On 28 April 2025 at 12:33 CEST, a power outage in continental Spain and Portugal caused a blackout that briefly affected a small part of southwestern France. This was the first complete blackout in the history of the Spanish system and the first blackout in history caused by overvoltage problems.
The event was not the simple story that many initially anticipated. It was not a cyber attack. It wasn’t a simple failure of solar or wind power plants. It wasn’t just caused by a lack of spin generation. It was a rapidly evolving stress event in a system where generation, grid management, security settings, regulation and visibility were not moving at the same speed as the energy transition.
Voltage regulation
Before we get into the consequences of the event and what they mean for Spain and for the wider European system, let’s start with the basics: what is voltage regulation. Everyone understands what a power shortage is, but what about reactive power? Voltage is the electricity grid equivalent of ‘pressure’ in a gas-hydraulic network. An energy system not only needs sufficient megawatts; it also needs megavars (reactive power) to stay within safe limits for the equipment to operate properly. If the voltage rises or falls too far, generators, transformers and protective systems can be disconnected to prevent damage, allowing a disturbance to spread quickly.
By comparison, frequency is the component of energy system stability that most people now associate with renewable energy sources, inertia, and rotating machines. Tension is less visible but just as decisive. It depends on reactive power, local grid conditions, generator behavior, security settings and the ability of grid operators to see and manage assets in real time.
This is why the Spanish blackout was not simply a matter of having sufficient power or spinning reserve available. The key question was whether the system had enough fast, local tools to manage voltage and reactive power as conditions changed. In a solar energy system this means that conventional power stations, renewable energy sources, batteries and network equipment must all be able to support the voltage dynamically and not just produce electricity.
Stress monitoring requires specific measurements and indicators to ensure the system is operating within acceptable limits, and research shows that the Spanish system was showing signs of stress for days and even months prior to the outage. A first alarm was recorded on January 31, when abnormal voltage variations pushed a Spanish nuclear power plant into the under-excitation limit; further abnormal voltage fluctuations appeared on April 22 and 24; and on the morning of April 28, instability was visible from about 9 a.m., increasing as the day progressed. What’s more important here is that the voltage can remain within formal operating limits and still vary too quickly to be considered safe.
As a result, the question arises: are modern, highly renewable energy systems ready to detect these types of rapid voltage movements before they become a cascading overvoltage event? The Spanish experience shows that this is certainly not the case from the point of view of the operating procedure and the security situation.
Furthermore, if the Spanish operator had properly identified the root causes of the event and wanted to address them quickly enough, would the technologies deployed at the time of the event have been able to solve the problem? It all seems like they would have failed. Why? Solar and wind energy can provide voltage support, but only if the rules, incentives, communications and factory settings require it, which was not the case. SolarPower Europe, UNEF, APREN and other renewable energy associations put it bluntly after the ENTSO-E report: “It’s not about renewables, it’s about voltage control.”
Expert reports
The aforementioned March 2026 ENTSO-E final report, which investigated the event, concluded that the blackout was the result of multiple interacting factors: oscillations, gaps in voltage and reactive power control, different voltage regulation practices, rapid production reductions, generator disconnections in Spain, and uneven stabilization capabilities. These factors caused rapid voltage increases and cascading disconnections of generation. The language of ENTSO-E is important because it shifts the debate from a single villain to system design.
The Spanish government report came to a similar conclusion earlier, in June 2025. It described the blackout as a multifactorial surge event. The system lacked sufficient dynamic voltage control; fluctuations between 12:00 and 12:30 conditioned system operation; and some revival resources have been disconnected in a manner that the government described as apparently inappropriate.
In other words, expert reports did not constitute a judgment against renewable energy sources. It was a judgment against running a modern, heavily renewable system with outdated operating assumptions. Solar and wind energy can provide voltage support, but only if the rules, incentives, communications and factory settings require it. SolarPower Europe, UNEF, APREN and other renewable energy associations put it bluntly after the ENTSO-E report: “It’s not about renewables, it’s about voltage control.” They also noted that Spain updated Operational Procedure (PO) 7.4 in June 2025 to allow renewable energy sources to contribute to voltage control.
Regulatory actions
This regulatory change is one of the most tangible reforms following the blackout. The Spanish independent competition regulator CNCM has published in the Spanish Official Gazette the resolution of 12 June 2025, which amends the operational procedures for the development of a voltage regulation service on the Spanish peninsula. The revised PO 7.4 creates a framework for the system operator to use reactive power from generation, demand and storage resources to maintain voltage within safe limits, and introduces a charge for the more demanding real-time setpoint following service.
The implementation has started. Red Eléctrica said in October 2025 that it had carried out authorization tests for the first renewable plants capable of providing dynamic voltage regulation under the new PO 7.4. In May 2026, Redeia’s chairman told shareholders that 14.5 GW already offered setpoint voltage regulation, including 6 GW of renewables. That’s a real change from the pre-blackout system, where the voltage toolbox was too reliant on conventional power stations and manual grid devices.
But the costs of stabilization have also become visible. Red Eléctrica’s ‘enhanced operations’ regime, introduced after the blackout to strengthen voltage control, cost €711 million ($827.3 million) through April 2026, according to Redeia. The same shareholders’ meeting showed how politically contentious the issue remains: some shareholders demanded resignations, while Redeia argued that official reports showed the incident was unprecedented, unpredictable and multifactorial.
Portugal has also moved. The government has announced a package that focuses on electricity grid resilience, faster planning, renewables, critical infrastructure and international cooperation. Measures include doubling the number of black-start power plants, a €137 million investment in network management and control, a 750 MVA battery storage auction and €25 million for critical infrastructure such as hospitals, water suppliers and utilities. Portugal also explicitly linked security of supply to stronger Iberian and European interconnections.
Conclusions
The deeper consequence is that ‘flexibility’ can no longer simply mean storage arbitrage or demand shifting. The Iberian market is entering a phase where solar production, negative prices, curtailment and voltage control all converge at the same hours. WSP’s Electricity Market Outlook for Iberia assumes solar-driven expansion until the late 2020s, with storage rapidly scaling up from 2026 and exceeding 30 GW by 2050. It also warns that solar faces serious erosion in shelter prices and that exposure to negative prices could be around 50% of solar generation in the short term. In that context, batteries, grid-forming inverters, dynamic voltage regulation and interconnection are not optional additions. They are the operating system of the near future.
The Spanish power outage serves as a warning for other European energy networks and for the future of system operations. Renewable penetration can be a success story, but it comes with an uncomfortable truth: too much, and at the same time, the use of outdated rules, guidelines, communications systems and network coordination can tap into problems never before experienced.
Author: Safa Sen, Market Engagement Lead for CWE at Ricardo, member of WSP.
Ricardo is a member of the professional services firm WSP Group, which unites technical, consulting and scientific expertise to shape communities and advance humanity. From local beginnings to a global presence today, it operates in more than 50 countries, providing solutions and delivering innovative projects across sectors: transportation and infrastructure, real estate and buildings, earth and environment, water, energy and mining and metals.
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