We have 18x33x52" and use 3 of the 2x20s - our pool at covering time 7:30pm last night was 87 degrees. We are as of this morning on the diverter because we do not want it any warmer. Like Matt, we ended up with a pool in the 90s last summer (one 2x20 panel on our old pool, an 18' round) and that is not refreshing. We are in FAR NW Minnesota - usually referred to as "The Nation's Icebox" - we would not be without our solar panels.