Another way to ponder this question (and thanks to the OP because it is a good one), is to ask:
"If the lights go out, to what pre-historical period would society revert and how quickly?"
I reckon a couple of hours before the looting started, a couple of days before the killing for food and water..
We'd be feral tribal hordes in no time; all the preppers who think they would be okay would simply become targets for those with nothing.
I believe they normally say that we would drop to the looting and killing stage within 72 hours.