Thanks for the kind words Terry. Not out of town today, but in Sacramento next week...

Psyclone,
To repeat what Tenax has answered,
1. If I purchase the model with the smaller cell for my 20,000 gal pool, when the cell needs replaced, can I replace it with any of the three sizes? Or does the control box only work with the size it's sold with?
--> They are interchangable, but I would recommend that you consider the SC-48 or SC-60, as the SC-36 may be borderline sized for your 20,000 gallon pool. When we (not just AutoPilot, we all do this) advertize that it's good for up to XX,000 gallons, that's usually with the pump running 24/7 and ideal conditions. A recipe for short cell life. My recommendation is to start of with the largest cell you can afford. You will get longer cell life and a longer warranty (prorated).

3. Does the contol box need to be protected from the sun and rain?
--> Although it is recommended, it is not required as the control box is approved for indoor or outdoor installation. There are vents on the unit, so it does need to be protected from any water penetration (read sprinkler system) from below or behind the unit (mounting it on a wall or board will protect it as opposed to mounting posts).

4. Does the unit wire into an existing time so that it switches on and off in parallel with a single speed pool pump? It appears so by the wiring diagram, but if that is the case, how does the control box clock retain the correct time?
-->As Tenax said, you can wire EITHER the Digital or Soft Touch to operated off your existing time clock. It should be wired to the same terminals that power your pump. The Digital does come with an internal relay and programmability to operate your circulation pump, but does not need to be used. The Digital has some benefits over the Soft Touch (and is the more desired of the two -sorry Terry). The Digital display gives you all the diagnostics on the digital display as opposed to flashing lights. The Digital provides salt ppm AND salt addition amounts (in lbs), based upon the pool volume programmed into it. The Digital allows more programming and inspection of the system that is most helpful when troubleshooting issues.

Thanks again Terry for answering these questions. And thanks Psyclone for considering Pool Pilot.