How big is your pool? How much do you want to heat it? How many hours of sunshine do you have?

Recommended panel size can be two to three times larger than you need. If you plan it correctly, should you need to expand it, you can, but you may be very happy with less.

To give you an idea of HOW minimal it can be, my parents had an 18'x33' oval AG. It got about 6 hours of direct sun a day, and rarely reach 80deg even with a solar cover. "The Rules" said I should heat it with 3 20'x4' panels. I used 1 20'x4' and 1 10'x4'. Even with only six hours of direct sun a day, the water stayed at 84deg. They worked for hours after the sun passed because they were on the ground and the ground baked and released its heat too.

Remember: if the water coming from the panels is even 1deg warmer than the pool, the panels are adding BTUs to your water, and BTUs are the key to heating your pool.

My pool has 660 sq ft of surface area and the panels have 220 sq ft and it's plenty--sometimes too much--I frequently have water in the high 90's during heat waves--and I live in northern NJ.