Like most people, I wonder what would happen in the case of the most likely emergencies. For example, how long would we have electricity if zombies started to take over? Cecil has the answers. In the case of a "28 Days Later" style fast-moving zombies, we're looking at hours or days. More traditional "Night of the Living Dead" slower, ambling, zombies might give us time to spin out power for weeks or even months depending on the method of generation. Pat Boone style zombies could mean the cessation of listenable music at a faster pace.
Of course there is a paradox that usually goes unanswered in zombie movies. Why do the zombies attack living people, but not each other? It cannot be brainpower allowing them to recognize living versus dead, as the zombified seem not to recognize loved ones as they attempt to dine on said loved one's brains. Perhaps they go by what meat smells fresher? One would have to wonder at the olfactory response in the decayed, ambling tissue of the zombie. Perhaps they know, they unlike the stars of the film, they are being paid as extras.