The main power room is the location of the cities main power distribution and control systems. This room is located in middle of the Atlantis control tower at least 6 levels down from Stargate Operations. This room also houses several Naquadah generators. This room is where the main power can be switched off locally for the entire city if there is not remote access.
In 2008, during a citywide malfunction, Dr.Radek Zelenka was able to use the ventilation shafts to gain access to this room and reset the systems. (SGA: "Quarantine").