The idea of a reserve is that when your softener is nearing its capacity, once you get into the reserve, the system will initiate a regeneration at your preset time, typically 2:00 A.M. when there is the least likelyhood of water usage. If you did an immediate regeneration, then a reserve would not be needed nor would it make any sense. The normal reserve is 1 days capacity. If a system were to go into the "reserve" capacity at 4:00 A.m. when you are getting ready for work, the system should have enough capacity to make it until the next regeneration window of 2:00 a.m., 22 hours away. This is why a variable brining can be beneficial, if your system is undersized and you do not use up the majority of your reserve. If your system regenerates less than weekly, the efficiency gains of variable brining are typically lost. A properly sized system can be difficult at times, high hardness, excessive water usage, etc, and a twin alternating system should be considered instead if you are unable to make a 5+ day regeneration frequency without using excessively large equipment which has its own unique efficiency problems.