Extra Heavy-Duty Solar Blankets - The cheapest and most effective solar-heating system for a swimming pool is a solar cover. Solar Covers Can: Prevent heat from escaping your pool: Retaining heat is especially important at night when the air is cooler than the swimming pool. Directly convert solar radiation into usable heat: In an in-ground pool, a cover can increase the water temperature by 5 degrees F for each 12 hours of coverage. Limit evaporation: Each gallon of evaporated 80 degree F water removes about 8,000 BTU's from the pool. Reduce chemical depletion: This advantage reduces cost, and you can consider it a form of energy conservation. Prices listed are for 12 mil Solar Covers.