I'm surprised no one mentioned the source of cataclysm itself. The fact that there was a civilization and despite all its advanced knowledge - it is now gone.
This would be especially important if Cataclysm was man-made.
This would be especially important if Cataclysm was man-made.
With no oil, people will just deal with the limitations of low wattage electrics until they get better battery and generation tech. Arguably, you could have a 19th century type society with a nuclear reactor; both uranium and graphite can be mined with simple tools.