Based on the top picture with the low-voltage breakers, there's nothing for the SWG so that particular ET panel was not spec'd to have a built-in intellichlor load center (power transformer). But, you can buy an external IntelliChlor power supply and hook it up to the ET panel. See the installation guide here -
http://www.pentairpoolme.com/uploads/document/file/540/Manual_IntelliChlor.pdf
You will want to wire it up so that the IC power supply is controlled with the main pump relay. This ensures that the SWG can never run without the pump running. You would then run a serial cable between the IC power supply and the ET panel serial communication plug so that the ET panel can send and receive information from the SWG cell. For your 20k gallon pool I would sincerely suggest you consider the IC-60 SWG cell. It is the largest cell Pentair makes and it will allow you to generate the most chlorine possible. This will allow your pool pump to run less (SWG's only produce chlorine when the pump is running) and the less you run the cell, the longer it lasts. Getting the largest SWG cell possible makes the most economic sense even though the upfront cost is higher.
If you buy everything online and do the installation all yourself (it's not really that hard) you'll save the cost of installation but your warranty will only be for 60-days. If you can find a PB or pool store to procure the units for you and do the installation on a professional basis, you'll get a 1-year warranty from Pentair. I can't really advise you on which way to go because ti all depends. Sometimes people get random cell failures 6 months down the line while most people never experience a failure. The risk you take in doing the install yourself is that if does fail 6 months down the line, you'll be left holding the bag for a new cell replacement.
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Oh, and I would also suggest that you consider upgrading your pump to a 2-speed at the very minimum. Single speed pumps waste a huge amount of electricity as most pools can very easily operate on low speed and don't really need the full power rating that a single speed pump delivers. With an SWG, you have to run the pump in order to generate chlorine and so you could wind up easily running the pump for longer periods of time. With a 2-speed pump, as long as the flow rate is satisfied on low speed, it's a lot more cost effective to run the pump and generate chlorine on low speed. Obviously a variable speed pump would be the best option BUT few people want to pay for that kind of upfront cost.