Entso-E final report on Iberian 2025 blackout

I was supposed to fly home from Santiago de Compostella when the blackout happened. Me and my girlfriend had checked out of our hotel and headed to the bus stop to take the bus to the airport. The blackout had already started but we didn't realise (in hindsight, I do remember the pedestrian crossing not working. But I didn't think much of it). Anyways our flight was cancelled and it was clear we needed somewhere to stay the night.

I immediately rebooked the same hotel, but when we got back there the receptionist had left so you had to check in over the phone instead. Except WhatsApp wasn't working. Then mobile data went down. And before long we were walking through the old town going hostel to hostel looking for a place to sleep, as everything got darker and darker (due to the lack of powered street lighting). The old town in almost pitch black was pretty scary!

We ended up breaking back into the hotel, borrowing a bunch of towels from a laundry cart in the hallway and sleeping in this lockable room we found in the basement.

Besides that somewhat stressful part, it was a really strange but fun experience to see the city without power: no traffic lights, darkened shops with lots of phone lights, cafés still operating just with only outdoor seating and limited menus, the occasional loud generator, and most of all the people seemingly having a great time in spite of it.

I would've loved to have stayed out all night exploring the city, but finding somewhere to sleep that night was a bit more pressing!

I think people underestimate how valuable these reports are, so I’m very glad that detailed investigation is done here. Every major grid operator around the world is going to study this and make improvements to make sure this doesn’t happen on their grid.

In a lot of ways it’s like investigations into airplane crashes.

As someone who lived through the blackout it was wild. I felt back into the pre-internet, pre-smartphone era. It was pretty cool actually. The rumor mill spread so fast that Within hours the official word on the street was that we were getting hacked by a foreign military and people were joking that we had nothing of interest to be conquered xD

The fact that there is not a single root cause but several ones makes me instinctively think this is a good report, because it's not what the "bosses" (and even less politicians) like to hear.

If someone wants a "quick and dirty" answers - there's presentation linked https://eepublicdownloads.blob.core.windows.net/public-cdn-c...

page 11 contains "Full root cause tree" - one image with all the high level info

472 pages. That's going to be a nice bit of reading this weekend. It is very nice to see such a comprehensive report as well as the fact that it was made public immediately.

0.63 Hz and 0.2 Hz grid instability. Oh my.

That was quite the interesting read!

Can’t read all of this since it’s 424 pages but i want to point out that Australia is beating Europe on grid connected storage. Not on a per capita basis. It’s beating all of Europe combined outright https://www.visualcapitalist.com/top-20-countries-by-battery...

We did have many many problems previously. The state of South Australia went out for a couple of weeks at one point in similar cascading failures. This doesn’t happen anymore. In fact the price of electricity is falling and the grid is more stable now https://www.theguardian.com/australia-news/2026/mar/19/power...

This price drop is inline with the lowered usage of gas turbine peaker plants (isn’t that helpful right now? No need for blockaded gas for electricity).

A lot of people say it can’t be done. That you can’t have free power during the day (power is free on certain plans during daylight due to solar power inputs dropping wholesale prices to negative) and that you can’t build enough storage (still not there but the dent in gas turbine usage is clear).

It’s one of these cases where you’ve been lied to. Australia elected a government that listened to reports battery+solar is great for grid reliability and nuclear was always going to be more expensive.

In a lot of ways it’s like investigations into airplane crashes.

472 pages. That's going to be a nice bit of reading this weekend. It is very nice to see such a comprehensive report as well as the fact that it was made public immediately.

The hack thing spread wildly, indeed. Weird experience.

I didn’t even know about it until the next day - totally off grid, and starlink for internet access - and no mobile signal where we live to give it away either.

The fact that there is not a single root cause but several ones makes me instinctively think this is a good report, because it's not what the "bosses" (and even less politicians) like to hear.

Frequently, when you see these massive failures, the root cause is an alignment of small weaknesses that all come together on a specific day. See, for instance, the space shuttle O-ring incident, Three-Mile Island, Fukushima, etc. These are complex systems with lots of moving parts and lots of (sometimes independent) people managing them. In a sense, the complexity it the common root cause.

There are ways to aggregate these into a single resilience score for policy makers with only moderate loss of detail but it's unpopular.

Yep, sounds like "This was bound to happen at some point"

I didn’t even know about it until the next day - totally off grid, and starlink for internet access - and no mobile signal where we live to give it away either.

The hack thing spread wildly, indeed. Weird experience.

There are ways to aggregate these into a single resilience score for policy makers with only moderate loss of detail but it's unpopular.

It's like the Swiss Cheese model where every system has "holes" or vulnerabilities, several layers, and a major incident only occurs when a hole aligns through all the layers.

https://en.wikipedia.org/wiki/Swiss_cheese_model

It usually starts with a broken coffee machine.

Yep, sounds like "This was bound to happen at some point"

It usually starts with a broken coffee machine.

Which on some level is exactly "what the bosses and politicians want to hear"

When it's everybody's fault it's nobody's fault.

It's like the Swiss Cheese model where every system has "holes" or vulnerabilities, several layers, and a major incident only occurs when a hole aligns through all the layers.

https://en.wikipedia.org/wiki/Swiss_cheese_model

I use this model all the time. It's very helpful for explaining the multifactorial genesis of catastrophes to ordinary people.

Which on some level is exactly "what the bosses and politicians want to hear"

When it's everybody's fault it's nobody's fault.

I use this model all the time. It's very helpful for explaining the multifactorial genesis of catastrophes to ordinary people.

We ended up breaking back into the hotel, borrowing a bunch of towels from a laundry cart in the hallway and sleeping in this lockable room we found in the basement.

I would've loved to have stayed out all night exploring the city, but finding somewhere to sleep that night was a bit more pressing!

If someone wants a "quick and dirty" answers - there's presentation linked https://eepublicdownloads.blob.core.windows.net/public-cdn-c...

page 11 contains "Full root cause tree" - one image with all the high level info

0.63 Hz and 0.2 Hz grid instability. Oh my.

That was quite the interesting read!

This price drop is inline with the lowered usage of gas turbine peaker plants (isn’t that helpful right now? No need for blockaded gas for electricity).

You need grid connected storage where you have (unpredictable) renewables. That doesn't negate the benefits of Nuclear baseload power. In an ideal mix, you need both, and also Gas for emergencies. One is not better than the other, they have different roles in a balanced grid.

> Australia elected a government that listened to reports battery+solar is great for grid reliability and nuclear was always going to be more expensive

The report you mean (csiro) was wildly biased though. They based their nuclear power cost estimate on a nuclear reactor that was never deployed anywhere (Nuscale) instead of "normal" nuclear power plants that have been deployed for decades.

> Australia elected a government that listened to reports battery+solar is great for grid reliability and nuclear was always going to be more expensive

The CSIRO report appears to have cost estimates for "normal" nuclear power plants too.

    Large scale nuclear $155-$252/MWh.
    Solar PV and wind with storage $100-150/MWh.

https://www.abc.net.au/news/2024-05-22/nuclear-power-double-...

Was the Nuscale cost estimate somehow worse than AP1000 or EPR(2)? That seems very unlikely to me given the history of those programs.

The NuScale cost was what the project itself announced. And they hadn’t even started building yet. The latest reports also include large scale nuclear power.

I find it funny when people get outraged because all CSIRO does is use real world construction costs easily proving how unfathomably expensive new built nuclear power is.

Nuclear has the same issue as (unpredictable) renewables, it is incapable of cost efficiently following the demand curve. As a result, just like renewables, it requires a form of dispatch-able power to complement it (gas, batteries, etc). Solar and nuclear fill the exact same role in a balanced grid - cheap non-dispatchable power.

Or at least nuclear would if it was cheap, but since its costs haven't fallen the same way that the costs of other energy did... well new nuclear buildout really doesn't have a good role at all right now, it's just throwing away money.

Solar and nuclear complement eachother fine - because their shortfalls (darkness for solar, high demand for nuclear) are mostly uncorrelated... a mix of non-dispatcahble power with uncorrelated shortfalls helps minimize the amount of dispatchable power you need... but batteries have made it cheap enough to transform non-dispatchable power to dispatchable power that nuclears high costs really aren't justifiable.

Nuclear has a hard time existing in a net with dominant renewables during most of the year. Down-regulating nuclear absolutely kills its profitability. What you want is power plants with low capex that can be profitable with just a few hundred hours at full capacity per year. For example you can burn hydrogen.

Plus, related (storage), you do not want to put hydroelectric in water reservoirs targeted to population consumption, as you could find out one summer that the reservoirs are empty, the result of such water being used with the intention of generate electricity, or even used as inertial stabilizer for renewables.

This is the moment were at the news you read "There's a drought because it isn't raining" and similar excuses, when in reality your five years of water's reservoirs become reduced to half -or one third- due they focused the electricity production over the population real water demand.

I mean, hydroelectric needs at least two level’s reservoirs, one to generate electricity (or even exclusive two level's reservoirs with water pumps for this), and the next one, absolutely untouchable by the electric companies, targeted as water storage for the population/agriculture, the classic more than five years reservoir, for real.

Was the Nuscale cost estimate somehow worse than AP1000 or EPR(2)? That seems very unlikely to me given the history of those programs.

The CSIRO report appears to have cost estimates for "normal" nuclear power plants too.

    Large scale nuclear $155-$252/MWh.
    Solar PV and wind with storage $100-150/MWh.

https://www.abc.net.au/news/2024-05-22/nuclear-power-double-...

The NuScale cost was what the project itself announced. And they hadn’t even started building yet. The latest reports also include large scale nuclear power.

I find it funny when people get outraged because all CSIRO does is use real world construction costs easily proving how unfathomably expensive new built nuclear power is.

Summary

The final report of the Expert Panel on the 28 April 2025 blackout in continental Spain and Portugal identifies the causes of the blackout and outlines recommendations to strengthen the resilience of Europe’s interconnected electricity system. It was prepared by a technical Expert Panel of 49 members, including representatives from Transmission System Operators (TSOs), Regional Coordination Centres (RCCs), ACER and National Regulatory Authorities (NRAs), and was chaired by experts from two unaffected TSOs.

The investigation concludes that the blackout resulted from a combination of many interacting factors, including oscillations, gaps in voltage and reactive power control, differences in voltage regulation practices, rapid output reductions and generator disconnections in Spain, and uneven stabilisation capabilities. These factors led to fast increases of voltage and cascading generation disconnections in Spain, resulting in the blackout in continental Spain and Portugal.

Based on these findings, the Expert Panel sets out recommendations addressing each of the factors identified in the report to help prevent similar events in the future. These include strengthened operational practices, improved monitoring of system behaviour and closer coordination and data exchange among power system actors. The findings of the investigation also underscore the need for regulatory frameworks to adapt in order to support the evolving nature of the power system.

The 28 April blackout was a first of its kind event, and the recommendations aim to strengthen system resilience with solutions that are already technologically deployable. This blackout highlights how developments at the local level can have system-wide implications and underlines the importance of maintaining strong links between local and European system behaviour and coordination, while ensuring that market mechanisms, regulatory frameworks and energy policies remain aligned with the physical limits of the system.

Download the final report of the Expert Panel

Context

On 28 April 2025, at 12:33 CEST, the power systems of continental Spain and Portugal experienced a total blackout. A small area in Southwest France close to the Spanish border experienced disruptions for a very short duration and several industrial consumers and generators were affected.

The rest of the European power system did not experience any significant disturbance as a result of the incident.

This was the most severe blackout incident on the European power system in over 20 years, and the first ever of its kind.

Figure 1 – Geographic area affected by the incident of 28 April 2025.

Expert Panel

Following the blackout, on 12 May 2025, ENTSO-E set up an Expert Panel in line with Article 15(5) of the Commission Regulation (EU) 2017/1485 of 2 August 2017 establishing a guideline on electricity transmission system operation (SO GL) and the Incident Classification Scale (ICS) Methodology. The ICS Methodology is the framework for classifying and reporting incidents in the power system and for organising the investigation of such incidents and is especially relevant to the work of the Expert Panel. It is noted that the investigation of the Expert Panel was performed in line with the version of the ICS Methodology applicable at the time of the incident.

Under the legal requirements of both SO GL and the ICS Methodology, when the incident is classified according to the ICS Methodology criteria as scale 3 incident – blackout – the Expert Panel is tasked to investigate the root causes of the incident, produce a comprehensive analysis, and make recommendations in a final report, published on 20 March 2026.

The Expert Panel consists of representatives from TSOs, the Agency for the Cooperation of Energy Regulators (ACER), National Regulatory Authorities (NRAs), and Regional Coordination Centres (RCCs).

The Panel is led by experts from TSOs not directly affected by the incident and includes experts from both affected and non-affected TSOs. The Expert Panel is led by Klaus Kaschnitz (APG, Austria) and Richard Balog (MAVIR, Hungary).

ENTSO-E Bodies Convenors participating in the Expert Panel

Olivier Arrivé – as chair of the System Operation Committees
Robert Koch – as convenor of the Steering Group Resilient Operation
Rafal Kuczynski – as convenor of the Regional Group Continental Europe

Experts from TSOs and RCCs participating in the Expert Panel

Amprion (Germany)	Tilman Ringelband
EirGrid (Ireland)	Donna Kearney
Elia (Belgium)	Bernard Malfliet
Elia (Belgium)	Peter Van Meirhaeghe
Energinet (Denmark)	Bent Myllerup
Mavir (Hungary)	Andras Woynarovich
Red Electrica (Spain)	David Alvira
REN (Portugal)	Filipe Ribeiro
RTE (France)	Laurent Rosseel
SEleNe CC (Greece)	Maria-Faidra Katsiantoni
Swissgrid (Switzerland)	Bastien Grand
Terna (Italy)	Guido Coletta
Terna (Italy)	Giorgio Giannuzzi
Transelectrica (Romania)	Mihai Cremenescu

Experts from ACER and NRAs participating in the Expert Panel

ACER	Georgios Antonopoulos
ACER	Uros Gabrijel
ACER	Aleksander Glapiak
ACER	Maria Barroso Gomes
ACER	Domen Kodric
ACER	Zoran Vujasinovic
ACER	Jan Kostevc
ANRE (Romania)	Alina Poanta
ARERA (Italy)	Marco Pasquadibisceglie
ARERA (Italy)	Andrea Rosazza
BNetzA (Germany)	Jochen Gerlach
BNetzA (Germany)	Thomas Hoelzer
BNetzA (Germany)	Nicolas Krieger
CNMC (Spain)	Virginia Garcia Escoin
CNMC (Spain)	Teresa Ibanez Tarrago
CNMC (Spain)	Diego Roldan Cabanillas
CRE (France)	Martin Frédéric
CRE (France)	Pierrick Muller
CRE (France)	Sara Rami
CREG (Belgium)	Marijn Maenhoudt
CREG (Belgium)	Emilie Leroy-Biasutti
EI (Sweden)	Anna Carlen
ElCom (Switzerland)	Raffaele Bornatico
ERSE (Portugal)	Jose Capelo
ERSE (Portugal)	Paulo Oliveira
ERU (Czech Republic)	Pavel Simoncik
MEKH (Hungary)	Erika Szirmai
URE (Poland)	Lukasz Makos
URE (Poland)	Piotr Rak
URE (Poland)	Witold Zuchowski

Expert Panel Meetings

12 May 2025
3 June 2025
23 June 2025
15 July 2025
18 August 2025
2 September 2025
19 September 2025
14 October 2025
30 October 2025
18 November 2025
10 December 2025
15 January 2026
29 January 2026
12 February 2026
26 February 2026
16 March 2026

Investigation Steps

On 12 May 2025, the Expert Panel initiated its investigation into the causes of the blackout. In accordance with the ICS methodology, the investigation is conducted in two phases:

1. Data collection and factual report

In the first phase of the investigation, the Expert Panel collected and analysed all data on the incident available at the time to reconstruct the events of 28 April. At the end of this first phase, the Expert Panel delivered its factual report, released on 3 October 2025. It describes the system conditions that prevailed on 28 April 2025, provides a detailed sequence of events during the incident and describes how the system was restored after the incident.

The report highlights the exceptional and unprecedented nature of this incident - the first time a cascading series of disconnections of generation components along with voltage increases has been part of the sequence of events leading to a blackout in the Continental Europe Synchronous Area.

Download the factual report of the Expert Panel

2. Final report and recommendations

In the second phase, which started immediately after the finalisation of the factual report, the Panel focused on the identification and analysis of the root causes of the incident. The Expert Panel specifically evaluated cascading disconnections of generation in the system, the voltage control and the oscillations’ mitigation measures. The Panel also assessed the performance of generators in regard to protection settings and contribution to voltage control, as well as the performance of the system defence plans and analysed the various steps of the restoration phase.

Based on the findings, the Panel sets out recommendations in its final report addressing each of the factors identified to help prevent similar events in the future.

Download the final report of the Expert Panel

Information to Stakeholders

A dedicated joint workshop of the System Operations European Stakeholder Committee (SO ESC) and of the Grid Connection European Stakeholder Committee (GC ESC), chaired by ACER, was organised on 18 July 2025 to inform the stakeholders on the progress of the investigation of the Expert Panel. A second joint workshop of SO ESC and GC ESC took place on 13 October 2025 to discuss the factual report. Detailed information about the role, composition and work of these two Committees is available on the ENTSO-E website here.

Hacker Times

Hacker Times

Entso-E final report on Iberian 2025 blackout

Discussion

Discussion

Summary

Context

Expert Panel

ENTSO-E Bodies Convenors participating in the Expert Panel

Experts from TSOs and RCCs participating in the Expert Panel

Experts from ACER and NRAs participating in the Expert Panel

Expert Panel Meetings

Investigation Steps

Information to Stakeholders

Publications & Documents

Reports

Press releases

Presentations

ICS Methodology document

Letters