[PTTG]

Take a look at the book "One Second After" for a well-researched if slightly fatalistic example. Life support without a national power is an inevitable death; sooner or later, you're going to run out of bottled oxygen.

We probably wouldn't rebuild the same way we built initially. It would take more than a decade for the US to rebuild all the destroyed electrical transformers, and that's assuming that all the factories that make transformers were ready to go right off the bat. I'm assuming we'd use very localized electricity with hydro, tidal, and wind (they don't need any fancy minerals, only a motor that runs backwards). Some places could get those set up in a few months, but it'd be survival electricity.

I imagine that Hoover Dam would remain a useful site... after one or two years of concerted repair efforts.

Thermal focusing solar would also be useful in a lot of places, because it's just mirrors and boiling water. Photovoltaics, useful as they are in the real world, would require high-precision engineering that might be hard to replicate.

Submerged nuclear submarines would be kingmakers. The onboard computers alone would be incredibly valuable, and the weapons would have new meaning if they're the only long-ranged fighting implements and there's no way to accurately determine where it came from.