The bigger the unit, the more water will be used the regenerate it, but it will have to regen less often than a smaller unit. You will use less water by getting an electronic unit vs a mechanical one. Clack and Fleck make very good electronic unit that you can program for a specific amount of discharge water. The average discharge per cubic foot of resin may be backwash 10 min @2.4 GPM, brine 60 min @ .33 GPM and final rinse 10 min @ 2.4 GPM. I did not mention refill because you will reuse that water.