power not available for sale to my other cities in the region

Sounds like maybe your power city is a few months ahead of the other cities. If it took you a couple of (game) months or years to get the nuclear power plants up and running in the power city, you'll need to run the others until they catch up to the time you added the extra power source. If this is it, consider plopping a temporary windmill or two to cover the deficit until you catch up.