You can significantly reduce swimming pool heating costs by using a pool cover. Swimming pools lose energy in a variety of ways, but evaporation is by far the largest source of energy loss. Evaporating water requires tremendous amounts of energy. It only takes 1 Btu (British thermal unit) to raise 1 pound of water 1 degree, but each pound of 80 degree F water that evaporates takes a whopping 1,048 Btu of heat out of the pool. (Source: US Department of Energy)
We use a pool cover to cover our aviary. In winter, it helps keep the birds warmer; in summer - cooler. Wrap it around the aviary, sunshine fills the "pockets" and gives warmth during the night.