Hey Tim,

I recommend going with the largest size cell you can afford. Why? Because it gives you the potential to generate a higher level of chlorine in a shorter period of time, so you can run your pump however many hours you want to. The Pool Pilot should keep up. Also, your cell will last longer, due to being able to run at a lower output setting.

Also, if you want something that offers a bit of automation, the Total Control system will provide that. Hayward also makes a similar system called the sense and dispense.
This incorporates an ORP (the amount of active oxidizer) and pH sensor to control both levels. There is an acid tank and pump used to control the pH.
Not all pools require the pH adjustment as there are some pools where the pH holds steady, with very little acid demand.
Usually these are fiberglass and vinyl liner pools...soo you may want to consider this before adding the expense of the acid tank. If so, there is a specific DIGITAL controller part number that allows to you to upgrde to the acid tank system, if desired. The Digital Total Control Part Number is #75003. This can be used as a purifier only or upgraded to the Total Control. Cost is about the same.

Although you can use a smaller cell, typically rated to 20,000 gallons, you'll usually need to run at a higher setting to maintain proper chlorine levels.

For the Total Control, you will need the following:
75001, Pool Chemistry Controller (this is the box with the ORP and pH sensors)
75003, Digital Control unit
75005, 15 gallon acid tank with chemical feed pump, 220 volts.
94113, Manifold assembly with SC-60 (RC52 is our new part number) cell and trisensor.

Thanks for considering AutoPilot.