I have a 9k gallon pool. when I purchased the house it had a T-cell-3, which I knew was just undersized for the pool. it finally gave out on me end of last week. FC went to zero and started troubleshooting and its the cell that went bad. not producing chlorine anymore, cleaned it, reset the controller, etc. tested the water direct from return jets and no FC above background.
time to upgrade, but not sure to go with the T-cell-9 or the T-cell-15.
the T-9 can be found for $285, and the T-15 for $350.
I think the T-9 is plenty big for my pool, but I am leaning to the T-15. I can replaced 3ppm of daily FC loss with the T-15 by running my pump for 5 hrs and the SWCG at 65% or so.
to get the same from the T-9 I would have to increase the pump run time and/or %.