Though I live in southern Arizona, neglecting to put my solar cover on at night will lead to a loss of anywhere from 2-4 degress the next day. Probably a combo of extremely low humidity and cool evenings regardless of high day time temps. As an example, we've been averaging about 97 degree day time temps, but evenings/over night temps are around 68-70 degrees.

My solar cover has been on since 1 February; it took until about 15 June to get the temp to 82 in the pool. Since about 23 June, it's been 85-87 consistently everyday by 2pm. A friend of mine with a similar pool uses no cover, his water is at 78-80 right now...