“If hackers were to knock out 100 strategically chosen generators in the Northeast, for example, the damaged power grid would quickly overload, causing a cascade of secondary outages across multiple states. While some areas could recover quickly, others might be without power for weeks.
The scenario isn’t completely hypothetical. Lawmakers and government officials got a preview in 2003, when a blackout spread from the coastal Northeast into the Midwest and Canada.”
Source: Why a power grid attack is a nightmare scenario | TheHill