at 18 foot your surface area is 324 sq feet, to raise your temp with solar panels you would need 75 to 100% surface area of your pool. You would have to buy 4 to 8, 2x20 foot solar panels to reach 50 to 100% of your surface area.. your talking 900 for half the surface area of your pool just for panels, and those are the cheap ones that you will have to replace within 5 years.... the nice ones that will last will cost 3000 to 6000 dollars.. if that's not enough you will probably need a new pump setup to make solar panels work unless you place them on the ground.. if you really want or need them get some good ones.. I did a lot of research and decided to go with a 16 mil clear solar cover with reel..