Sorry for double post.
The US carried out dozens of such tests in the Pacific to see what would happen.
EM communications were knocked out for a few hours.
The EMP threat is vastly (and I mean way extrememly vastly) overestimated.
It is is not going to fry your computer or your car's electronics or the power systems. They are not going to work for the few seconds that EM radiation overwhelms them when a bomb explodes but that is it.