mean.
So if you are not going to go to the extremes like I did to cut heat loss, you HAVE to use a natural gas/lp pool heater or heat pump pool heater to do any good. There is just not enough BTU's with an ordinary hot water heater or point of use heater. I went resistive because I didn't think I would have a pool much over a year or two (which is now 4), and if I go bigger, then I will dig the trench and install the gas line for the big natural gas heater. To keep myself from going broke, I insulated the pool, something some if not all of you wouldn't undertake.
Finally, we were out in the pool a couple of weeks ago, but here in Oklahoma, wind is our biggest enemy. The pool was nice, everything was great except for the cooling effect of the wind. It can be cold or cool here and getting out of the pool with no wind is tolerable. My pool is more like a giant hot tub more than a swimming pool.
Bob E.