The Tcell940 and T Cell 15 are both chlorine generators for pools, but the Tcell940 is for smaller pools, while the T Cell 15 is for larger pools. The Tcell940 can generate up to 40,000 gallons of water while the T Cell 15 can generate up to 60,000 gallons of water. Which one is better for you depends on the size of your pool. If you have a smaller pool, you may want to go for the Tcell940, and if you have a larger pool, the T Cell 15 would be your better option.