A power unit was a type of energy production device fuelled by power rods. The Constitution-class starship USS Enterprise had a power unit, as did the Robot civilization encountered by the Enterprise in 2266. The Enterprise' unit was a relatively dangerous area of the ship, requiring the use of engineering suits to access it and decontamination procedures after working with it.
After a million years of operation the Robots' power unit had depleted its power rod supply and the Robots took remote control of the Enterprise hoping to cannibalise it for a new energy source. The Robots found themselves unable to access the Enterprise's power unit so using the threat of the life of his crew blackmailed Captain Kirk into retrieving power rod from it for them. Kirk however had other ideas and sabotaged the power rod, placing in it in a cobalt container, making it an explosive. Fifteen minutes later when the rod was placed in the Robots' power unit it exploded, destroying the unit and subsequently disabling the Robots. (TOS comic: "UK comic strips, first story arc")