A coordinated attack on several key substations at once could bring down large portions of the power grid plunging large portions of the country into darkness for days if not weeks. In 2003 a failure in a single power plant in the Niagara Falls region, set off outages that spread over an area of 9,300 square miles with a population of roughly 50 million people.
I remember the Iraqis advising those who lost power to sleep on their roofs. The outage was in summer.