If your pool is in direct sunlight, the sun helps to raise and maintain the water temperature free of charge. In many cases, you can add a solar system to your existing filter pump and plumbing heating the pool water as it is filtered. Buying and installing a solar pool heating system can be a significant investment, so be sure to shop around and get the best combination of price, performance and warranty.
One final thought, while heating your pool can be a costly investment, you can save money on your solar pool heating costs and solar collector sizing by simply using a also known as a pool warmer cover. A pool warmer cover acts like a solar blanket helping to minimize night time heat loss as well as preventing chemical loss and water evaporation when it is windy, making the pool and solar system more efficient.
There are basically two types of pool covers: Thermal/opaque covers which completely cover the pool surface and help prevent heat and water loss. If these covers are left on during the sunny part of the day, these covers can also transmit some heat into the pool. The second type are disc type solar pool covers which float about on the waters surface absorbing the suns rays helping to transmit the suns heat to the water below. These floating disk covers also help prevent heat and water loss during both the day and night helping to reduce your solar pool heating system size and complexity.