Outdoor swimming pools absorb up to 85% of direct solar energy from the water's surface. However, when not in direct sunlight, pools lose energy and warmth in cooler air. A solar pool cover optimises solar energy absorption and conserves heat by sealing it in the water. It can raise the temperature of a pool as much as 20° Fahrenheit within a week. According to the Energy Star organisation, covering a pool when it is not being used is the most effective way to save on heating costs.