I read at one time that a single EMP air burst over the center of the continental US would bring down not only everything digital, but also everything electrical, from generation facilities to long distance transmission as well as the local transformer at the end of your neighborhood.
Basically, the US, and its entire population, would go back to the late 1800's. Life would instantly become just as hard, and just as short. There will be some that may adapt to that life, but there will certainly be many, many more that won't be able to, and will just die off, unable to cope.
Building everything back up to present levels, replacing all the damaged electrical equipment and distribution, would probably take at least a lifetime or two, perhaps more. The lost population, probably even longer.
But then again, I think that NK would be a smoldering radioactive cinder in exchange, and they know it, so I don't think that the chances are very high that they'll do it, nor that they have the means to deliver such a weapon; at least not yet. The same could be said for Iran, at this point in time.