There is a great deal of uncertainty concerning EMP. Just how strong would the effect be and what impact would it have on modern-day micro-electronics?
The effect was “discovered” mainly through the Starfish Prime high-altitude test in the Pacific in 1962, but open-air nuclear testing was banned within a year and that put a halt to further research. The Soviets had their own EMP nuke tests, with interesting results.
An EMP shot is expected, even assumed, to take out the commercial power grid, since a surge of electrical current would be induced in those miles of transmission lines. Relays, switches and transformers would be destroyed and they are very difficult to replace. (Few if any spares are on hand, so expect long outages. As in years.)
The worst-case scenario would see micro-electronics fried instantly. Every vehicle today has an on-board computer, so fuel and food deliveries would stop if the trucks were all disabled. Would the EMP pulse knock out the circuitry in your generator? We don’t know. Would every cell phone and laptop go dark forever? We don’t know. What about aircraft and satellites? Again, we don’t know.
And we don’t wanna find out, either. Incinerating North Korea (or Iran, which is also very interested in EMP) might be satisfying but it wouldn’t get the trucks to your supermarket any faster.
The worst case scenario includes 200 or so nuke plants melting down when they can no longer maintain cooling. That alone should justify spending a little money to harden the grid. Everyone seem more concerned with an unlikely EMP attack which might or might not "break" the grid. I am more concerned about the certain to occur solar flare with will "break" the grid. Hopefully the flare will break someone elses grid and then maybe we will take the threat seriously.