Solar heating works. I can get as much as 8 to 10 degrees in a day, if it's a hot clear sunny day. If I put on the solar cover that gets me that much.

I find if you have solar panels with a surface area equal to 1/5 the surface area of the pool you'll get a noticeable effect. Go up to 1/3 to 2/5s of the pool's Surf area and you'll get results like mine. Go up to 1/2 and your water should be VERY comfortable. Go to 1:1 and you can swim all summer in Canada!