When the zombies take over, how long till the electricity fails?

I've always wondered about this. How long would the power last if some kind of plague, zombiefication etc wiped out most of the people. In The Stand by Stephen King it takes a simple old trip up to the power station and the pulling of a few levers (once you read the manual) to get it happening again. Although this report is based upon US/Canada I'm sure its applicable pretty much anywhere. Plus its kinda scary when you think about it, I thought these things pretty much ran themselves these days. Anyway if you're interested in how long you have to cook up microwave burritos before the juice goes off look here.

Via Boingboing