I don't have the correct answer, but from past experience with three kids & wife, we had minor problems with a 30 gallon. We switched to a 50 &
never had any problems like, running out of hot water, flushing toilet while
in the shower. You can adjust the heat on the heater itself. You can also
adjust your toilet to about 3/4 gallon to save on water. Last bit, I found
cold water washes clothes just as good as hot.