I’ve read about EMP stuff from someone who actually has worked with them and what they can actually do is pretty localized, apparently.
EMP is dependent not on the energy budget of the weapon,but of the hard X-ray and gamma ray flux, which induces Compton scattering (to near- or mild-relativistic speeds) of electrons which are then directed towards the ground by the Earth's magnetic field.
Moving electrons create magnetic fields; magnetic fields induce short-lived currents in all kinds of electronic circuits.
Major fun ensues.