For a 24' AG, most companies recommend a total of 4' x 20' of heating area. Some companies sell 2' x 20' sections so you would need two of those. A diverter valve is helpful as once you get the water as warm as you want, you can bypass the heater. If your pool (and heater) gets decent sun exposure, the pool will warm pretty quickly.

I have a 24' round AG and got a Fafco solar heater because it was a single 4' x 20' mat and it has the bypass valve built in. It was not the cheapest, but if you had to buy the diverter kit as an option then it propably works out the same or less.

Peter