If you have an Autopilot Digital Nano, the PPC1 is the same as the old RC-35/22 (for 22,000 gallons with a Nano control box). The PPC2 is the same as the old RC-28, which is a more powerful cell and is used with the Digital Nano+ control box, so you wouldn't want to downgrade to the PPC1 if you use the PPC2 on a Digital Nano+.