What is the best way to heat the water in inflatable swimming pools?

There are many ways to heat the water within an inflatable swimming pool. Ofcourse there are the standard electrical pool heaters, but often these are costly and are best for long term investment. There is a more cost effective way to heat the pool that is using a Solar Cover that goes above and covers the pool. It uses the heat of the sun to trap the heat and gradually the water in pool should become warmer.