From the Super Mario Wiki
The Hydro Generator is a room inside of the Haunted Towers in Luigi's Mansion: Dark Moon. The Hydro Generator can be accessed by taking the right door in the Courtyard. Professor E. Gadd explains that the Hydro Generator is used to give electricity to the entire mansion. The main feature of the small room is the water-powered generator in the middle. The two wheels are connected to a mechanism that produces electricity. To the right of this machine is a file cabinet, a treasure chest, and a barrel. The door on the left leads back to the Courtyard while the door to the north leads to the East Hall.
During A Job for a Plumber, Luigi needs to bring back power to the Hydro Generator room by going to the Water Supply. When Luigi first enters the Hydro Generator room, a Slammer can be seen messing with the machine, but soon disappears. In the same room, there is a missing treasure chest to the right of the machine. Revealing it with the Dark-Light Device causes this mission's Boo to appear. In The Pinwheel Gate, Luigi needs to go to the Hydro Generator room to activate the door to the East Hall. The treasure that contained the Boo is missing again, but yields coins, bills, and Gold Bars when the Spirit Balls have been captured. In Graveyard Shift, there's a Gold Greenie in the room. In Doggone Key, the Hydro Generator room is one of the rooms that the Polterpup goes through. In Hostile Intrusion, the Hydro Generator may be one of the rooms that is infested with ghosts.