Sorry, Mark, but I have simply found that the 75-100% rule to be INCREDIBLE over-kill. I have NO trouble heating my pool with far less than that. My water surface area is 640 sq.ft. and my panels are 240 sq.ft. (37.5%) and I have nicely warm water all summer--usually ranging from 86 to 94 degrees. I live in NE New Jersey.

I'm not sure what that recommendation is based on--if the sun isn't shining the panels don't work anyway. I'm not really interested in swimming if it's not warm out, so more panels do no good.

I suppose if you want to swim when it's cooler, then you'll need more. But where I live, my panels extend our swim season to May and September (normally non-swimming months here). In April and October, there isn't enough sun, and too much rain to bother with solar heating.