In the late afternoon of April 28, 2025, the Iberian grid was operating under relatively high renewable penetration, with wind and solar together accounting for more than half of instantaneous generation. A disturbance in the transmission system triggered a cascade of voltage instabilities. Because the majority of renewable plants in Spain and Portugal are equipped with grid‑following inverters, they were unable to provide the reactive power and voltage support needed to stabilize the system.
Within minutes, oscillations spread across the network. Conventional plants attempted to compensate, but their reduced share of generation left insufficient inertia and reactive power reserves. The result was a total system collapse, with both Spain and Portugal losing supply. Restoration took several hours, with priority given to hospitals, transport hubs, and critical services. The event was the most severe system collapse in Europe in over two decades, affecting tens of millions of people, halting transport, and disrupting industry across the Iberian Peninsula.
On October 3, 2025, the European Network of Transmission System Operators for Electricity (ENTSO‑E) released its long‑awaited factual report on the April 28 blackout. While the report stops short of assigning root causes—those will come in a final analysis due in early 2026—it provides a detailed chronology of events, identifies structural weaknesses in the Iberian grid, and highlights the urgent need to integrate grid‑forming inverter technology into renewable generation fleets.
The ENTSO‑E report emphasizes three key findings:
- High Renewable Penetration Without Ancillary Services
- At the time of the blackout, renewables supplied more than 50% of Iberian demand.
- Nearly all of these plants used grid‑following inverters, which can only operate when a stable grid frequency and voltage are already present.
- As a result, they withdrew from the grid during the disturbance rather than helping to stabilize it.
- Regulatory Restrictions on Renewables
- Until June 2025, Spanish grid codes (Operational Procedure 7.4) did not permit renewable generators to provide voltage support.
- This regulatory lag meant that even technically capable inverters were not configured to deliver reactive power or synthetic inertia.
- Restoration Challenges
- Black‑start capability was limited to conventional plants.
- The absence of grid‑forming renewables slowed restoration, as operators had to rely on a small number of synchronous units to re‑energize the system.
Grid‑Following vs. Grid‑Forming
The report underscores the fundamental difference between grid‑following and grid‑forming inverters:
- Grid‑following inverters synchronize to an existing grid waveform. They are inexpensive, widely deployed, and optimized for maximum energy delivery. However, they cannot set voltage or frequency on their own.
- Grid‑forming inverters can establish a stable voltage and frequency reference, mimicking the behavior of synchronous machines. They can provide reactive power, synthetic inertia, and black‑start capability.
ENTSO‑E notes that the vast majority of Iberian renewable capacity—well over 95%—is grid‑following. This imbalance left the system vulnerable when synchronous generation was scarce.
Transitioning Spain and Portugal’s renewable fleets from grid‑following to grid‑forming capability will require significant investment. The report does not provide cost estimates, but industry analyses suggest that grid‑forming inverters currently carry a premium of about $100 per kilowatt compared to conventional units. For a 100‑MW solar plant, this translates into an additional $10 million in upfront costs.
Given that Spain has more than 80 GW of installed renewable capacity, the potential system‑wide cost of retrofitting or replacing inverters could run into the billions of euros. While some existing inverters can be upgraded via software, many older units would require hardware replacement.
ENTSO‑E stresses that these costs must be weighed against the economic and social losses of large‑scale blackouts. The April 2025 event disrupted commerce, transport, and healthcare, with damages likely exceeding the incremental cost of deploying grid‑forming technology.
Policy and Market Responses
In the wake of the blackout, Spain moved quickly to revise its regulations. In June 2025, Operational Procedure 7.4 was updated to allow and encourage renewables to provide voltage and frequency support. New projects are now expected to include grid‑forming capability, particularly when paired with battery storage.
Portugal is following a similar path, with its regulator signaling that future renewable auctions will include ancillary service requirements. At the European level, ENTSO‑E is preparing recommendations for harmonized grid codes that mandate grid‑forming capability in all new inverter‑based resources.
Industry players are also responding. In August 2025, Galp announced five new battery projects in Spain and Portugal using Sungrow’s PowerTitan 2.0 grid‑forming inverters, explicitly designed to provide fast frequency response and voltage regulation. These projects are seen as early examples of the transition ENTSO‑E envisions.
The Iberian blackout highlights a challenge facing all high‑renewable systems: how to maintain stability when synchronous machines are displaced by inverter‑based resources. While Spain and Portugal were the first to experience a continent‑wide blackout under these conditions, other regions with high renewable penetration—such as Germany, Denmark, and parts of the United States—are also grappling with the same issue.
ENTSO‑E’s report frames the April 2025 event as a wake‑up call. Without rapid deployment of grid‑forming technology, Europe risks further instability as renewable penetration rises. The costs of conversion are significant, but the costs of inaction, as demonstrated in April, are far greater.
References (APA)
- ENTSO‑E. (2025, October 3). Factual report on the April 28, 2025 Iberian Peninsula blackout. European Network of Transmission System Operators for Electricity. https://www.entsoe.eu.
- SolarPower Europe. (2025, October 4). Spain blackout shows need for grid‑forming renewables. https://www.solarpowereurope.org.
- Thunder Said Energy. (2024). Grid‑forming inverters: islands in the sun. https://www.thundersaidenergy.com.
- Sungrow. (2025, August). Galp selects Sungrow PowerTitan 2.0 for Iberian storage projects. https://www.sungrowpower.com.
