I have a dark blue vinyl pool cover that is on the pool at night. (electrically operated, so easy to open and close).
The pool is in-ground about 20x40.
The question is if it is a hot day outside (90 degrees plus) should I have that cover on or off if I want to heat the pool?
On the one hand, when it is on during the day and then I later open the pool, you can feel that the first several inches of water is very hot from the cover.
On the other hand, it seems that if the pool were open that it would be heated directly by the sun.
I know there is evaporation if I keep it open and chlorine used up, but I am mainly concerned with heating the pool.