Here in middle TN, we had a warm April then cold May. I run a 4 x 20 panel on my 18' AG pool. Panels are elevated on racks in the yard (about 3 feet up) and get sun from about 9 am until 5 pm. They run from north to south. With the highs in the low to mid 80's, my pool gained from 75 degrees to 92 in about a week. I know this is not typical. (Friends pool in same town is just now hitting 82.) They were definately the best $200 I ever spent. I also use a 16mil clear solar cover, and it definately helped. For 2 weeks our highs were only in the 60's with almost no sun (40's at night) and alot of rain yet my pool was still at 75.