According to the US Department of Energy, 70% of total energy loss in a pool (both indoor and outdoor) is due to evaporation http://energy.gov/energysaver/articles/swimming-pool-covers Evaporating water requires a tremendous amount of energy. It only takes 1 BTU (British Thermal Unit) to raise lib of water 1°F, but each pound of 80°F water that evaporates takes a whopping 1,048 BTU of heat out of the pool.* (1 gallon= 8.33 pounds). Therefore, for every gallon of 80°F pool water that is lost due to evaporation we lose 8,729 BTU of energy.
One solution that some choose is to use a physical solar cover to protect the pool water from heat loss and evaporation. This approach does work well when the cover is actually in place at the surface of the pool. A liquid solar product, however, works all the time. Coverfree is a liquid solar product that significantly reduces heat loss and evaporation. Just 4 ounces per week will help protect a 20,000 gallon (or 675 square foot) pool from these costly issues.