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(Bloomberg) — The largest US grid operator is leaning on more than 65 million people to conserve electricity to keep the lights on as a frigid winter storm moves over the East Coast, boosting heating demand and forcing power plants to trip offline. 

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Rotating blackouts continue to be “a real potential risk” and people are being asked to help the grid until Sunday afternoon, said Michael Bryson, PJM Interconnection LLC’s senior vice president of operations.  

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The magnitude of the cold weather that blew in from West was so fast and intense that PJM, which manages the grid stretching from Illinois to Virginia, had under forecast demand during the peak hour by more than 7 gigawatts. That’s the equivalent of 7 million homes on a typical day. The grid initiated a rare emergency requiring certain customers to curtail their usage. 

While frigid temperatures were expected heading into the weekend, “one of the things we under forecast was how cold it would get and how much customer demand there would be on a holiday weekend,” Bryson said in a telephone interview Saturday. “We are doing everything possible to keep from having to do any rotating outages.” 

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The warning of rotating blackouts is especially stunning because PJM has a massive surplus of power plants. Old coal and natural gas plants have lingered even as a wave of big new gas plants were built with the shale boom in the previous decade. 

PJM’s excess supply was such an outlier that it was considered one of the safe zones that wasn’t at elevated risk of blackouts this winter, according to the North American Electric Reliability Corp. seasonal assessment. The other region was the Southeast, which had seen rolling blackouts in this storm. 

The number of power plant outages also snowballed to more than 31 gigawatts on Saturday. These “forced outages” are triple what had been predicted on Friday, according to PJM data. Generators’ access to natural gas “is being affected” and a couple of plants had emissions restrictions, Bryson said. The causes of these outages appeared to be varied and it was too early to give a breakdown by fuel type, he said. 

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“Conservation works,” said Bryson. “We saw benefits from that today and we will be leaning on our customers between now and tomorrow afternoon.”

PJM isn’t alone in its forecasting woes. Most grids underestimated how much demand would climb in response to bone-chilling temperatures. “Generally holiday weekends have lower customer demand,” Bryson said.

One reason for the power woes is the country, especially the Sunbelt region, moved too swiftly in the last two decades to electrify residential heating, said Pat Wood, chief executive officer of Hunt Energy Network and former chairman of the Texas Public Utilities Commission and Federal Energy Regulatory Commission. 

“With a generation fleet that is more nat gas heavy than ever before, we are using twice as much gas to heat homes through electricity as we do with gas furnaces,” Wood said.

Gas supplies have been constrained across large swaths of the US by this storm. In New England, oil was generating 40% of the power on Saturday afternoon while gas was a mere 15%. Typically gas is the dominant fuel and there’s no oil burn.