What Caused the Spain and Portugal Blackout? A Preliminary Analysis

[Written with significant assistance from Claude 3.7 Sonnet]

Executive Summary

Spain’s April 28, 2025 blackout reveals a complex interplay of energy systems during transition. While initially blamed on high renewable penetration (59% solar at the time), this preliminary analysis suggests multiple contributing factors. Nuclear plants’ automatic disconnection significantly extended the outage by removing 4–5 GW of capacity that couldn’t quickly restart. Spain’s energy storage capacity — approximately 6–7 GW of pumped hydro and 1.8–2.5 GWh of batteries — proved insufficient for the cascading failures. Geographic evidence suggests the newly commissioned Valdecañas hybrid pumped-hydro facility in Extremadura (southwestern Spain) may have played a role in the initial disturbance. The experience contrasts sharply with Texas and California, which have eliminated similar blackouts through massive battery deployments (13+ GW each). As Spain progresses toward its 22.5 GW storage target by 2030, this incident demonstrates that successful energy transition requires not just renewable deployment but synchronized investment in flexible storage technologies and grid management systems.

Spain experienced a nationwide blackout on April 28, 2025 that raised questions about the country’s energy system resilience, particularly as it transitions toward renewable energy. While initial reactions blamed the high penetration of solar power, a deeper analysis reveals a more nuanced picture of Spain’s energy storage capabilities, grid management practices, and the interaction between conventional and renewable power sources during the disturbance.

This preliminary analysis explores the landscape of Spain’s energy storage systems as of April 2025 and examines their role in grid balancing compared to conventional generation sources. As this represents an early assessment based on available information, I intend to provide a more comprehensive analysis once additional data becomes available from Spanish authorities and grid operators. The current findings should be considered initial observations that may evolve as the investigation continues and more details emerge about the precise sequence of events.

I conducted this analysis in light of my recent study of California and Texas’s remarkable growth in battery storage, which seem to have made blackouts and brownouts a thing of the past in those states.

The Nuclear Disconnect: Why Spain’s Reactors Shut Down

The role of nuclear power plants during the April 28 blackout has been widely misunderstood. Spain’s seven nuclear reactors, with 7.4 gigawatts of installed capacity, typically provide about 20% of the country’s electricity and contribute significant grid inertia.[¹] However, when the grid frequency disturbance occurred, four of these reactors disconnected from the grid completely.

This was not a cause of the blackout but rather a consequence of grid protection mechanisms. Nuclear plants employ sophisticated safety systems designed to disconnect from the grid when frequency or voltage falls outside acceptable parameters. When the grid disturbance began in southwestern Spain, these protective systems detected abnormal frequency oscillations that triggered automatic safety protocols.

“Spanish authorities reported that the country’s nuclear power plants were taken off the grid automatically due to the loss of grid power… Backup generators automatically supplied cooling to keep all seven reactors safe,” according to reports from the event. Four reactors were generating power at the time of the incident, while three were already offline for scheduled maintenance.

The nuclear plants’ disconnection created a secondary challenge: once offline, nuclear reactors cannot quickly reconnect to the grid. Unlike gas turbines that can restart relatively quickly, nuclear plants require a complex, methodical restart process to ensure safety. This is why, as Prime Minister Pedro Sánchez pointed out the day after the blackout, “Spain’s nuclear power stations still hadn’t resumed operating… which showed they were no more resilient than renewables” in terms of recovery time.

The nuclear disconnect highlights an important aspect of modern grid management. While nuclear power provides stable baseload generation with valuable inertia, its protective systems can paradoxically contribute to cascading failures during significant grid disturbances. This is not a fault of nuclear technology but rather a challenge of integrating different generation technologies with varying response characteristics into a cohesive system.

The event also dispelled the notion that simply having more nuclear power would have prevented the blackout. As energy expert Adam Bell noted, “A lack of inertia was therefore not the main driver for the blackout. Indeed, post the frequency event, no fossil generation remained online — but wind, solar and hydro did” according to reports following the outage.

Nuclear Power: Part of the Problem?

A compelling case can be made that Spain’s reliance on nuclear power significantly contributed to the prolonged nature of the blackout, if not its initial cause. The automatic disconnection of four nuclear reactors during the grid disturbance instantly removed about 4–5 GW of generation capacity from the system, exacerbating the initial problem.

The extended recovery time inherent to nuclear power then became a critical factor. As Prime Minister Sánchez pointed out, Spain’s nuclear plants still hadn’t resumed operating the day after the blackout, demonstrating they were “no more resilient than renewables” in terms of recovery. While safety systems appropriately protected the reactors, they also locked a significant portion of the country’s generation capacity offline during the critical recovery period.

Energy expert Carlos Cagigal suggested the outage “probably happened because Spain’s nuclear plants weren’t operating at the time” according to media reporting. Though this conflates cause and effect (as the reactors were operating before the disturbance), it highlights a key insight: once tripped offline, nuclear plants couldn’t contribute to grid recovery efforts due to their lengthy restart procedures.

This suggests that an energy system with more flexible, rapidly recoverable generation sources might have recovered more quickly. While nuclear power wasn’t the primary cause of the initial disturbance, its protective responses and slow restart capabilities were major factors in extending the blackout’s duration and complicating recovery efforts.

The Atmospheric Phenomena Theory

Portugal’s grid operator, RNA, offered an interesting explanation that points to unusual weather conditions as a potential trigger for the cascading failures. According to their initial assessment, the blackout may have been caused by “a rare atmospheric phenomenon which caused ‘oscillations’ and ‘vibrations’ in the high power lines, which in turn resulted in ‘synchronisation failures’” as reported by ITV News.[²]

Specifically, RNA suggested that “extreme temperature variations in the interior of Spain” created “anomalous oscillations in the very high voltage lines (400 kV), a phenomenon known as ‘induced atmospheric vibration’” according to their initial statement.[³] This phenomenon reportedly caused synchronization failures between electrical systems, leading to successive disturbances across the interconnected European network.

In certain weather conditions, transmission lines can experience a phenomenon known as “galloping” where they visibly swing. This can potentially cause physical breaks or, more significantly, changes in the electrical characteristics of the lines. If the ionized air around the cables interacts with the cable itself, it can alter the frequency inside the wire, creating discrepancies with the rest of the grid.[⁴]

However, weather data from Spain on April 28 showed calm and sunny conditions with average spring temperatures, making this explanation questionable. A technical expert interviewed by ITV News stated it would be “really really weird” for this weather to have caused induced atmospheric vibrations based on the conditions that day.[⁵]

While the atmospheric phenomena theory remains under investigation, its plausibility has been questioned by several experts. The calm weather conditions and the specific nature of the grid failure pattern suggest other factors likely played more significant roles in triggering the initial disturbance.

Spain’s Pumped Hydro Storage Landscape

Spain’s primary large-scale energy storage comes from its extensive pumped hydroelectric storage (PHES) system. As of April 2025, Spain operates approximately 6–7 GW of pumped hydro capacity across 18 facilities. These engineering marvels function by moving water between reservoirs at different elevations, effectively storing energy during low-demand periods and releasing it when needed.

The jewel in Spain’s pumped storage crown is the Cortes-La Muela complex, recognized as “the largest pumped hydroelectric storage plant in Continental Europe with a total installed capacity of 1767 MW” according to industry sources. This facility alone can generate power equivalent to supplying 400,000 homes by exploiting the 500-meter elevation difference between its upper and lower reservoirs.

Spain’s pumped hydro capacity represents a significant energy reserve of 100–120 GWh, allowing for many hours of grid support. Iberdrola, Spain’s leading utility, reached “101.2 gigawatt hours (GWh) of storage capacity by the end of 2022, with plans to expand to 119 GWh by 2026” across its pumped hydro portfolio.

Recent additions to this capacity include the 225 MW Valdecañas pumped hydro plant, which began commissioning in early 2025. This project exemplifies the modernization of pumped hydro technology in Spain, as it has been “hybridized with a 15 MW/7.5 MWh battery energy storage system” to enhance its responsiveness and flexibility. The innovative hybridization approach allows for faster response times and greater versatility in grid support services.

The Emerging Battery Storage Ecosystem

While pumped hydro dominates Spain’s storage capacity in terms of sheer size, battery energy storage systems (BESS) are rapidly gaining ground. As of early 2025, Spain had between 1.8–2.5 GWh of installed battery capacity, growing from “1.823 MWh of cumulative storage capacity at the end of December 2023” according to industry data.

The battery storage landscape in Spain is diverse, with both behind-the-meter installations at homes and businesses, as well as utility-scale projects. Iberdrola, for instance, has been deploying “six Battery Energy Storage Systems (BESS) with a combined capacity of 150 MW” across several regions. Each of these systems has 25 MW of power and 50 MWh of energy capacity, providing both power and duration to support the grid.

Renewable energy developer Grenergy has also been active in the storage space, developing “twin storage projects in Oviedo — BESS Asturias 2 and BESS Asturias 3 — each with a capacity of 48.64 MW and an energy storage capacity of 238.528 MWh” according to project filings. These larger batteries represent the growing scale of Spain’s ambitions in energy storage.

The Spanish government has recognized the critical importance of energy storage for its renewable energy transition, setting “a new 2030 energy storage target of 22.5 GW in an energy strategy submitted to the European Commission” in late 2024. This represents a significant scale-up from current levels and demonstrates Spain’s commitment to building a more resilient, renewable-dominated grid.

Learning from Texas and California: Battery Storage Success Stories

Spain’s blackout experience offers striking contrasts with recent successes in the United States, where Texas and California have effectively eliminated rolling blackouts through massive deployments of battery storage. By the end of 2024, California achieved over 13 gigawatts of battery storage capacity, with Texas close behind at 13.5 GW according to the document “Why rolling blackouts are a thing of the past”.[⁶] This represents a dramatic scale-up from 2020 levels, when Texas had just 0.5 GW of storage.

The Texas experience is particularly instructive, as it demonstrates how market forces alone can drive rapid battery adoption. In Texas’s competitive ERCOT market, private investors build generation assets based on economic returns rather than policy mandates. The fact that batteries have won in this market-driven environment underscores their economic competitiveness. In 2023, ERCOT issued 11 conservation notices during extreme weather, including seven consecutive days in August. By contrast, 2024 saw no conservation calls despite similar peak demand — the difference being massive battery capacity providing crucial grid flexibility.[⁷]

The financial benefits have been substantial. In Texas, August 2024 power prices were approximately $160 per megawatt-hour lower than the same month in 2023, with savings totaling approximately $750 million according to the document.[⁸] Beyond economic benefits, batteries have significantly reduced renewable curtailment — wasted clean energy that could otherwise have been used.

Spain’s current battery storage capacity of 1.8–2.5 GWh falls far short of these American success stories. While Spain has ambitious targets for 2030, the Texas and California experiences suggest that accelerating battery deployment could yield immediate reliability and economic benefits. The speed of their deployments also demonstrates that rapid scaling is possible when economic incentives align with grid needs.

Comparing Storage to Conventional Generation

To understand the significance of Spain’s storage capacity, it’s helpful to express it in terms of conventional generation equivalents. The combined power capacity of Spain’s storage fleet — approximately 8–9 GW when adding pumped hydro and battery resources — is comparable to the country’s entire nuclear fleet, which has “a combined electrical power capacity of some 7.4 gigawatts across seven operating nuclear reactors” as of 2023.

When viewed through the lens of Spain’s natural gas generation, which comprises about 25–27 GW of combined cycle gas turbine (CCGT) capacity, the storage resources represent roughly one-third of this conventional fleet’s power rating. This proportion is significant but highlights the continued importance of gas generation for flexibility and backup.

The energy dimension paints a more nuanced picture. Spain’s total energy storage capacity of approximately 105–125 GWh can provide power equivalent to about 14–17 hours of the nuclear fleet’s daily output. Compared to the gas fleet, current storage can replace about 17–20% of the daily potential output of Spain’s CCGTs, or roughly 4–5 hours of operation at full capacity.

For grid balancing purposes, response time is as crucial as capacity. Battery storage systems excel in this regard, capable of responding “within milliseconds to grid signals, providing crucial services for regulating the grid frequency to within a millisecond and providing back-up capacity at peak energy periods” according to system operators. Pumped hydro typically responds within minutes, with newer facilities incorporating batteries or static starters to improve their flexibility.

This rapid response capability gives storage resources a distinct advantage over conventional generation for certain grid services. While nuclear plants provide steady baseload power and system inertia, they lack operational flexibility. Gas plants offer better ramping capabilities but still require minutes to hours to respond, depending on their operational state. The storage fleet, particularly batteries, fills a critical niche by providing ultra-fast response for short-duration grid balancing needs.

The April 28 Blackout in Context

When examining the April 28, 2025, blackout, it’s important to consider the energy mix at the time of the incident. Spain was operating with “solar PV accounting for 59% of electricity at the time of the blackout, wind nearly 12%, nuclear almost 11% and combined cycle gas turbine (CCGT) plants 5%” according to grid data. This high renewable penetration was not unprecedented — Spain had successfully operated with 100% renewable energy just 12 days earlier.

The challenge arose not from the high renewable percentage itself but from a combination of factors. The system was operating with “very little inertia, which is energy stored in a large rotating steam or gas turbine driving and rotating generators which acts as a buffer” as noted by grid experts. When a significant disturbance occurred in the southwestern region, the system lacked sufficient stabilizing resources to prevent cascading failures.

Spain’s storage capacity, while substantial compared to European peers, could not fully compensate for the rapid loss of generation. In a span of just five minutes, “solar PV generation plunged by more than 50% to 8 gigawatts (GW) from more than 18 GW” according to operational data. This precipitous drop, combined with the subsequent disconnection of nuclear plants, created a deficit too large for the available storage and remaining generation to address.

The incident demonstrates that while Spain has made remarkable progress in building storage capacity, its energy transition still faces challenges in ensuring system stability during extreme events. Grid experts suggest that what happened was not a failure of renewable energy technology but rather “the management of renewables in the modern grid” as highlighted in post-blackout analysis.

Looking to the Future

Spain’s energy storage landscape is poised for dramatic growth in the coming years. Battery storage capacity is projected to “increase from 56 megawatt-hours in 2023 to approximately 5.4 gigawatt-hours by 2027” according to industry forecasts, representing nearly a 100-fold increase in just four years.

This expansion is part of Spain’s broader strategy to have “22 GW of storage capacity in place by 2030” as outlined in national energy plans, which would transform the country’s ability to manage high renewable penetration. Spain’s approach aligns with wider European trends, with forecasts suggesting “an additional 128 GW and 300 GWh of electrochemical storage by 2030” across the continent.

The April 2025 blackout, while disruptive, has provided valuable lessons for Spain’s energy transition. It has highlighted the need for more synchronized planning between renewable deployment and grid reinforcement, including storage resources. It has also sparked discussions about maintaining adequate inertia and stability services as conventional generation decreases.

If Spain were to accelerate its battery deployment to match the successes seen in Texas and California, it could potentially achieve similar reliability benefits. In particular, the Texas approach demonstrates that even without climate-focused policies, the economic case for batteries can drive rapid adoption when market signals are properly aligned.

As Spain moves forward, its experience offers insights for other countries pursuing ambitious renewable energy targets. The combination of diverse storage technologies — from conventional pumped hydro to cutting-edge battery systems — alongside improved interconnections and advanced grid management systems, provides a pathway to a more resilient, renewable-dominated electricity system.

Rather than abandoning its renewable ambitions, Spain is doubling down on the infrastructure needed to support them. This approach recognizes that the future lies not in choosing between conventional and renewable generation, but in creating sophisticated systems that leverage the strengths of diverse technologies while mitigating their limitations.

References

[¹]: World Nuclear Association. (2025). Nuclear Power in Spain. Retrieved from https://world-nuclear.org/information-library/country-profiles/countries-o-s/spain [²]: ITV News. (2025, April 28). Oscillations and vibrations: What caused the power outage in Spain and Portugal? ITV News. [³]: Martin Stew. (2025, April 28). Oscillations and vibrations: What caused the power outage in Spain and Portugal? ITV News. [⁴]: Carbon Brief. (2025). Q&A: What we do — and do not — know about the blackout in Spain and Portugal. [⁵]: ITV News. (2025, April 28). Oscillations and vibrations: What caused the power outage in Spain and Portugal? ITV News. [⁶]: “Why rolling blackouts are a thing of the past — and why President Trump is dead wrong on green energy.” Document #74. [⁷]: “Why rolling blackouts are a thing of the past — and why President Trump is dead wrong on green energy.” Document #74. [⁸]: “Why rolling blackouts are a thing of the past — and why President Trump is dead wrong on green energy.” Document #74.