I jut had a solar company out and they told me with my pool facing north under a screen enclosure NOT using a pool blanket that they can put up about 500 sq feet of panels (1/2 on the north roof, 1/2 on west roof - full sun all day) I can get about 13 degree increase in the temp of the water over the temp of the air (13,500 gallon pool).

They stated that I can get more by placing more panels on east/west/south facing side or by using a pool blanket.

This is with the helio-panels (sp?) without any risers (bad for hurricanes).

BTW - neighbor had these installed and they have to turn off the solar already because it's too hot! Avergage daily temp day = 90's and avergage nightly temp = 70's