The Biggest Blackouts in U.S. History

Key facts

• Most people at once: the August 14, 2003 blackout, with about 50 million people and 61,800 MW of load in eight U.S. states and Ontario.
• Longest and largest by lost service: Hurricane Maria (2017). All 1.57 million Puerto Rico utility customers lost power, and full restoration was declared about 11 months later.
• Largest controlled power cut: Winter Storm Uri (February 2021), when grid operators shed 23,418 MW. More than 4.5 million people in Texas lost power.
• Most hours without power in a decade: in 2024, driven mostly by Hurricanes Beryl, Helene and Milton. Helene alone cut power to 5.9 million customers across 10 states.
• Lasting reforms: the 1965 blackout led to NERC's creation in 1968. The 2003 blackout led to mandatory, enforceable reliability standards, which took effect June 18, 2007.

The biggest U.S. blackouts at a glance

Event	Date	Where	Scale	Main cause
Northeast blackout	Nov. 9, 1965	N.Y., Conn., Mass., R.I., parts of Pa., N.J. and Ontario	30 million people; 20,000+ MW; up to 13 hours	Protective relay tripped a key line; cascade
New York City blackout	July 13–14, 1977	New York City	9 million people; 6,000 MW; up to 26 hours	Lightning knocked out 345 kV lines
Northeast blackout	Aug. 14, 2003	8 states and Ontario	~50 million people; 61,800 MW; up to 4 days in parts of U.S.	Lines into trees, alarm failure, poor situational awareness
Southwest blackout	Sept. 8, 2011	Arizona, Southern California, Baja California	~2.7 million customers	Loss of one 500 kV line; system not secured for it
Hurricane Maria	Sept. 20, 2017	Puerto Rico	All 1.57 million PREPA customers; ~11 months	Hurricane disabled 80% of the grid
PG&E safety shutoffs	Oct. 9–12 and Oct. 26–29, 2019	Northern and Central California	~729,000 customers, then ~968,000	Deliberate wildfire-prevention shutoffs
Winter Storm Uri	Feb. 15–18, 2021	Texas and South Central U.S.	4.5+ million people in Texas	Frozen equipment and fuel shortages
Hurricane Ida	Aug. 29, 2021	Louisiana	~900,000 Entergy customers	All 8 transmission lines into New Orleans lost
Hurricane Helene	Sept. 26–27, 2024	10 states, mostly in the Southeast	5.9 million customers	Wind and flooding
Hurricane Milton	Oct. 9–10, 2024	Florida	3.4 million customers	Hurricane winds

Sources are linked in each section below.

People, customers and customer-hours measure different things. A "customer" is a utility account, so one household counts once. A long outage for fewer people can outweigh a brief one for many.

November 9, 1965: the Northeast blackout

According to the 2003 Task Force report's historical review, this disturbance cut more than 20,000 MW of load and affected 30 million people. Nearly all of New York, Connecticut, Massachusetts and Rhode Island lost power, along with parts of Pennsylvania, New Jersey and Ontario. Outages lasted up to 13 hours.

The trigger was small. A backup protective relay opened one of five 230 kV lines carrying power from a plant in Ontario toward Toronto when its loading passed the relay's 375 MW setting. Operators didn't know about that setting. Within 2.5 seconds the other four lines tripped too, and power swings cascaded across the Northeast. The event led directly to the creation of the North American Electric Reliability Council in 1968.

July 13–14, 1977: New York City

Lightning struck two 345 kV lines on a shared tower in northern Westchester County. Over the next hour, Consolidated Edison's system separated from its neighbors and collapsed, because the city's own generation could not carry its load. The blackout cut 6,000 MW and affected 9 million people, with outages lasting up to 26 hours, the Task Force report notes.

August 14, 2003: the largest by population

Just after 4:00 p.m. Eastern, a cascade blacked out an area with about 50 million people and 61,800 MW of load. It hit Ohio, Michigan, Pennsylvania, New York, Vermont, Massachusetts, Connecticut, New Jersey and Ontario. Some U.S. areas were without power for four days, and parts of Ontario had rolling blackouts for more than a week. Estimated U.S. costs ranged from $4 billion to $10 billion.

The U.S.-Canada Task Force's April 2004 final report grouped the causes into four categories:

1. FirstEnergy and its reliability council (ECAR) failed to understand the inadequacies of FirstEnergy's system, especially its vulnerability to voltage instability around Cleveland and Akron.
2. FirstEnergy lacked situational awareness. Its control-room alarm and logging software failed at 2:14 p.m., and operators did not realize it.
3. FirstEnergy failed to manage tree growth in its transmission rights-of-way. That was the common cause of the loss of three 345 kV lines and one 138 kV line.
4. Grid reliability organizations, including MISO, did not provide effective real-time diagnostic support.

The report's first recommendation was to make reliability standards mandatory and enforceable. NERC's standards became mandatory and enforceable across the contiguous U.S. on June 18, 2007, per FERC.

September 8, 2011: the Southwest blackout

On the afternoon of September 8, a fault took out Arizona Public Service's Hassayampa–North Gila 500 kV line, a link in the main path carrying power from Arizona into Southern California. Power rerouted, other paths overloaded, and about 11 minutes later an automatic separation scheme at the San Onofre nuclear plant split San Diego from the grid. The result was a complete blackout of San Diego and Mexico's Baja California system.

About 2.7 million customers, which FERC says equals "five million or more individuals," lost power, some into the next day. A joint FERC-NERC inquiry found that the entities planning and running the grid "were not prepared to ensure reliable operation or prevent cascading outages in the event of a single contingency." APS later agreed to a $3.25 million civil penalty, according to the FERC settlement order.

September 2017: Hurricane Maria, the longest

Maria made landfall in Puerto Rico on September 20, 2017, as a Category 4 hurricane. The island's utility, PREPA, reported that all 1,570,000 of its customers were without power, EIA reported. The storm made 80% of PREPA's transmission and distribution network inoperable, EIA later found.

Five weeks in, Rhodium Group estimated the outage at about 1.25 billion customer-hours and still climbing. It called the outage in Puerto Rico and the U.S. Virgin Islands "the largest blackout in U.S. history," as reported by The Weather Channel. For comparison, it put Hurricane Sandy at 775 million customer-hours and Katrina at 681 million.

PREPA announced in mid-August 2018, nearly 11 months after landfall, that its restoration work was done, NPR reported. Some damaged homes still couldn't be reconnected, and experts said the rebuilt grid remained fragile.

October 2019: PG&E's wildfire safety shutoffs

These outages were deliberate. To keep power lines from igniting wildfires during extreme wind, PG&E cut power to about 729,000 customers in 35 counties from October 9 to 12, 2019. It called that event "the largest to date," according to its report to the CPUC. Two weeks later, the October 26 and 29 events together cut power to about 968,000 distribution customers, according to PG&E's amended report. The average outage was about 55 hours.

February 2021: Winter Storm Uri

Extreme cold forced 1,045 generating units in Texas and the South Central U.S. offline, derated or unable to start. ERCOT shed 20,000 MW at the worst point, and SPP and MISO shed more. The combined 23,418 MW was the largest controlled firm load shed in U.S. history, according to the FERC-NERC staff report.

More than 4.5 million people in Texas lost power, some for up to four days. Texas health officials later put the death toll at 246, the Texas Tribune reported. For how controlled outages like these work, see rolling blackouts explained.

August 2021: Hurricane Ida and New Orleans

Ida knocked out all eight transmission lines that deliver power to New Orleans. A transmission tower spanning the Mississippi River, one that had survived Hurricane Katrina, fell, AP reported. About 900,000 Entergy Louisiana and Entergy New Orleans customers lost power at the peak. Entergy counted 30,679 damaged or destroyed poles, more than Katrina, Ike, Delta and Zeta combined, per Entergy.

September–October 2024: Helene and Milton

Helene made landfall near Perry, Florida, late on September 26, 2024, as a Category 4 hurricane. It then moved north through the Southeast, bringing strong winds and flooding rains. EIA reports that Helene left 5.9 million customers without power across 10 states, including at least 1.2 million in South Carolina. DOE put the region-wide peak at 4.6 million customers on September 27.

Two weeks later, Milton came ashore near Siesta Key, Florida, on October 9 as a Category 3 hurricane. Outages peaked at about 3.4 million Florida customers on October 10, according to DOE. More than 50,000 workers from at least 42 states, D.C. and Canada helped restore power.

Partly because of these storms, U.S. customers averaged about 11 hours of interruptions in 2024, the most in a decade. Hurricanes Beryl, Helene and Milton accounted for about 80% of those hours, according to EIA. See how your state compares in our average outage duration by state report.

International comparison: the 2025 Iberian blackout

At 12:33 p.m. local time on April 28, 2025, mainland Spain and Portugal lost power. The Iberian Peninsula has nearly 60 million people. Spain's grid operator had restored more than 99% of demand by 6:30 a.m. the next day, and Portugal had restored 6.2 million of its 6.5 million households overnight, Al Jazeera reported.

The ENTSO-E expert panel's final report, presented March 20, 2026, called it Europe's most severe blackout in more than 20 years and "the first ever of its kind (overvoltage)." It found "not a single root cause" but several factors. Voltage rose too fast, and generators disconnected in a cascade of overvoltage trips that automatic load shedding could not stop.

What these events taught grid operators

• Small failures can cascade. A relay setting in 1965, trees in 2003 and a single line in 2011 each triggered a regional blackout. Planning for the loss of any one element, known as the "N-1" standard, is now central to reliability rules.
• Rules need teeth. NERC began as a voluntary council after 1965. Mandatory standards came only after 2003.
• Operators need to see the whole system. Alarm failures and poor visibility of neighboring systems were central to both 2003 and 2011.
• Weather is now the dominant threat. Hurricanes, cold snaps and wildfire risk drove most of the recent events above, and 2024 set a decade high for outage hours.
• Restoration is a test of its own. Maria showed how a weak grid and hard-to-reach damage can stretch an outage into months.

Today's monitoring tools reflect those lessons. Our live outage map and grid threats monitor track outages and the weather and grid alerts that tend to come before them. If your own lights go out, here's how to check if the power is out near you.

The bottom line

The biggest U.S. blackouts fall into two groups. Grid cascades like 1965, 2003 and 2011 hit tens of millions of people and were fixed in hours or days. Weather disasters like Maria, Uri, Ida and Helene hit fewer people but lasted far longer. Several led to concrete reforms, from NERC's founding to mandatory standards and new winterization recommendations. The best lesson for households is to prepare for a multi-day outage, which is now the more common risk.

PowerOutageTracker.com. "The Biggest Blackouts in U.S. History." Accessed November 2, 2026. https://poweroutagetracker.com/guides/biggest-blackouts-in-us-history