I have simulated EMPs and I can tell you anything electronic that isn't massively sheilded is quite vulnerable, as well as most transformers.

A conventionally fired EMP is really easy and cheap to build. It may take out a small city.

A nuclear EMP at altitude will knock out all consumer electronics in the CONUS, right down to the wristwatches and cell phones all of us use. I've mentioned before what the sheilding requirements would be to keep tech tools from being zapped.

Anyone read the prologue (intro) to the story "Lights Out". That seems like a pretty feasible scenario for delivery.

However, anyone who would attack us with an EMP of that nature would well realize that a retaliatory strike would not be preventable, and would not be so limited. Sure they might hurt us, but they would cease to exist. Even the Ayatollah prefers to keep what power he has to being vaporized.