What lessons have we learned from Spain’s power outage?

A year after the 28 April 2025 blackout across the Iberian Peninsula, new details on its causes have emerged. The combination of technical failures and real-world impacts triggered a wake-up call for the design and operation of critical infrastructure in an increasingly renewable-powered world. We now have insights from the ENTSO-E final report, and engineers must use these lessons to better design critical infrastructure and enhance societal resilience in a rapidly changing world. 

What actually happened
The event, dubbed "El Apagón" (“the blackout” in Spanish), lasted for almost 24 hours in some areas and affected millions of people and businesses across Spain, Portugal and parts of Southern France. According to the aforementioned ENTSO-E report, this originated due to a combination of four events, which are summarised as:

1. Two significant oscillations in voltage, power, and frequency occurred on the 28th of April 2025: the first from 12:03-12:08 CEST, the second from 12:19-12:22 CEST.
2. In order to mitigate these oscillations, Spanish transmission system operators (TSOs) took planned contingency actions – principally, increasing the interconnectivity between the Spanish grid’s “branches” and reducing the connections with France.
3. This successfully eliminated the oscillations and reduced voltage variations; however, it increased the overall voltage.
4. Due to this overvoltage, transformers supplying power from power generation plants tripped (i.e., disconnected from the grid) to protect themselves. 

At a household level, the experience is like your washing machine or air conditioner short-circuiting in your own home; when this happens, your main distribution board trips to protect you and your equipment. If a short circuit accumulates more energy over a longer period, the power loss in your flat / house can spread to your building, street, or entire neighbourhood.

The event on the 28th of April 2025 created this experience on a larger scale and impacted the entire Iberian Peninsula. Due to the overvoltage, the transformers that transmit energy from power generation plants (solar, wind, nuclear, and gas) to the wider grid tripped (i.e. disconnected themselves from the system) to protect themselves. The first trips triggered by this overvoltage were wind and solar (thermosolar and PV) plants across Southern Spain, and it then spread across the entire Spanish electricity grid. This cascade of trips happened extraordinarily quickly; the second, near Badajoz, occurred within 30 seconds of the first in Granada. It created a supply-demand imbalance (as more and more power sources went offline) that the grid's protection systems couldn't contain, and the Iberian Peninsula’s electrical grids simply collapsed.

It should also be noted that ENTSO-E concluded that the high percentage of renewable energy generation in the grid (the Spanish grid was powered by 55% solar and 11% wind at the time of the blackout, according to data from Red Electrica, the Spanish TSO) was not the cause; the grid had previously operated, multiple times, with a higher share of renewable power generation. The failure to control the oscillations without causing overvoltage tripped the transformers. The resilience of these transformers to those oscillations should be improved, but the root cause was not the high use of renewables in the Iberian grid; it was the systematic failure of voltage control.

The immediate effects on society during the failure tested a key concept of resilience: passive survivability. This is the ability to function without connection to the wider world, such as to the electrical grid. Affected areas restricted interactions to what could be reached on foot, by bike, or by car. In cities like Madrid, where fewer people have cars, residents rely on their local neighbourhoods (barrios) and cooperation within their community. These behaviours are essential for resilience when facing the growing risks of climate change – such as heatwaves, floods, and droughts – that increasingly challenge infrastructure and social systems. The blackout demonstrated that strong community bonds and preparedness are central to managing and adapting to future disruptions.

The infrastructure resilience question
As the root causes of the blackout were investigated over the last year, a clear resilience takeaway emerged for developers and operators of critical systems: if even national grids with deep engineering expertise and substantial investment can experience systemic failure, enhancing resilience must be a proactive focus for every critical facility. The question now is how to ensure your infrastructure remains operational and connected, even when surrounding systems fail.
Traditional approaches to critical infrastructure design focus on N-1 redundancy, 2N, 4M3, etc, various levels of redundancy that ensure no single component failure can cause an outage. However, the blackout’s failures weren’t confined to any single substation or transmission line; the entire Iberian Peninsula, Portugal, and parts of Southern France were affected. It demonstrated that very low-probability scenarios can still occur, and we need to consider them when designing critical infrastructure.

Commercial implications
The failure had commercial implications. Insurance models, SLA commitments, and operational risk assessments operate on the assumption of a baseline level of grid reliability. A failure changes that baseline, and the Iberian blackout showed a real business planning scenario. This was a lesson to the operators of critical infrastructure who needed to reassess the resilience of their investments. 
For data centre operators, the blackout taught them to evaluate their current backup power strategies. Industrial facilities needed to understand the impact it could have on their supply chains. Energy developers were taught to develop assets that support grid stability, not just exporting power. There were lessons to be learned across all sectors regarding resilience. 

Designing for and living in the new reality
At Cundall, our approach to critical infrastructure resilience has evolved through the design of over 1,000MW of hyperscale data centre capacity and increasingly complex energy sector projects. The blackout reinforced three core principles we've embedded in our design methodology:

1. Understand your actual risk profile
   The assumptions on grid reliability from five years ago aren’t necessarily relevant today. To effectively design with resilience in mind, there needs to be an honest assessment of outage probability and impact and how these factors align with the real costs of protection. We are already seeing clients move beyond the standard N+1 backup power and instead towards a more sophisticated risk modelling process that accounts for different grid situations. 
   This is key to resilience in the face of climate change, as designing for current conditions is a shortcut to obsolescence within 5-10 years, if not sooner, as climate change impacts intensify. Lived experience in the 2020s is already exceeding predictions for those same years from data modelled in the late 2010s. While this trend is more prevalent in Southern than Northern Europe (e.g., Spain far more than the UK), it is still occurring in locations further north, such as the UK and Ireland.
2. Design islands, not dependencies
   The most resilient facilities are those that can island entirely from the grid when needed. To do so, though, requires designing private substations with true black-start capability, as well as integrating meaningful energy storage, and ensuring that protection systems can coordinate across both grid-connected and islanded operation modes. 
   When climate risks materialise, public authorities will have to make hard decisions about who to prioritise. It is entirely possible that residential areas will be prioritised over data centres when, for example, heatwaves spike cooling demand. Other typologies are similarly vulnerable; for example, droughts may cause hotel swimming pools to be drained, which was a key part of London’s contingency plan if the drought of Summer 2022 had worsened.
3. Balance resilience with decarbonisation
   The response to grid instability is to design better integration and controls. Our BESS projects increasingly include synchronous condensers or other inertia sources. Hydrogen generation facilities are being designed with grid-support capabilities. Even traditional backup power is transitioning to renewable fuels as data centres continue to consume an ever-increasing share of the electricity grid.
   Climate change adaptation and mitigation are two sides of the same coin. While we work to adapt to climate change risks, we must continue to decarbonise, or the effects of said risks will not decrease over the course of the 21st century. A solution that increases resilience but also emissions, such as powering your data centre with off-grid private gas turbines, is simply not acceptable. Regardless of whether this can be converted to another energy source, such as biomass, in the future, it locks in significant emissions.

What this means for your next project
Our Madrid team lived through this and, by applying our technical skills to our lived experience, we can provide critical insights so that critical infrastructure and society as a whole can be more resilient in the face of similarly disruptive events in the future. This is particularly relevant for critical infrastructure, such as hospitals, data centres, energy storage, and other industrial facilities, as they form the foundation for operations.
Every major infrastructure project now needs to consider three resilience scenarios:

1. Normal grid operation: standard operating conditions, with no significant disruptions.
2. Degraded grid operation: significant disruptions to the level and regularity of grid supply.
3. Complete grid failure: no grid supply whatsoever.

The design implications span everything from utility interface strategy through to control system architecture; the technical solutions aren't theoretical. We are already assessing these risks for critical infrastructure and developing solutions to increase your assets’ resilience through both physical designs and operational procedures. We analyse the risks from these three scenarios for your operations, depending on the infrastructure in place on-site and the local area’s characteristics, such as local power transmission lines; the design response for these is the same, whether they’re caused by climate risks such as heatwaves or man-made risks such as a failure to control voltage.

To design out these risks, we are already studying the integration of systems that can transition from grid-connected to islanded operation. Their use is increasing and, one day, it could be as typical to see them in data centres as it is for generators to be currently, such as Battery Energy Storage Systems (BESS) that provide both grid services revenue and enhanced resilience. We're also specifying protection and control systems that coordinate across multiple generation and storage technologies.

All of these build resilience against both climate and man-made risks; as the world responds to climate change, the risks of climate change itself, such as wildfires, and societal changes, such as electrification, increase demand.

But the starting point isn't technology selection – it's an honest commercial assessment of what level of resilience your operation requires, balanced against the real costs of protection, in a rapidly changing world where the asset’s resilience, determined by design variables within the operator’s influence and societal resilience, determined by factors often beyond the operator’s influence. Determining the feasibility and cost of the level of resilience requires integrating the aforementioned three core principles into your projects, starting with a complete risk assessment and then designing to reduce those risks in a low-carbon manner.

Moving forward
Spain's blackout will most likely not be the last systemic grid event of its kind as energy systems undergo a global green transition. The question for critical infrastructure operators isn't whether to prepare for these scenarios, but how to ensure resilience in a cost-effective manner while maintaining commitments to decarbonisation and contributing to the local area’s resilience.

The answer lies in treating resilience as an integrated design challenge, not an add-on protection layer. The challenge for cooperation between grid and data centre suppliers is to create a resilient infrastructure that can simultaneously generate revenue, continue to decarbonise, and ensure operational continuity, all of which feed into the central question of societal resilience.

Electricity grids are changing in response to climate change, and risks from those changes and climate change itself are likewise increasing. Is your critical infrastructure ready for what comes next? Reach out to a member of our team to find out how we can support the resiliency of your next project.