Penguin supplies two main solutions to heat the pool using solar - although there must be a caveat that solar alone can only be used to 'extend' the season for the months either end of the season and is best used with a pool cover to stop the evaporation of water at night which accounts for about 70% of the total heat loss. If you want all year round heating it is recommended to use a supplemental form of heating using either an electric 'inline' heater which has no efficiency gains (like an element in a kettle) or a pool heatpump - either gas or electric.
1. Using traditional solar panels to heat your pool
The solar panels used for domestic hot water generation are very suitable to use and as a rule of thumb you should require about half the surface area of your pool in panel surface area - although in practice, space and economics makes this about about 5 panels for a standard 5x10 pool - with a proviso that the customer can always add more panels if needed.
These panels do not survive well in direct contact with the corrosive pool water so the heated water is passed through a heat exchanger which transfers the heat to the pool water. The installed solution works out to about the same price as a suitably sized heatpump - but the heat generated is free!
2. Using plastic panels - Helocoil - to heat your pool
Another more recent innovation is the use of black plastic panels which are UV resistant and the pool water circulates directly through the piping making it about 30% cheaper to install. But because they are less efficient the customer requires a far greater surface area to have the same heat gain as a traditional panel which takes up a lot of space and makes them very noticeable.