Welcome to TFP!!
Have you done reading here on the pros and cons of SWCGs? If not, let me tell you a couple things to consider.
1) like Bama and Blake mentioned a cell that can 'handle' a 40,000 gallon pool will need to run ~ 24/7 to produce enough chlorine to do so (IMS {if memory serves} one for a 40,000 gal. pool produces ~1.5 lbs. chlorine when run at 100% for 24 hours). if you have to run your pump 20+ hours/ day and have the cell at 100% output it will cost you in electricity and cell life is based on 3 of hours it's energized and producing so, as mentioned, a cell rated for more gallons would run less often and need less frequent replacing ($)
2) how many returns do you have? As part of the conversion from salt to chlorine the unit produces extra hydrogen gas which ends up as bubbles coming out of the returns. This can cause foam or a foamy crust, if you use certain algaecides (like the ammonia based 'linear quats). There is also the pH rise caused by these bubbles, which would require regular additions of Muriatic acid - not a huge deal, but something to consider.
3) I can't recall any posts on how salt water 'plays' with brick, but certain coping stones are degraded by the saltwater. there is also the effect on non-stainless steel exposed to the brackish water (as well as on improperly made stainless steel).
Please don't get me wrong, I really like SWCGs and am just trying to make sure you don't waste money on something that you'll hate and not want to use.
