If you go play with the swimming pool energy calculator provided by the Department of Energy (your tax dollars at work, find it through Google), you will learn that a solar cover really doesn't do that much in terms of "insulating" the pool at night. How it helps is in preventing evaporation of the pool water - around 90 percent of the energy lost is due to evaporation. That's why it doesn't make sense to insulate the sides of an AG pool - free or mildly forced convection can't compete with the evaportation. Next time you swim after the cover has been off for while at night, notice that the top couple of inches are much cooler than the rest of the pool, even colder than the outside air. That's Thermodynamics in Action!

And yes, I have a Ph.D. in Mechanical Engineering, so I'm kind of a geek about this sort of stuff. My apologies if your eyes are glazed over!

Doug