When a local power company goes down, it must draw power from the grid to reboot its system. This takes up to six times normal power. It's inertia in action.
The problem is obvious: in 2000, if numerous local plants shut down, getting them back on line may overload the grid.
A city would have to reboot in sections, no doubt: dispersing the incoming power among parts of its local grid. It will try to get one section up, then the next. The problem comes because of the simultaneity factor. They will all want to go on-line and get power from the others.
This is from "Earth in Space," Vol. 9, No. 7, March 1997, pp. 9-11 © 1997 American Geophysical Union.
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Extensive blackouts are the nightmare of the power industry. Once power is interrupted in large metropolitan areas, diversity of electric use on the network is lost. When power is restored, all thermostatically controlled electric loads come back on simultaneously. This stress, added to the higher demands of many devices such as motors and transformers, can draw up to 600% of normal load during restoration procedures.
Such a blackout is also likely to cause transient voltage stresses and permanent damage to network equipment such as high-voltage breakers, transformers, and generation plants, which makes them unavailable for restoring power. Hours or days may pass before power can be restored. Oak Ridge National Laboratory assessed the potential impact of a widespread blackout in the northeastern United States from a geomagnetic storm event slightly more severe than the March 1989 blackout as a $3–6 billion loss in gross domestic product. This figure does not account for the potential disruption of critical services such as transportation, fire protection, and public security. Other assessments placed the 1989 and 1991 geomagnetic storm effects in a category equivalent to Hurricane Hugo and the San Francisco earthquake in their relative impact on the reliability of the electric power grid.