Are we talking a continental magnitude emp hit, regional, or local? I could well see a continental hit causing 500,000 deaths, as there are likely that many on life support in this country at any given time. Depending on the region, I'd say the number would be close to that too. At the local level, there are only one or two cities big enough that an emp could put in peril that many people in that short a time.

If a pulse is strong enough to catastrophically disable airplanes, the half a million number would be a conservative figure.