Power generators are large human structures that serve as external power supplies for other facilities.
Power generators were also used to power the psi disrupter on Braxis. They were only lightly-defended and located a considerable distance from the disrupter. They were destroyed by Raynor's Raiders, cutting the disrupter's power.
Power generators are also the only known terran structures having no fire animation when damaged, though they gradually decrease in health like all other terran buildings when the hitpoints are in the red.