I was having issues with my hot water heater (electric) and I am a service tech and I install about 9 water heaters annually, so when I came across a few water heaters that (whose combined age was less than my current water heater) still had some good years left to them. I decided to use them to solve my water heater issues.
Prior setup was one 50 gallon electric water heater, running on a 240 volt 30 amp breaker, to which I added a second 50 gallon electric that was only 6 years old and had a failed lower thermostat (failed in the on position) that caused the temperature pressure valve to vent and make a mess of the customers laundry room and a 10 year old 80 gallon hot water heater that had a failed dip tube.
Both of these were reconditioned and delimed, replaced the lower thermostats with upper thermostats so the could be linked together (normal electric water heaters only have one element on at a time, which is how they do everything from a single 30 amp breaker. But replacing the lower thermostat with an upper one allows me to wire them all in sequence, so all three water heaters only require one 30 amp circuit without resorting to number 2 awg wire, way too expensive.
But I went a little further. I now have nearly 4 times the storage capacity of normal households, why do I need the quick recovery times a 240 volt setup offers?
Obviously I don't, so I converted everything over to 120 volt, halving the voltage halves the amp draw, but quarters the watts, but with the capacity I now have is no big deal. I also like this because in my experience replacing innumerable lower water heater elements, I noticed that the ones that fail are usually shorter linearly than the ones that last longer. In fact this is what I encourage others to do when replacing an element, the lower one should always have a longer linear run than the upper one, the reasoning is less wattage per linear inch , which translates into a difference in how hard of a life the element has, and I just quartered the wattage usage on all my water heater elements, they may fail, some day, but I don't think it will be anytime soon.
The system has been up an running for a month now and has only had the issue of lower hot water flow than the cold. This is due to the need for a check valve on the cold inlet to the first water heater in the system, I have a circulation pump set up to keep all the hot water even throughout the three tanks, and didn't want the hot water to back up into the cold water line. But apart from that, it works fine. I also have a second timer to turn the heaters one during none peak hours, we don't currently have a setup for here, but that doesn't mean it won't make a difference.