Evaporative Losses
Water at the surface is constantly changing into water vapor. This evaporation requires energy, known as “the heat of evaporation”. The heat required to turn liquid water into water vapor is removed from the body of water that remains, lowering its heat content and thus its temperature.

Pool water evaporation is increased by high wind speeds, high air temperature, low relative humidity, and high water temperature. As evaporation takes place, the pool operator must replace the water, which further lowers the pool or spa water temperature. Evaporation causes about 50% of all energy losses.

The energy loss for one gallon of water evaporation is 8,730 British Thermal Units (BTUs).For example, if 55,000 gallon pool lost one inch of water due to evaporation, the loss would be 781 gallons. The total BTUs lost would be 781 times 8,730 or 6,188,130 BTUs. 55,000 gallons is 458,150 pounds of water, which is also the number of BTUs necessary for 1 °F temperature change. Dividing 6,818,130 by 458,150 represents a BTU loss that is equivalent to a temperature decrease of 15°F, for one inch of water loss due to evaporation.