I just completed a 21600 gal. pool. PB put in an IC20. I was not satisfied with the performance and am moving up to the larger cell. I spoke to a Pentair technician and was told that they both use the same power pack. I'm able to move up to the larger cell for an additional $50.

There some good photos on this website:

http://ths.gardenweb.com/forums/pools/