I appreciate the possibility of passive feeds but from outside to engine room buried deep in the bowels is unlikely, which is why the engine control room used the telephone system which can be passive or very low power built in to a PABX. But UPS would be more useful for emergency exit lighting (UK law demands premises have these in buildings) - this emergency exit lighting was not visible in the film, the emergency "power" (unspecified as to its extent) was cut as a plot line. The engine room was in total darkness save for the torch carried by the pirate.
either way there are two plot features that conflict IMNSHO.

Don't let it spoil your enjoyment - it was a gripping film, and the Somali captain deserves an Oscar, and it is not just my opinion.