The Robelle 20 x 40-Foot In-Ground Heavy Duty Solar Cover is made of heavy-duty polyethylene material that works with the sun to catch and retain heat. Solar covers float "bubble side down" on the surface of your pool water when the pool is not in use, allowing solar energy to pass through and trapping it as retained heat in the pool. Helps raise the water temperature by absorbing sunlight during the day and retaining the heat at night, extending the pool season. Savings come from a reduction in water evaporation decreasing chemical consumption. Material is highly resistant to ultraviolet rays which inhibits deterioration. 3 year warranty.