All components of the power plant are accommodated in the building of a power house.
Wikipedia definition (similar term):
The Volta Powerhouses are hydroelectric generation plants located in southern Shasta County, California, between Shingletown and Manton. Volta Powerhouse I 40°27′33″N 121°51′56″W / 40.45917°N 121.86556°W was built in 1901 by Keswick Electric Power Company; Volta Powerhouse II was built in 1980 . Manton locals claim that Volta Powerhouse I is the oldest operational hydro-electric generation plant in California.
Source: Wikipedia - Volta powerhouses