The best performance
A properly sized system will give you 10 to 12F better than an unheated pool. That spread will shrink after a number of extremely warm, sunny days, and after a number of cool, rainy days.
A solar system should be sized to give you 80F (28C), or whatever your heating objectives are, on the first sunny day, after a cold spell, whenever the daytime high reaches 72F (22C) under sunny skies, as long as nightime lows stay above 50F (10C). Such a system will give you the right conditions for swimming, as soon as air temperatures become inviting for a swim. You can expect 80F from May 1st to September 5th in your above-ground pool; you will get the same spread with an inground pool, only one week later at both extremes of the season.