I did some preliminary research 3 years ago into this to see if we could get our pool warmer in the spring and autumn. It seems, as a rule of thumb, that you need the same square meter's of solar panels as you do the square meter of the surface of your pool, plus the associated pipework bracketing etc. This was to keep the pool at around 24 degrees c. When I did the costings of this and all the work required we decided against it. Obviously there is more to it than this; depth of pool, length of pipework, hours of sunlight in the spring/autumn etc etc. If you could get actual costs I am sure many people would be interested.