I am running 2 2x20 panels on the ground next to the pool. Last year I seen a jump in temp of about 4 to 8 f using a solar cover. The units I am using have a rated output of 1000 btu per sq ft per hour. 1 btu = 1 pound of water 1 degree per hour. I gal of water = 6.3 to 6.6 lb ---> 151 gal of water 1 degree per hour * number of sq ft. For my self that is about 12000 gal of water 1 degree per hour based on 80 sq ft. I can get about 7 hours of sun where I have my panels set up so I the math lets me get 6.28 deg f each day. But in the real world it is not so predictable. Wind, ground temp, and other effect the panel.
Steve