Duke Energy's rolling blackouts across the Carolinas Christmas Eve were the result of malfunctioning equipment - and a lack of power to buy from elsewhere.
Duke had been counting on supplementing its own power generation with electricity from PJM, which operates the electrical grid from the mid-Atlantic to the Midwest.
But PJM reported this week that it lost 23% of its power generating capacity due to storms and cold weather.
As with Duke, most of PJM's problems came from cold-related malfunctions at coal- and gas-fired power plants. That meant there was no extra power for sale.
Duke has said the December 24th blackouts affected a half-million customers in the Carolinas.