Blast has it. An EMP doesn't need the power turned on because it has all the power required. The only issue is how effectively it can couple (get inside) your electronic systems. If the system is protected by some form of Faraday cage (car body) and is insulated you may be lucky. On the whole though, a well executed EMP strike 300 miles above Omaha is likely to be extremely effective in taking out a high percentage of U.S. transportation and communication, as well as most of the electric grid. Ugly.