The big generators that we rely on for power are 1) Expensive; 2) No longer made in the US; 3) Require months of lead time; or 4) require months to be fabricated and shipped.
Motivated attackers could hack into an interconnected system powering multiple communities to wreak the same kind of havoc experienced for days during the Northeast Power Outage of 2003.
An affected power site could be unprepared for several months, leaving several hundred thousand or even millions of people without a centralized power source for a good portion of a year. Food stocks requiring refrigeration would go bad. Institutions with back-up independent energy sources may not be able to afford to produce their own energy for a length of time. The inevitable result would be food shortages and the deaths of people requiring electricity to stay alive but unable to gain access to affordable energy.