Most outdoor pool  owners already use the sun to heat their pool water. Sunlight is absorbed by pool water, and thus the water is warmed. Pretty straightforward stuff. But there’s more you can do to exploit the 100% free solar power of the sun to heat your pool. How?
We asked Everett Barber, co-author of Convert Your Home to Solar Energy , who advised: “The homeowner can extend the benefit of that direct solar heating  by using a pool cover to hold the solar heat in the pool during the night time, when the evaporative losses are the greatest and there is no solar gain to offset those losses.”
According to Barber, if a typical pool cover is used every night when your pool is open, it will actually extend your swimming season. Think about how many weeks you can comfortably swim in your pool. Now, add 2-3 weeks onto the beginning and end of that season. In other words, use a solar pool cover nightly, and you’ll increase the average temperature of a pool by 4 to 5 degrees over a pool that doesn’t have a cover.
Meanwhile, during the day, it’s important to expose the pool water to direct sunlight. Here’s how:
Daylight Exposure. For maximum benefit, Barber says the pool water should be in full sun between about 9:00 AM and 4:00 PM.
Trim Trees. If there is shade on the pool water, the first step is to cut down or trim nearby trees, so the ideal solar exposure is attained. Barber warns that trimming can be costly, depending on how much has to be cut.
Pool Cover. Use a pool cover to capture and hold in that solar heat, and use it every night – at least during the cooler parts of the season.
Barber says that per dollar spent, implementing the steps above can make a very large reduction in (if not completely eliminate) the need for additional pool water heating. However, if you want additional heat, here’s the advanced step to take:
Solar Pool Heating Collectors. Install an array of solar pool heating collectors to heat the pool water. Arrays are groups of solar panels that collect solar energy. In solar-powered  homes and other buildings, you’ve probably seen these arrays lined up on a rooftop. There are several kinds of solar collector arrays that heat pools, which are described in Convert Your Home to Solar Energy . Besides the aforementioned solar pool cover, they include:
Solar Thermal Pool Heating System. These systems have been available since at least the early 1970s, and they are proven and widely used. They use liquid-cooled collectors to heat water of glycol. However, instead of storing the captured heat in a tank like solar domestic water heating systems, the swimming pool itself becomes heat storage.
Solar Electric-Assisted Heat Pump Pool Heater. This kind of system is a relatively new way of heating pools , but it’s growing in popularity. The solar array power a heat pump heater (either air source of ground source), which then heats the pool water. If you don’t like the appearance of big solar thermal arrays, you may find the look of solar electric collectors more to your liking.
The installation cost varies from one part of the country to another. It also varies depending on whether the solar array can be mounted on a roof adjacent to the pool, of if it must go on a mounting frame that was built especially for the solar array. The cost of the solar system also depends on the size of the pool. The larger the pool, the larger the cost of the system. A detailed chart on how much it will cost to keep your pool warm using solar energy is provided inside Convert Your Home to Solar Energy , and it offers estimated costs for various U.S. cities, for both heat pump heaters and natural gas.
As long as you’re going to the expense of using and maintaining a swimming pool, you might as well get as much use out of it as possible. Don’t be deterred by cooler temperatures. Consider harnessing the solar power to heat your pool and keep it warmer, longer.