You are going to start seeing power failure pretty quickly. Coal power is very hands on, and they just won't keep chugging along for very long without humans keeping them running. Maybe 48 hours assuming power demand falls too.

The transmission lines are the real issue. They are constantly failing and being repaired. Without people fixing the lines, power can't get where you need it anyways.

In a post apocalypse type scenario, most of North America is going to be on fire pretty damn quick, and that is going to take out long distance power transfer. Our entire modern society is built around having rapid response to any fires. Our cities will burn down within days or weeks without firefighters.