Comment by cyberax

You need at least hundreds of kilometers for the effects to become significant (as in "tens of volts"). Nothing on the small scale will be affected.

Power lines might be the most vulnerable part, actually. The geomagnetic field can induce current that will bias the core of transformers, causing them to overheat. This can lead to blackouts if the networks are close to capacity, and it's suspected that the 2003 North East Blackout was at least partially caused by them.