I found that not using my solar cover on the pool overnight during the warmest part of the summer months did not matter much, maybe losing a degree or two, but the sun each day would return that lost warmth. In the spring/early summer/late summer months, using the solar cover on the cooler nights helped maintain the warmer temps for quite a while, allowing a longer swim season. In the middle of Sept, I gave up on the swim season and stopped using the solar cover. The pool cooled off much quicker with the cool nights, losing several degrees each night.