Rolling blackouts explained

What a rolling blackout is

A rolling blackout (grid operators call it firm load shed or rotating outages) is a deliberate, controlled cut in electricity to some customers so the whole system does not collapse. The grid has to balance supply and demand every second. When demand outruns available generation and imports, frequency starts to fall. If operators do nothing, generators trip offline to protect themselves, and a manageable shortfall can turn into an uncontrolled, region-wide blackout that takes days to restart.

Shedding load is the emergency brake. The grid operator tells utilities how many megawatts to drop, and utilities switch off blocks of customers, ideally rotating them so no one is out too long. Hospitals and other critical facilities are usually protected, which means the remaining burden falls more heavily on everyone else.

Why they happen

• Extreme weather on the demand side. Heat waves push air-conditioning load to records; deep cold pushes electric heating load up.
• Extreme weather on the supply side. Cold can freeze equipment and interrupt natural gas supply to power plants. Heat and drought can reduce output from thermal and hydro plants.
• Not enough reserves. When an area has a thin margin between available capacity and peak demand, a few unexpected plant outages can tip it into shortfall.
• Limited imports. If neighboring regions are stressed by the same weather, they have little to share.
• Fast demand growth. NERC's 2025 Long-Term Reliability Assessment says new data centers account for most projected peak demand growth over the next decade.

Texas, February 2021

The most severe recent example is Winter Storm Uri. The joint FERC, NERC and Regional Entity staff report (November 2021) found that ERCOT had to shed 20,000 MW of firm load at the worst point, and that the event as a whole was the largest controlled firm load shed event in U.S. history, at 23,418 MW across affected areas. More than 4.5 million people in Texas lost power, and at least 210 people died.

The report's central finding was that large numbers of generating units froze, tripped or could not get fuel, at the same time demand spiked. The "rolling" part broke down: so much load had to be shed for so long that many customers could not be rotated back on and were out for days.

California, August 2020

During a West-wide heat wave, the California Independent System Operator (CAISO) ordered rotating outages on two evenings. The final root cause analysis by CAISO, the CPUC and the California Energy Commission (January 13, 2021) reports that about 491,600 customers were affected on August 14 and about 321,000 on August 15. These outages were shorter, closer to the textbook version of a rolling blackout, and happened around sunset as solar output fell while demand stayed high.

Winter Storm Elliott, December 2022

During Winter Storm Elliott, the FERC-NERC staff inquiry (October 2023) found 90,500 MW of coincident unplanned generating unit outages, derates and failures to start. Firm load shed exceeded 5,400 MW, which the report calls the largest in the history of the Eastern Interconnection.

Where the risk is today

NERC publishes seasonal and long-term reliability assessments. In its 2025–2026 Winter Reliability Assessment, seven assessment areas were rated elevated risk, meaning supply could fall short in extreme, widespread cold. US areas included New England, the Carolinas (SERC-East), SERC-Central, ERCOT in Texas, WECC-Basin and the Pacific Northwest. No area was rated high risk. Our Grid Risk Outlook lists each region and the reason NERC gives.

"Elevated" is not a forecast that blackouts will happen. Most winters and summers pass without load shed. It is a signal that, in your region, the cushion is thin enough that extreme weather could exhaust it.

What happens during a rolling blackout

1. 01The grid operator issues conservation appeals and energy emergency alerts as reserves shrink.
2. 02If reserves fall below a set threshold, the operator orders utilities to shed a specific amount of load.
3. 03Utilities switch off circuits, typically for set intervals, while trying to protect critical facilities.
4. 04When supply recovers, load is restored in stages.

Warning time can be short. Follow your utility and grid operator on their official channels, sign up for utility outage alerts, and keep a battery or hand-crank NOAA weather radio. Our best emergency radios guide compares options.

How to prepare

• Charge everything when alerts start. Phones, power banks, laptops and any portable power station.
• Pre-cool or pre-heat your home before the expected peak, then keep doors closed.
• Know your heating dependency. Most gas furnaces need electricity for the blower and controls; a heat pump stops entirely. See winter power outage survival.
• Protect medical devices. If someone depends on a CPAP or oxygen concentrator, size backup power for at least one night. The backup power calculator can help.
• Keep food safe. Keep the fridge and freezer closed: about 4 hours for the fridge, about 48 hours for a full freezer (Ready.gov, FDA).
• Unplug sensitive electronics to protect them from surges when power returns.
• Use generators safely. Outdoors only, at least 20 feet from doors, windows and vents, with CO alarms inside.

Then work through the 72-hour blackout checklist so the basics are covered regardless of the cause.