A reasonably sized family pool could contain four thousand gallons (18000 liters) of water! That is a lot of water to heat up every day... Luckily a lot of pools are in areas that enjoy plenty of sunshine (that is why they were built in the first place), so there is an economically viable solution that means you can enjoy your pool in the mornings, at night or on colder days too.
Pool solar heating is fairly straightforward in theory - your pool water is piped round a big black heating element, and the heating source is the good old sun.
If swimming pool solar heaters have a disadvantage it is the big black thing - a lot of people find it ugly and do not want to see it in their garden. If you want what is called 100% solar heating then you need solar panels that equal the surface area of your pool (that could be, for example, 40 to 50 square yards).
Hence a lot of people try to hide the panels away by placing the solar pool heating panels on the roof of the house. This is not a bad idea, but be careful - these things are designed to get hot, so they expand and contract quite a lot, and they can also be heavy. So make sure whoever fits them knows what they are doing - ask them to explain how they are going to stop the panels causing your roof to leak. And if you choose to do it yourself, make sure you get your calculations right...