A power plant is a ground-based facility built to provide cheap and reliable energy to a city or even region.
Earth's power plants were based on nuclear fission up until the 22nd century, when they were replaced with more efficient fusion plants. (Star Trek IV: The Voyage Home)
The power plants on Cardassia Prime were sabotaged during the final battle of the Dominion War, playing a critical part in the battle's outcome. (DS9: "What You Leave Behind")
A planet in the Delta Quadrant had polaric ion power plants, the use of which were protested by Pe'Nar Makull and Ny Terla. (VOY: "Time and Again")
In the late 2370s, the planet Quarra in the Delta Quadrant suffered from a labor shortage in its industrial power plants due to the low level of automation in Quarren technology. As a result, the Quarren military began a secret program of commandeering passing alien starships, erasing their crews' memories, and putting them to work. (VOY: "Workforce", "Workforce, Part II")