Electric water heater upper thermostat never comes on because its differential temperature control is too high between 17F to 27F (it is operating at the high end of 27F). This is the temp difference range when the thermostat turns the heating element on and off. The lower thermostat differential temperature is 5F to 15F and is averaging about 10F. When testing each individual thermostat the lower thermostat comes on first at about 110F and off at 120F (10F differential). The upper thermostat comes on at 98F and off at 125F. (27F differential). This data was taken using contact thermocouples that attached to the tank next to the upper and lower thermostats. The following test was done with the lower tank temperature at 120F and the upper tank temp at 125F. A hot water sink faucet was turned on at 1.4 GPM and in 5 minutes the lower thermostat came on first at 110F. The water temperature slowly drops and over a period of 30 minutes the lower tank temp was at 88F and the upper tank temp was at 112F which still was not low enough for the upper thermostat to come on at its turn on temp of 98F. I had to run the hot water for 40 minutes before the upper thermostat turned on its heating element. It is my understanding that the benefits from having the upper heating element come on as early as possible is for faster recovery and less water temp drop.