It's electricity 101, find a motor, any motor, and turn it backwards= generate electricity, then send to battery bank. Done. Sure you may(ok, will) need some electrical management in there somewhere, but again such components should be abundant in a post-apocalyptic setting based on the modern world. Building robots? Harder but still do-able, but I don't push tooo hard for that, even though I'd love to see it. Asking for basic electricity generating techniques is however very trivial by comparison.
I just want late-game electricity to be worthwhile once achieved, if we are stuck with gas generators forever then saying it's lame is being generous. Please at least make solar banks acquirable or better yet introduce windmills or some other form of, likely similar, electricity generation. Heck you could even install a workout bike and let players convert player/food energy into electrical energy in a pinch. I just think such things would need to be balanced right, not omitted altogether. FFS we can turn coal, gas, nuclear explosions, wind, tide, and even falling ♥♥♥♥ing water into electricity. Relying on gas, in a world that no longer extracts it, is a dead end regardless(yeah yeah, we can mine oil shale, which is silly, refining usable gas is leagues more complex than that. But ok, and I will do this as the game intended, fine). Anyway, that's my 2 cents. Carry on.