Can anyone tell me the power source of electricity in the wasteland? Megaton lit up at night and other settlements seem to have electrical power but there is no mention of it's source.

I believe the lore talks about Fission based technology used in reactors, generators and batteries (and cars....that's why the blow up so well) that is suppose to be self-regenerating to some respect. It's supposedly part of the alternate reality branch that is different from OUR timeline.

If you look at Megaton from the front a ways back, there is a propeller on the front that spins. Perhaps that is a wind turbine.

Also, most lights I see have a battery wired to them. And, there are all those power sources for the weapons. Weapons take orders of magnitude more energy to fire enough energy to do damage as they would to say power a light bulb.