Introduction
On April 28th 2025, the Iberian Peninsula experienced one of the most significant power blackouts in European history, affecting Portugal, Spain and parts of southwestern France. More than 60 million grid users were impacted, with outages lasting around ten hours in most regions, and even longer in some areas. The blackout disrupted communication, transportation, hospitals and other essential services.
Now, five months after the lights went out in Iberia, important questions remain. Do we already know what caused this major blackout? Could it have been avoided? How did transmission system operators manage to restore the network so quickly? And what was the impact of the blackout?
Investigation by expert Panel
On May 12, 2025, two weeks after the blackout, the European Network of Transmission System Operators for Electricity (ENTSO-E) established a joint expert panel composed of representatives from transmission system operators (TSOs), the Agency for the Co-operation of Energy Regulators (ACER), national regulatory authorities (NRAs), and regional coordination centres (RCCs). The detailed composition of the panel is provided in ENTSO-E’s official webpage on the blackout [1]. The panel is mandated to investigate the root causes of the incident and to publish its conclusions.
The investigation is conducted in two phases. In the first phase, the panel gathers all available data from the affected TSOs and relevant authorities. This material is analysed to produce a factual report that reconstructs the events of April 28 and provides an initial explanation of the causes of the blackout. On the moment of writing, the report is scheduled for publication on October 3, 2025. However, ENTSO-E has already released preliminary findings [1], which will be discussed in the following section.
In the second phase, the panel will issue a final report containing a more in-depth analysis and a set of recommendations. This report will examine the root causes of the incident in greater detail, evaluate the actions taken to mitigate system oscillations, assess the performance of generators with respect to protection settings, and review both the system defence plans and the measures implemented during the restoration phase [1].
Timeline
On April 28th, prior to the disturbance, the Iberian Peninsula was operating with a grid dominated by renewables electricity. According to the Spanish TSO, REE, the load was covered for 59.8% by solar power, 10.6% by wind power, and 10.5% by nuclear power and Spain was exporting energy to all its neighboring countries [2].
It is worth noting that the high share of renewable energy in the generation mix resulted in lower grid inertia [2], [3]. Grid inertia refers to the tendency of the power system to remain in its operating state. It can therefore be seen as a resistance to frequency changes. This inertia is provided by the kinetic energy stored in the rotating mass of the synchronous generators connected to the grid, all rotating at synchronous speed [4]. A lower grid inertia implies a higher sensitivity of the grid to disturbances and means the system requires faster control mechanisms to maintain stability.
From 09:00 Central European Summer Time (CEST) voltage variabilities were observed in Spain. However the voltage in the transmission system remained under its upper operational limit, and at 12:00 CEST the transmission network was around its nominal working conditions again [1]. REE concluded in its report [5] that these oscillations were neither significant nor the cause of what happened subsequently. Before 12:03 CEST, the system is said to be within acceptable voltage and frequency limits.
Between 12:00 and 12:30 CEST, the Iberian grid experienced two periods of oscillations, signalling the growing instability of the system. An analysis performed by ENTSO-E allowed to determine that the first of these two periods, taking place from 12:03 to 12:07 CEST, was a forced oscillation induced by an external source, such as a power plant.
In order to mitigate the voltage oscillations, the system operators took measures like reactive power adjustments and switching in lines to decrease the system impedance and improve generator stability. To reduce the risk of system oscillations spreading, operators lowered the AC power flow between Spain and France. This effectively reduced the phase angle differences between the Iberian Peninsula and the rest of Continental Europe, helping stabilize both regions [1].
The second period of oscillations was inter-area and took place between 12:19 and 12:22 CEST. These oscillations were successfully mitigated by additional countertrading measures. These measures further reduced power flows between Spain and France and strengthened the internal connections of the southern Spanish grid [1].
While grid operators had successfully contained earlier instabilities, the system remained fragile and new issues soon emerged. As shown on Figure 2, at 12h32 CEST Spain’s net active power exchange began to decrease which in turn led to a rise in voltage levels. According to the Expert Panel, the primary driver of this development was the loss of more than 500 MW of wind and solar generation connected to Spain’s distribution networks. The exact cause of these disconnections remains unclear, largely due to limited observability at the medium- and low-voltage levels [1].
After the periods of oscillations, three successive generation losses occurred within only twenty seconds. First, more than 350 MW of wind and solar power were lost following the tripping of a generation transformer in the Granada area. Subsequently, around 725 MW of injection was interrupted at two 400 kV substations. Finally, at 12:33:16, in less than one second, wind farms, solar-thermal plants, and other generators with a combined capacity exceeding 1100 MW tripped. The causes of these events are still under investigation, with further details expected in the Expert Panel’s factual report to be published in early October. No generation trips were observed in Portugal or France during this
timeframe [1].
As a result of the generation trips, the grid frequency dropped and the voltage increased drastically in the South of Spain, and, as a consequence, also in Portugal. This overvoltage triggered a cascade of more generation losses, which in turn caused the frequency of the Spanish and Portuguese power systems to decline. The Iberian system lost synchronism with the wider European grid. Between 12:33:19 and 12:33:22 CEST, Spain’s and Portugal’s automatic load-shedding mechanisms and Systems Defence plans were activated, but were unable to prevent the collapse. At 12:33:24 CEST, the Iberian power sys-
tem experienced a full black-out. Consequently, the AC interconnection between Spain and Morocco tripped due to underfrequency and the AC overhead lines with France were disconnected against loss of synchronism [1].
The restoration process
The TSOs initiated the restoration of the Iberian grid through a series of coordinated steps. Shortly after the blackout, REN requested start-ups for the Castelo de Bode hydropower plant (138 MW installed capacity [6]) and the Tapada do Outeiro combined-cycle gas turbine plant (990 MW installed capacity [7]) in Portugal [1]. These are powerplants with black-start capacity, meaning they can deliver electricity without requiring electric power from the grid themselves [8]. REE in Spain relied on multiple hydropower plants equipped with black-start capabilities as well, but also on the quick re-energizing of inter-
connections. Morocco supplied 900 MW and France was able to provide from 700 MW in the beginning up to 2 GW [2].
At 18:36 CEST, a first (220 kV) line between Spain and Portugal was re-energised, later followed by the first 400 kV tie-line at 21:35 CEST. The Portuguese power system was fully operational at 00:22 CEST and the Spanish grid followed at 04:00 CEST, both on 29th of April.
Impact of the blackout
The International Energy Agency (IEA) emphasizes on its website [9] that the Iberian Blackout illustrates how deeply electricity is woven into modern society. When a failure occurs, millions of citizens and businesses are immediately affected as power outages disrupt homes, schools, communication networks, financial services, and much more. The world is rapidly entering a new age of electricity. Traditional uses of power continue to grow, while emerging demands, such as electric vehicles and AI-driven data centers, are becoming increasingly significant. At the same time, the supply side of the electricity sector is undergoing profound change, driven by the rapid deployment of renewable tech-
nologies like solar photovoltaics and wind power worldwide. In this context, safeguarding the security and resilience of power systems is not merely a technical challenge but a strategic imperative.
One of the most critical sectors affected by the blackout was the health system, although it received surprisingly little public attention. Hospitals were able to sustain essential services such as emergency departments, intensive care units, and life-support systems through backup generators. Nevertheless, the simultaneous shutdown of mobile networks, landlines, and internet services created severe communication bottlenecks. Most hospitals were unable to coordinate effectively with each other, civil protection authorities,
or emergency services. Even the national emergency number (112) became temporarily inaccessible, leaving citizens without a vital lifeline at a critical moment.
The consequences extended beyond healthcare facilities themselves. Failures in traffic lights, road congestion, and disrupted communication between ambulance dispatch centers likely caused delays in emergency transport and patient transfers. In its perspective article, Frontiers [10] argues that healthcare resilience must be recognized as a fundamental pillar of national security. It calls for regulatory reforms, decentralized energy solutions, digital redundancies, and integrated command structures linking healthcare to other strategic sectors. According to Frontiers, the blackout acted as a decisive stress
test for Portugal’s National Health Service, exposing a convergence of structural, digital, and operational vulnerabilities that critically undermined healthcare delivery during the crisis.
The blackout also had economic consequences. CaixaBank estimates that the the impact on Spains GDP was less than a tenth of a percentage point, amounting just under 400 million euros [11].
But the most tragic consequences of the blackout were deaths potentially linked to the event. According to the BBC [12], three people died from carbon monoxide poisoning caused by a faulty generator used in their home. Other fatalities remain under investigation, such as a woman in Madrid who died in a fire, possibly started by a candle used during the outage. Additional deaths were reported that day, though the causes are not always agreed upon.
Could blackouts happen elsewhere in Europe?
The Iberian Peninsula faces challenges due to limited interconnection, but even highly connected countries like Belgium or the Netherlands are not immune to blackouts. While Spain’s interconnection capacity is 8% of peak demand, the Netherlands reaches nearly 48%, according to Rabobank [13]. However, strong interconnections do not fully eliminate blackout risks.
The Dutch grid is among Europe’s most reliable, with households experiencing only 22 minutes of outage in 2023, and a nationwide blackout has never occurred. Its resilience is supported by larger battery storage, interconnectors, and thermal capacity compared to Spain. However, concerns remain about the future: after 2030, the planned phaseout of some thermal capacity may leave demand uncovered at times, forcing temporary load shedding. Additionally, grid congestion has become a pressing issue in some regions. Therefore, regular short-term outages are now considered increasingly likely [13].
Conclusion
Although we are waiting on the factual report for definitive answers, we can state that the Iberian blackout of April 28th 2025 was caused by a complex series of events, which made it extremely hard to prevent. Reduced system inertia was not a direct cause, however strengthening grid inertia will likely be a crucial safeguard against future disruptions. The use of grid-forming inverters combined with storage systems can provide synthetic inertia and thus substantially enhance system stability.
Following the blackout, electricity grids became a priority on political agendas. Portugal’s government already announced a €400 million investment in the electricity system [14]. Spain’s TSO also pledged greater investment, though with more debate on the specifics. It seems inevitable that other governments will follow suit, recognizing that grid stability is not just a technical matter but a cornerstone of energy security in a decarbonizing Europe.