I just installed a pair of 2' by 20' panels (80 SqFt total) to supplement the heatpump I have in my 20' x 40' IG (800SqFt surface). I realize that this is only 10% and that to provide all the heat needs I would need 400-600 sqft. But I am only looking at this to provide extra heat and save me some utilities. So far it seems to be doing a reasonable job. I figure this setup can add about 0.3 degrees per day to the pool. While that is not a huge number, it does come out to over two degrees per week which is not bad for almost no cost. The entire setup cost me under $200 for the panels and piping.