Wow. I guess there is no definative answer. I am in Western NY-Buffalo area. My pool is in direct sun morning til night and we're in for a few cloudless days so I want to take advantage. I am just concerned about getting my water up to a comfortable temp. It was 70 yesterday. My youngest couldn't stand it any more and went in! Way to cold for me. We always have it on at night. Just wasn't sure if on/off during day was better. I have a heater, but would rather not pay to heat if I don't have to.