The power room was the location of a starship's power generators.
Aboard the superdreadnought HMS Manticore, the power room once suffered an explosion. Honor Harrington risked her life to save crewmembers trapped in the room, earing two medals for her actions. (HH1)
In Reliant-class battlecruisers, the fore and aft power room with the fusion plants were located along the central axis of the hull. This protected them from damage, but also made them difficult to access in case major repairs or refits needed to be made. (HH3)