I would guess that there would be some radio chatter ... but in an ideal execution - we wouldn’t know until our lights went out.
This is why the administration is talking to google. Google wants to connect us all to an electrical grid so our electricity can be monitored and rationed down the road.
By doing that, a smaller weapon could fry the grid. They wouldn’t have to launch something to impact the entire country. Feasible it could be done by sabotage of a few monitoring locations.
China could already knock out satellites. A coordinated attack that blinds us and then hits selected targets has the same impact and it could be brought back relatively quickly.