Disaster Movies and the Electric Power Grid

So let’s assume it’s the End Times, the Wrath of Gods has descended upon us heavily in the form of flesh-eating ghouls rampaging in the streets. The question is asked, “If zombies attack, how long before the power grid fails?” Of course the question is rather silly, perhaps only for escapist horror movies, but it might have some application in development of advanced nanoweapons too. For example, suppose someone builds and seeds a plague of sleep?