We know that big solar flares and CMEs can cause EMP on Earth, knocking out power grids and frying electronics.
We also know that some sun-like stars (e.g. Kappa Ceti, Groombridge 1830) can erupt into "superflares", which makes the biggest CME Sol's ever had look like a popgun. Fortunately these don't seem to have happened at Sol (it's effects would show up in the historical record and/or on the surfaces of the other planets and moons). If they happened at Sol, these superflares would destroy Earth's ozone layer completely, wipe out all our electronics, and melt the surfaces of the icy moons of Jupiter and Saturn.
What I'd like to know is whether there's a way to figure out the maximum effective range of the EMP that results from a Kappa Ceti-like superflare. Would it knock out electronics beyond say, 50 AU from the star? Could it knock them out from a range of several lightyears? I know that a superflare was observed at Groombridge 1830 in 1968, and that didn't have any effect here (it's about 30 lightyears away), so presumably the EMP generally wouldn't have any effect at that range, but what about closer?
See these links for more info:
More about superflares:
http://www.tmgnow.com/repository/solar/superflares.html (which says that they affect the whole star, and are not directional)