This is one of those things that isn't IF it happens it is WHEN it happens. The Sun regularly throws off huge coronal mass ejections that would mess up our electrical systems. Depending on its level it could be a small piece of the planet like the one that hit Eastern Canada a few years ago. If it is big enough the Earth will turn in it like a pig on a rotisserie spit over a fire and it will be worldwide.
If all power were to be shut down in the US there would be an unimaginable death rate. The CIA has estimated that it could be in the area of 90% if it lasted for a year. there would be some places that survived and had power. Some of the government refuges and ships would probably survive. The thing is that there just isn't any way for a few to make any real difference when 350 million people are in the dark without food, water, sewage system, garbage pick up or a means to leave their areas. The big cities will ROT and if you look at what is happening on Long Island right now it will give you some idea of how bad it will be.
A major Astroid strike is another event that is an IS versus MAY happen thing. There are actually a lot of them Our Sweet Mother Earth is basically a cruel IQ test. A species is allowed just so long to evolve, get smart, and then get off this rock. When the time is up she will wipe the slate clean and start again. We are past due and the clock is ticking.