Just a solar cover will do very, very, little to increase the temperature of the pool. The best utility of the cover is if you have a heater and the water temperature is heated to be higher than the night time temperature of the air. That results in a large loss of heat due to the evaporation, which is greatly mitigated with the use of a solar cover.

If you get a cover without any other heating, I will virtually guarantee that after a bit you will hardly use the cover.