For me today, I powered my heater down at 1:30pm, I had about 300 litres in my tank of 55 degree water.
That lasted me till 4:30 with nice hot water.
I really think having a recirculating pump is a good way to go.
Although I’m restricting the heat at one side, the other side is running flat out pumping hot water back to the tank. This means my heater isn’t going into shut down mode, and my coolant is linked to both heat exchangers, so less heat one side, more heat the other.
When there’s two of us working we both work on minimum and have nice hot water. If we run both sides on max temp, we still get 45-50 degrees at both brush heads.
If I just ran one pump off this it would be scorching.