I can't speak for everybody else, but I can speak of us. Ours is a 13.5 and it kept up acceptably in Vegas at 100+ temps. It may have something to do with level of insulation rather than just plain size of unit. It got up to almost 120 during the day and we never had truly "cold" water, just from the hose exposed to the heat. Even if you let it run and run, it was still warm. But, the AC seemed to keep us comfortable in spite of the ambient temps. We've been in 100 degree + , and humid here at home, and I have no complaints so far.