On 3 October 2025 the expert panel set up by ENTSO-E to investigate the blackout of 28 April published its factual report. At 12:33 CEST that day continental Spain and Portugal lost all power. A small area of southern France was briefly affected, but the rest of the European system saw no disturbance. ENTSO-E called it the most severe blackout on the European power system in more than 20 years and the first of its kind, caused by overvoltage. The panel of 45 experts from transmission operators and regulators across Europe gathered data from generators, grid users and network operators to reconstruct the sequence. It noted significant difficulties obtaining high-quality data from several Spanish distribution operators and generation companies. A final report with root causes and recommendations is due in the first quarter of 2026.
The factual report does not assign blame or reach conclusions on root causes. But the sequence it describes is already clear enough to draw lessons. Our view is that the blackout was fundamentally a failure of voltage control in a system where the tools and rules for managing voltage had not kept up with the change in its generation mix. It was not caused by renewables as such, and the right response is to change how inverter-based generation is required to behave, not to slow its growth.
What the report shows
The morning of 28 April looked like a typical spring day in Spain, with a high concentration of solar and wind output in the south-west at midday. Voltage variations became significant from about 10:30 CEST, though still within normal operating ranges. The upper voltage limit at 400 kV is 435 kV in Spain and 420 kV in Portugal. The report notes that on the evening of 27 April a generation unit in Andalusia was declared unavailable until 29 April. It was replaced overnight, and on the morning of 28 April the Spanish transmission operator performed security assessments and concluded it was not necessary to replace it for the rest of the day.
Before the collapse, the system experienced oscillations: a local oscillation at 0.63 Hz and an inter-area oscillation at 0.21 Hz. Spanish and French operators took standard mitigating measures, switching in lines to reduce system impedance, setting the HVDC link between Spain and France to fixed power mode and reducing the flow between the two countries. These measures damped the oscillations, but voltages rose.
Then the cascade began. Milliseconds after 12:32:57 a generation transformer in the Granada region tripped on overvoltage protection while injecting 355 MW, with 417.9 kV on the 400 kV side. Next, photovoltaic and solar thermal facilities connected to two 400 kV substations in Badajoz tripped, removing about 725 MW. Between 12:33:17 and 12:33:18, further trips disconnected wind and solar generation in Segovia, Huelva, Badajoz, Seville and Cáceres, about 930 MW or more. By 12:33:18 Spain had lost about 2.5 GW of generation. Each loss removed reactive power absorption and pushed voltage higher, causing further disconnections. Frequency fell, the peninsula lost synchronism with continental Europe, and at 12:33:21 all AC lines to Morocco and France disconnected. Automatic load shedding was activated but could not stop the blackout because of its overvoltage nature.
Why voltage control is the core issue
Voltage on a transmission grid is managed by injecting or absorbing reactive power. Traditionally, large synchronous generators such as gas, coal, nuclear and hydro plants do much of this work, and transmission operators supplement it with reactors, capacitors and other devices. The report shows a difference between the reactive power that conventional units in central and south-west Spain were expected to provide and what they actually provided. It lists among the areas for further investigation the voltage management instruments available, the behaviour of grid users in voltage control and disconnection, and the performance of the system defence plan.
Many solar and wind plants are connected through inverters and, under the rules in force in Spain, operated with a fixed power factor rather than actively controlling voltage. That means they do not adjust their reactive power to help stabilise voltage when it rises. Modern inverters are technically capable of doing so. The problem is regulatory and contractual: the rules did not require it, and the market did not pay for it.
The policy lessons
Our view is that three lessons are already clear, even before the final report. First, inverter-based generation must be required, and paid, to provide dynamic voltage control. Spain's regulator had been working on reforms to allow renewable plants to participate in voltage control. That work should be completed quickly, and other European countries with high solar penetration should check whether their own rules are adequate.
Second, system operators need enough voltage control resources online at all times, which means scheduling synchronous plants or equivalent devices based on voltage needs and not only on energy and reserve needs. Synchronous condensers, static compensators and grid-forming inverters with batteries can provide these services without burning fuel. Investment in them is cheap relative to the cost of a national blackout.
Third, data. The panel's difficulty in obtaining data from Spanish distribution operators and generators is itself a finding. Operators cannot manage a system they cannot see. Real-time visibility of distributed and transmission-connected generation, including its reactive power behaviour, should be a requirement.
Interconnection and isolation
The Iberian peninsula is weakly connected to the rest of Europe through France. Stronger interconnection would not have prevented a voltage collapse that began inside Spain, but it would provide more support during disturbances and speed restoration. In the event, restoration in Spain began with support from France and Morocco and black-start plants, while Portugal restored from black-start units and then links to Spain. The long-delayed Bay of Biscay interconnector and other projects should be treated as resilience investments, not only as market integration projects.
Our assessment
The Iberian blackout was a warning to every European system with fast-growing inverter-based generation. The factual report shows a cascade driven by overvoltage, in which generators disconnected to protect themselves and in doing so made the problem worse. The answer is not less solar and wind. It is rules that make them part of the solution for voltage control, enough dynamic reactive resources on the system, and the data to operate it. The final report should be used to set those requirements across Europe, not just in Spain.