Industrial-grade diesel electric generator, which is basically a diesel engine hooked up to an alternator.
There are notes attached to the prefab in the form of info_null entities, for how to incorporate the prefab to your level. These can be safely deleted.
Also, if you do an internet image search for "diesel generator diagram" the results would give an idea on how to realistically build a generator room to house these generators.
halflife.wad from Half-Life
barney.wad from Blue Shift
nw.wad from Night Watch
Polys: around 600
Added NGIN-fied version, using mostly NGIN textures