@jpmckane - the Master valve is only energized when the Rachio is calling for water for a zone. That way if there is a pipe break between the master valve and the zone valve or the zone valve doesn’t completely shut off, the master valve will prevent/reduce wasted water and damage.
The dedicated AC ports will have voltage present all the time.
The Rachio power supply is I believe a 1,000 milliamp power supply. If the power supply has a “brown out” due to too much current pull then the Rachio will throw an alert.
Another design option would be to have the city water come into the rain barrel - with an air gap. So a low water sensor would open a valve to the city water and a high water sensor would turn off the city water. The two sensors could be in a tight range to prevent too much city water from being put into the rain barrel. And if the city water input rate was equal to the maximum flow from all the zone’s it shouldn’t run dry.
Edit: After thinking about it overnight the simplest solution would be to come out of the master valve and through a float switch that is normally open when the water is high (so closed when low) and feed a sprinkler zone type valve on the city water supply to fill the tank. I might also use a pump start relay also on the master valve, or a master valve after the pump, to prevent the tank from running dry due to a leak and ruining the pump by running it dry.