The elements and thermostats are wired so only ONE element can be on at
any one time. When first turned on, the top thermostat powers the top
element and disconnects the bottom element. This heats the only top
few gallons, so for that few gallons recovery is fast. Then when the
top part gets hot, the upper thermostat switches power to the botom
thermostat. The bottom thermostat powers the bottom element till the
entire tank has hot water. Then the bottom thermostat opens and the
power is off to both elements. When you start to use hot water, the
cold water comes into the bottom of the tank through the dip tube and
the bottom thermostat closes first. Since the top is still hot, the
bottom element comes on and you heat the water in the bottom, which
will convect up and therefore heat the entire tank. The hottest water
stays at the top and the coldest water stays at the bottom. If you use
all the hot water in the tank, the top thermostat switches power on to
the top element for quick recovery in the top of the tank, then
switches power back to the bottom again.
NOTE: Because there is no flue or flue losses, the efficiency is 100%,
not 30%. Electricity usually costs more than gas, so a gas water
heater that is 75% efficiency is usually cheaper to operate and
recovers faster because the burner usually provides more BTUs than an
Your breaker should be sized at 125% of the continuous load.
That means that you need at least 25 amps. If your voltage is a little
high, you may need a 30 amp breaker, because with high voltage, the
amps will be higher as well.
Hope this helps.