I'm in southern california and my batch heater, that supplies my 'garden shower' (which I just got out of) gets to 140*F after a day in the sun. I just got this going a few months ago, so I don't know about the fall temps. Without closing the insulated panel at night I still have readings of 115* in the morning. This is a 40 gal. waterheater tank inside an insulated plywood box, angled toward the sun, with a patio door covering the top.