Solar Pool Covers heat above ground pools by day and insulate them at night. When pool solar covers are left on pools, while not in use during the day, the above ground solar cover allows the sun's rays to pass through the bubbles to heat the pool water during the day. At night, the bubbles of the above ground pool solar cover act as an insulator minimizing heat loss. Pools lose the majority of their heat from the pool water surface and having one of these solar pool covers on the pool is like keeping a hat on your head. It stops precious free heat from escaping the above ground pool.