Rolling Blackouts Explained: Why Grid Operators Cut Power on Purpose
By the PowerOutageTracker team · Editorial policy · Updated · 8 min read
A rolling blackout is a controlled, temporary power cut that a grid operator orders when demand is about to exceed available supply, so that a few neighborhoods lose power for a while instead of the whole grid collapsing. Utilities rotate the outages among groups of customers, usually while protecting hospitals and other critical loads. They are rare, but in February 2021 ERCOT had to shed 20,000 MW at the worst point of Winter Storm Uri, part of the largest controlled load shed in U.S. history.
Key facts
- A rolling blackout, also called rotating outages or manual firm load shed, is a deliberate, temporary power cut ordered to keep a stressed grid from collapsing.
- Grid operators warn through Energy Emergency Alerts (EEA 1–3). EEA 3 means "firm load interruption is imminent or in progress" under NERC standard EOP-011-4.
- During Winter Storm Uri (February 2021), ERCOT shed 20,000 MW at the worst point. Combined with SPP and MISO, that came to 23,418 MW, the largest controlled firm load shed in U.S. history. More than 4.5 million people in Texas lost power, according to FERC and NERC.
- Texas health officials put Uri's final death toll at 246.
- During Winter Storm Elliott (December 2022), utilities in the Southeast shed more than 5,400 MW, the largest controlled load shed in Eastern Interconnection history.
- In August 2020, a heat wave forced CAISO to order rotating outages in California. About 491,600 customers of the state's regulated utilities were affected on August 14 and 321,000 on August 15.
What is a rolling blackout?
Electricity has to be produced at the same moment it is used. When demand climbs above what power plants and imports can supply, the grid's frequency starts to fall below its normal 60 hertz. If it falls too far, generators trip offline automatically to protect themselves. That makes the shortage worse and can cascade into a blackout across the whole interconnection.
A rolling blackout is the controlled way to prevent that. The grid operator orders utilities to cut a specific amount of demand, measured in megawatts. The FERC-NERC report on Uri describes it as operators ordering "a percentage of the demand or load to be temporarily disconnected, to avoid system instability or other system emergencies."
How close can it get? Early on February 15, 2021, ERCOT's frequency fell so low that operators had "only nine minutes to prevent approximately 17,000 MW of generating units from tripping," which "could potentially cause a complete blackout of the ERCOT Interconnection." Frequency stayed below the generator trip level for over four minutes before it recovered.
How utilities carry it out
- Rotation. Utilities switch off groups of circuits and then rotate to other groups. Southern California Edison says CAISO-ordered rotating outages last "only about one hour" per group.
- Protected circuits. Critical loads such as hospitals and police stations are protected from manual load shed. The FERC-NERC Uri report describes this practice in ERCOT.
- Reserved circuits. Some circuits are held back for automatic underfrequency load shedding (UFLS), the grid's last line of defense. In ERCOT, at least 25% of load is reserved for automatic shedding.
- Short notice. SCE says it may have "less than 10 minutes" before it must begin, which is too little time to notify customers one by one.
When the shortage is deep and long, rotation breaks down. During Uri, the protected and reserved circuits "hampered" ERCOT operators' ability to rotate outages, according to FERC and NERC, and some Texans went without power for as long as four days.
Energy Emergency Alerts: the three levels
NERC's EOP-011 standard defines three alert levels that grid operators declare as conditions get worse.
| Level | NERC definition | What it means in practice |
|---|---|---|
| EEA 1 | "All available generation resources in use" | Every available plant is running or committed, and the operator is concerned about keeping reserves. ERCOT: "Low reserves. Tools deployed to meet demand." |
| EEA 2 | "Load management procedures in effect" | The operator can no longer meet expected energy needs, has activated its emergency plan, and is asking neighbors and market participants for help. |
| EEA 3 | "Firm Load interruption is imminent or in progress" | Minimum reserves can't be maintained. ERCOT: "Extremely low energy reserves. Controlled outages possible." |
Sources: NERC EOP-011-4, Attachment 1; ERCOT TXANS.
Most EEAs end without blackouts. An EEA 1 or 2 is a signal to conserve, not a sign that outages are certain.
Notable U.S. rolling-blackout events
| Date | Grid operator / region | Firm load shed | Customers or people affected | Main cause |
|---|---|---|---|---|
| December 1989 | ERCOT (Texas) | — | ERCOT's first system-wide rolling blackouts | Severe cold |
| February 2, 2011 | ERCOT (Texas) | 4,000 MW | About 3.2 million customers | Cold weather generator failures |
| August 14–15, 2020 | CAISO (California) | Two 500 MW blocks on Aug. 14; one on Aug. 15 | About 491,600 and 321,000 customers | Extreme heat; evening supply shortfall |
| February 15–18, 2021 | ERCOT, SPP, MISO South | 23,418 MW combined (ERCOT 20,000) | More than 4.5 million people in Texas | Winter Storm Uri: frozen equipment, fuel supply problems |
| December 23–24, 2022 | TVA, Duke Energy, LG&E/KU, others | More than 5,400 MW | About 500,000 Duke customers on Dec. 24 alone | Winter Storm Elliott: cold-related generator outages |
Sources: FERC-NERC 2011 report; CAISO Final Root Cause Analysis; FERC-NERC Uri report; FERC-NERC Elliott report.
For the bigger historical picture, including blackouts that were not controlled, see the biggest blackouts in U.S. history.
Winter Storm Uri, Texas (February 2021)
From February 8 to 20, 2021, extreme cold and freezing precipitation caused 1,045 generating units in Texas and the South Central U.S. to suffer 4,124 outages, derates or failures to start. From 7 a.m. February 15 to 1 p.m. February 17, ERCOT averaged 34,000 MW of unavailable generation. That is nearly half its all-time winter peak.
ERCOT ordered its first 1,000 MW of load shed at 1:20 a.m. on February 15. Shedding peaked at 20,000 MW by 7 p.m. and lasted nearly three days. Because ERCOT can import only slightly more than 1,000 MW over its direct-current ties, it could not lean on neighbors the way MISO and SPP did.
The FERC-NERC report found:
- 75% of the outages, derates and failures to start came from freezing problems (44.2%) or fuel problems (31.4%).
- 81% of freeze-related outages happened at temperatures above the plants' stated design temperatures.
- 26 ERCOT units had no winterization plans at all.
The human cost was severe. The Texas Department of State Health Services counted 246 deaths between February 11 and June 4, 2021, according to the Texas Tribune. Close to two-thirds were from hypothermia.
Winter Storm Elliott, Southeast (December 2022)
Elliott hit from December 21 to 26, 2022. At its worst point, 90,500 MW of generation was out unexpectedly across the Eastern Interconnection at the same time. Several Southeastern grid operators shed more than 5,400 MW, according to the FERC-NERC inquiry:
- TVA: more than 3,000 MW. TVA ordered its 153 local power companies, which serve 10 million people, to cut 10% of their load.
- Duke Energy Carolinas and Duke Energy Progress: 1,000 MW and 961 MW. Duke said about 500,000 customers were affected on December 24.
- LG&E/KU, Dominion Energy South Carolina and Santee Cooper: smaller amounts.
TVA's after-action report said these emergency steps were needed for the first time in its 90-year history, WVLT reported.
California's rotating outages (August 2020)
During a West-wide heat wave, CAISO called two 500 MW blocks of load shed on August 14, for a total of one hour, and one 500 MW block on August 15, for 20 minutes. The final root cause analysis called the heat wave a 1-in-30-year event. It named three causes: extreme heat, resource planning that had not kept up with early-evening needs after solar output fades, and some day-ahead market practices.
The outages ran longer than CAISO's orders. Some PG&E customers were out for about 150 minutes, because PG&E had less than 10 minutes' warning and had to switch circuits by hand.
Rolling blackouts vs. PSPS wildfire shutoffs
| Rolling blackout | Public Safety Power Shutoff (PSPS) | |
|---|---|---|
| Why | Not enough supply to meet demand | Wind and dry conditions could let power lines start a wildfire |
| Who decides | Grid operator orders it; utilities carry it out | The utility decides |
| Warning | Often minutes | SCE begins notices up to three days ahead, though sudden wind changes can cut that short |
| Duration | Typically about an hour per rotation; longer in severe emergencies | Can last over 24 hours; power returns only after crews inspect the lines |
The CPUC describes PSPS as utilities temporarily turning off power "to reduce the risk of fires caused by electric infrastructure," as "a measure of last resort." Southern California Edison says its line inspections "typically take up to eight hours" before power can be restored.
How to know if rolling blackouts are coming
Rolling blackouts almost always come after a run of warnings:
- Texas. ERCOT's TXANS issues a Weather Watch, a Voluntary Conservation Notice and a Conservation Appeal before any EEA. EEA notifications need a separate sign-up. ERCOT also posts alerts through its app and social media.
- California and other regions. Watch for conservation requests and EEA declarations from your grid operator.
- Your utility. Sign up for outage text or email alerts and bookmark your utility's outage map.
- PowerOutageTracker. Our grid threats page tracks active grid and weather alerts. Our Texas and California outage pages show where customers are already out.
When an EEA 2 or 3 is declared, conserve power, charge phones and medical devices, and review what to do during a power outage.
What changed afterward
Each event led to formal recommendations. After Uri, FERC and NERC staff recommended revising mandatory reliability standards. Generator owners would have to identify and protect cold-weather-critical components, retrofit units for local extreme temperatures, train staff on winterization plans every year, and write corrective action plans after freeze-related outages.
Elliott, less than two years later, was the fifth event in 11 years in which cold-related generator outages put grid reliability at risk. That shows how much work remains. Growing electricity demand is now adding pressure too. See our explainer on AI data centers and the power grid.
The bottom line
Rolling blackouts are a controlled last resort, not a failure of the grid's safety systems. Operators accept short, rotating outages to avoid an uncontrolled collapse that could take days to recover from. They are rare and usually come after public warnings. The 2021 and 2022 winter storms showed they can last much longer than an hour when supply shortfalls are deep. The best preparation is simple: sign up for alerts, conserve when asked, and keep a basic outage kit ready.
Check live outages where this matters most
Frequently asked questions
What is the difference between a rolling blackout and a regular power outage?
A regular outage happens when something breaks, such as a line downed by a storm. A rolling blackout is deliberate: the grid operator orders utilities to disconnect a set amount of demand because there is not enough supply, and utilities rotate that cut among groups of customers. FERC and NERC describe it as operators ordering 'a percentage of the demand or load to be temporarily disconnected, to avoid system instability.'
How long do rolling blackouts last?
They are meant to be short. Southern California Edison says CAISO-ordered rotating outages last 'only about one hour' before power rotates to another group. But they can last much longer in a severe emergency. During Winter Storm Uri in 2021, some Texans went without power for as long as four days, because the shortage was so large that utilities could not keep rotating outages.
What does an Energy Emergency Alert Level 3 mean?
Under NERC's EOP-011 standard, EEA 3 means 'firm load interruption is imminent or in progress': the grid operator can no longer meet its minimum reserve requirements. ERCOT describes EEA Level 3 as 'extremely low energy reserves' with 'controlled outages possible.' EEA 1 and EEA 2 come first and use other tools, such as all available generation, emergency imports and demand response.
Is a Public Safety Power Shutoff (PSPS) the same as a rolling blackout?
No. A rolling blackout responds to a supply shortage and is ordered by the grid operator, often with little warning. A PSPS is a wildfire-prevention measure: a utility turns off power in high fire-risk areas during dangerous weather to keep its equipment from starting a fire. Southern California Edison says it begins PSPS notices up to three days ahead, that events may last over 24 hours, and that power returns only after crews inspect the lines.
How will I know if rolling blackouts are coming?
In Texas, ERCOT's TXANS system issues a Weather Watch, Voluntary Conservation Notice and Conservation Appeal before an Energy Emergency Alert. Elsewhere, watch for conservation requests and EEA declarations from your grid operator and sign up for your utility's outage alerts. Our grid threats page tracks active alerts, but the final warning before a rotation can be very short: SCE says it may get less than 10 minutes' notice.
Sources
- FERC, NERC and Regional Entity Staff — The February 2021 Cold Weather Outages in Texas and the South Central United States (2021)
- FERC, NERC and Regional Entity Staff — Inquiry into Bulk-Power System Operations During December 2022 Winter Storm Elliott (2023)
- CAISO, CPUC and CEC — Final Root Cause Analysis: Mid-August 2020 Extreme Heat Wave (2021)
- FERC and NERC Staff — Report on Outages and Curtailments During the Southwest Cold Weather Event of February 1-5, 2011 (2011)
- NERC — Reliability Standard EOP-011-4, Emergency Operations (2024)
- ERCOT — Texas Advisory and Notification System (TXANS) (accessed 2026)
- Southern California Edison — What Is a Rotating Outage? (accessed 2026)
- California Public Utilities Commission — Public Safety Power Shutoffs (accessed 2026)
- Southern California Edison — Public Safety Power Shutoffs (accessed 2026)
- Texas Tribune via KSAT — Texas puts final estimate of winter storm death toll at 246 (2022)
- WVLT — TVA releases report detailing December 2022 blackouts (2023)
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PowerOutageTracker.com. "Rolling Blackouts Explained: Why Grid Operators Cut Power on Purpose." Accessed November 2, 2026. https://poweroutagetracker.com/guides/rolling-blackouts-explained