It would happen quickly. You have to remember that with a power system collapse, your personal store of fuel would be whatever is in your tanks - likewise every delivery vehicle. Without electricity to run the pumps, fuel would be locked just a few feet beneath the surface of every gas station. Considering every food store has a continuous stream of food delivery and stock turnover, the supply pipeline is about two weeks. With a power failure, two days
If you picked out twenty power plants in a region....say California....then total chaos would be in effect within forty-eight hours. Once people get the message of the effect and the word “years” for recovery...the guns come out, and civilization will no longer exist. You need to find a vehicle, and kill if necessary to get fuel. You plan an exit from this affected region and will encroach on anyone in the safe region...killing them if necessary to get food, fuel and shelter. I could see a 100,000 Americans per day....getting whacked.
Not for long. People are clever.
/johnny