If we only need to power a dark-age "amount" of technology, windmills and water-wheels (or small hydroelectric dams if you're lucky in choice-of-location) would cover more than enough, no? And it's not like any of those are hard to build, or require any advanced principles; they get more efficient as your tech-base improves, but people living on frontiers in the 1800s were knocking them together for their family homes as soon as electricity became a thing that anyone had any use for.

And that's assuming we don't still have access to, say, the hardcopy US Patent filing records. You could get very far indeed with basic metalworking + modern knowledge of the optimal energy-collecting shapes for rotor blades.