Just like any open body of water, swimming pools experience evaporation. Water molecules at the surface absorb heat from your pools heater, the sun and surrounding air, gaining enough energy to escape as vapor taking a lot of heat with it.
This Results in Higher Bills: As water evaporates, you need to top up the pool or spa with unheated water more frequently, leading to increased water usage and higher water bills, especially in areas with water scarcity.
Higher Maintenance Cost: As water evaporates, it takes dissolved chemicals (chlorine, sanitizer, etc.) and heat with it. You'll need to add more chemicals more often to maintain proper water balance, increasing your spending.