By Deric Tilson
On April 28, 2025, at 12:33:24 CET, a blackout encompassed Spain, Portugal, and parts of southwest France, leaving over 50 million people without power. The loss of electricity cost Spain an estimated $1.82 billion in economic output and damages. When the Iberian grid collapsed, people who were going about their day moments before were now stuck in elevators, food in freezers and refrigerators began to thaw or spoil, and non-urgent medical needs were delayed as hospitals dealt with scarce backup power supplies.
Quickly, pundits, experts, and posters on social media descended on the details of the blackout, grabbing what information they could, and spewing hot takes: “Solar is to blame.” “Why did the nuclear power plants go offline?” “Green energies and renewables did this.” “Aha, nuclear plants were in planned outage.” Schadenfreude abounds when systems begin to break, especially when aspects of those systems are politically charged.
Fifteen hours later, the transmission grids of Spain and Portugal were restored to full operation. Electrons once again flowed to people’s homes, charging their phones, illuminating their rooms, cooling their food, and providing them with modern comforts. The question of how the blackout happened remained.
In June, Red Eléctrica de España (REE), the Spanish grid operator, released a report detailing the events that led to the blackout. The European Network of Transmission System Operators for Electricity (ENTSO-E) is currently undertaking a more thorough investigation of the incident.
Electric grids form the backbone of our economies. If a grid fails, it is necessary to understand why, learn from the failure, and take precautions so that it does not happen again or on another system. Doing so entails finding the true cause of the failure. If politics or opinions obfuscate the results, it may solve nothing and may make the system more vulnerable than it was before. In this case, a malfunctioning solar plant disrupted the Spanish grid and led to a feedback loop that ultimately caused it to collapse. As more variable renewable energy sources are being added to the grid, it is worth considering that our old systems for ensuring reliability might not be enough.
The Blackout
7:00:00 AM CET, 5 hours 33 minutes 24 seconds until grid collapse
The morning of April 28th in the Iberian Peninsula was ideal for mid-spring. It promised to be a windy day, and the sun would be out in full force. Grid operations were normal, and a variety of natural gas, combustion, and nuclear generators were running smoothly.
Most of the electricity generation around the world—Spain included—is done by heating a fluid and sending the vapor into a turbine, which then spins the components inside a generator. The large, spinning generators, whether steam, combustion, or water driven, also create frequency and voltage as they output energy. Most of the power being generated throughout the early morning was from these spinning generators.
Between 7:15 and 7:20, rays of sunlight began to hit the solar photovoltaic panels across the peninsula. Solar photovoltaics and wind often transmit their energy through inverters. An inverter changes power from DC to AC power before heading to the grid, and also makes sure the outgoing power matches the parameters of the grid. Unlike spinning resources, which “form” the grid, solar and wind “follow” the grid and lack the inertia to help dampen grid fluctuations.
By 9:30, the solar radiance had increased significantly, sending more energy to the grid. As wind and solar provided more power to the grid, other generators, like natural gas turbines and nuclear power plants, were asked to reduce output or go on standby. As the spinning generators turned off, there were fewer and fewer grid-forming resources on the grid.
12:00:00, 33 minutes 24 seconds until grid collapse
Between 10:30 and 11:45 that morning, the operator saw voltage fluctuations on both 220 kV and 400 kV transmission lines, but none of the fluctuations were outside the limits of safe operation. It is not unusual for grids to see oscillations throughout normal operation; most of them are dampened within seconds and have little-to-no impact on the grid as a whole. By noon, the fluctuations had calmed to normal conditions.
Three minutes later, at 12:03, a voltage and frequency swing disrupted the grid. The oscillation originated in the province of Badajoz at what REE refers to as “Photovoltaic Plant A.” At the time of the oscillations, it was generating approximately 250 MW. The oscillation continued for four minutes and forty-two seconds.
As power is generated, spinning generators create frequency, which allows for synchronization of the generation fleet. All of the online generators have to be spinning at the same frequency across the grid, no matter how many hundreds or thousands of miles of cable are in between them.
As generators spin and people demand electricity by turning on electronics, the differential between the two creates pressure, like a push and pull of electrons, across the wires. This is called voltage. Much like frequency, voltage needs to be maintained and must be regulated across the grid. Transformers will step up and down the voltage as necessary for the power line rating and to deliver power to consumers safely.
Fluctuations and oscillations occur on all grids during normal operations in response to the loss of one generator or wire, for example. When this happens, a generator and/or the power flow on a part of the grid may go slightly out of sync with the rest of the grid. Most of the time, these fluctuations disappear within seconds. If unnatural oscillations persist, then the grid will begin to experience problems, and the generators may go out of sync with each other or the interconnections. Large fluctuations may cause safety systems to engage, causing lines, transformer substations, and generators to disconnect from the grid.
To dampen the oscillations caused by Photovoltaic Plant A, REE adjusted the power operations between Spain and France, coupled power lines within Spain, and reduced the flow of energy being exported into the country. These operations helped lower the disruption caused by the oscillations
When the operator reduced the flows between Spain and France, the voltages across the system were also increased. Between 12:07 and 12:15, 600 MW of generating resources disconnected from the grid. Most of it was likely wind and solar resources that used inverters not sufficiently equipped to handle the fluctuations and increasing voltage plaguing the grid.
By 12:15, power flows across the border with France had increased, negating the previous actions and setting the stage for more oscillations.
12:19:00, 14 minutes 24 seconds until grid collapse
At 12:19, another oscillation hit the Spanish grid, lasting for three minutes. Photovoltaic Plant A in Badajoz had increased its output from 250 MW to 350 MW. REE reported that its electricity output was stable, but the voltage, or pressure, being placed on the grid was not. This time, the oscillation caused the frequency of the whole Iberian grid to be slightly offset by 0.21 Hz.
The oscillation and its repetitions on the Spanish grid were not natural to the system, but were forced, or exogenous to the transmission lines. A forced oscillation occurs when something external to a system causes a disruption to the frequency of that system. Inverter-based resources, like wind and solar, can cause these kinds of forced oscillations on electric grids. For reasons that remain unclear, the inverter sending power from Photovoltaic Plant A was malfunctioning.
REE was able to get the oscillations under control once again by reducing power flows between Spain and France while coupling more power lines within Spain. In the meantime, 500 MW more of small, behind-the-meter resources disconnected from the grid.
As of 12:30, over half of the 32 GW being generated in Spain was coming from solar. Electricity prices had gone negative, and Spain was exporting over 3.5 GW to neighboring countries; the electricity was in the realm of “too cheap to meter,” but cracks were beginning to show. REE sent start instructions to the combined cycle gas turbines and a nuclear power plant to help stabilize the grid. These units would not materialize in time.
12:32:00, 1 minute 24 seconds until grid collapse
Between 12:32:00 and 12:32:57, an additional 500 MW of wind and solar disconnected from the grid. The loss of these units caused the transmission line voltage to begin increasing and exceeding the system parameters.
Voltage on the broader transmission network then began to rise. The distribution transformers had built-in “safety valves” to release the excess voltage, which were configured to maintain proper voltage levels in those networks. When the voltage increased, the safety mechanisms within the transformers failed to respond quickly enough, causing overvoltages in the distribution networks, even though primary transformer voltages remained within acceptable limits.
The transmission and distribution infrastructure found itself in a feedback loop. The oscillations from Photovoltaic Plant A caused a rise in voltage, which then caused solar and wind to trip offline. This, in turn, increased the voltage further, causing even more generation to be disconnected.
Three more generation loss events happened in rapid succession:
12:32:57, 27 seconds until grid collapse
A transformer connected to a generator tripped on the low-voltage side in Grenada. 355 MW of solar PV, wind, and thermal solar were lost as a result.
12:33:16, 8 seconds until grid collapse
725 MW of solar PV and thermal solar were disconnected in Badajoz.
12:33:17, 7 seconds until grid collapse
In less than one second, 1,100 MW of generation was lost. This included wind farms in Seguira and Huelva, solar PV in Badajoz, Seville, Cáceres, and Huelva, and thermal solar in Badajoz. 930 MW of the 1,100 MW have been identified as belonging to wind and solar; the remaining 170 MW are still under investigation.
Unlike the large spinning turbines and generators, solar panels and wind turbines do not produce inertia. When oscillations occur, the inertia of the spinning generator, or the kinetic energy of the rotor’s mass, helps dampen the fluctuations and prevents the grid from destabilizing. Wind and solar instead rely on inverters that match, or “follow,” the state of the grid as they produce power. Most inverters used in solar are grid-following. If the frequency or voltage on the line connected to the solar farm goes outside a preset range, then a safety mechanism will disconnect the farm from the grid to protect the power electronics. Without more advanced inverters, which are only just beginning to enter wider use, renewables cannot tolerate fluctuating conditions on the grid.
By this point, 2,200 MW of generating assets had disconnected from the grid in the past 20 seconds.
12:33:18, 6 seconds until grid collapse
The voltage along the transmission corridors continued to increase sharply. Overvoltage triggered a cascade of generation losses. The frequency began to drop.
12:33:19, 5 seconds until grid collapse
Automatic load shedding and system defense plans, both grid protection measures, were initiated to save the grid from collapse. Automatic load shedding is a measure in which the computers controlling the grid disconnect non-essential demand, like residential load, from the grid in an attempt to keep the larger grid from failing. Usually, this occurs when either frequency or voltage falls out of range. The load shedding can be targeted in a few areas or shared across a grid in what is known as rolling blackouts and rolling brownouts. The load shedding is supposed to allow generators more time to come online and balance the grid.
System defense plans are a series of actions that go into effect to prevent system-wide failure. These include specific thresholds for which actions to take and when. They are modeled in advance and comply with best known practices. ENTSO-E has developed plans for REE and other European grid operators to follow.
12:33:20, 4 seconds until grid collapse
The AC connections between Spain and Morocco tripped due to the Spanish grid’s frequency dropping below the acceptable limits.
Grid interconnections and electricity imports and exports enhance reliability by allowing regions to share power during shortages or emergencies, reducing the risk of blackouts. Interconnections improve economic efficiency by enabling cheaper electricity from surplus regions to flow to areas with higher demand and prices. They also enable other generators and resources throughout the region to assist in dampening voltage and frequency fluctuations. However, a sufficiently large fluctuation or failure in one region of a larger interconnected grid risks spreading the emergency across the entire large-scale system.
12:33:21, 3 seconds until grid collapse
The AC ties between France and Spain were disconnected. The frequency of the Spanish grid went so far outside its bounds that safety equipment engaged to prevent the disruption and oscillations from spreading outside the peninsula.
12:33:22, 2 seconds until grid collapse
Automatic load shedding and defense measures failed.
12:33:23, 1 second until grid collapse
Generation losses continued to cascade.
Spinning generation also began disconnecting from the grid. Discrepancies between the grid frequency and the spinning rotor can risk severe mechanical damage, forcing generators to shut down. However, inertia in spinning generators acts as a damper, smoothing out small oscillations and resisting larger ones to slow the rate of change. Spain’s spinning generation stayed connected to the grid until the conditions exceeded their tolerances.
Safety measures caused nuclear units to disconnect from the grid.
12:33:24, grid collapse
Blackout.
27 seconds after generation began disconnecting from the grid because of overvoltage, 50 million people across the entirety of Spain and Portugal were left without power.
12:35:00, 2 minutes after grid collapse
Quickly following the initial blackout, blackstart efforts began. The Castelo de Bode hydropower plant and the Tapada do Outeiro combined cycle gas turbine power plant turned on in blackstart mode. Minutes later, a 400kV transmission line interconnected with France was re-energized. Slowly over the next 15 hours, the grid was brought back online, line-by-line and section-by-section.
Blackstarting is a rare occurrence. Most outages are localized and can be resolved with the help of neighboring unaffected parts of the grid, as opposed to blackstarts, which require starting up generators and sending power to dead lines. Traditionally, hydro units and combustion turbines are used to start up a power grid from scratch; both have enough inertia to keep producing power and synchronizing with each other. Nuclear is also capable of blackstarting; Bruce Nuclear Power Plant, hosting CANDU reactors, helped the blackstart efforts during the 2003 Northeastern Blackout. Grid-forming inverters with battery systems could, in theory, also help blackstart, but that technology has just started to come on the market. External interconnections can also be used to help prop up the failed system during a restart.
April 29th, 04:00:00, 15.5 hours after grid collapse
The Spanish and Portuguese grids were fully restored and operational.
Could a Similar Blackout Hit the American Grid?
The physics underpinning the grid remain the same, no matter the country or location. The Iberian blackout caused a flurry of concern amongst grid operators and reliability organizations. A month after the blackout, the North American Electric Reliability Corporation (NERC) gave a presentation to the Federal Energy Regulatory Commission in which several potential areas of concern were identified:
Insufficient voltage regulation to handle large oscillations
Unreliable voltage regulation to prevent a system collapse
Poor tolerance of inverter-based resources to handle voltage oscillations
Potential gaps in operations planning
The key lesson learned by US grid operators and NERC was that if increased voltage leads to generators tripping, which then results in a lowering of frequency, load shedding measures meant to protect the grid will cause voltages to increase further if there is not enough spinning generation. If the US grid had seen a similar situation to the Iberian grid before April 2025, load shedding would have commenced, voltages would have increased, and parts of the American grid could have collapsed. It is unlikely that the whole American grid would have collapsed, but millions of people could have been without power. Now, operators are more aware of this scenario, but it is not apparent that a cascading failure could be prevented.
The main culprit of the Iberian blackout looks to be overvoltage of the transmission lines as generators were disconnected from the grid. Could additional spinning resources, like nuclear and natural gas, have prevented the blackout? Was more than half of all generation coming from renewables too much for the grid to handle? REE maintains that additional inertia would not have saved the grid from failure, and only delayed the eventual failure of the grid due to increasing voltage and tripping renewables. This may be true, but even a few additional seconds made available by more inertia could have been enough to allow for automatic load shedding or other protection measures to keep at least parts of the grid operating.
April 28th was not the first time the Spanish grid was operating with over 50% wind and solar; on April 16th, 2025, wind and solar output met over 73% of demand. Other grids routinely operate on large amounts of inverter-based resources; Australia, Texas, and California all generate large portions of their energy mix from wind and solar. Australia uses grid-forming inverters to maintain reliability and keeps a number of gas turbines spinning to help regulate the grid, but not producing much power. Texas has a market for fast frequency response. California relies on the flexibility of gas generators and help from its neighboring grids.
One tool missing from Spain’s grid was battery storage. Spain only had 60 MW of battery energy storage. As a comparison, ERCOT in Texas has 11,000 MW of battery storage, and the California ISO has over 13,000 MW; with increased battery storage, Texas and California have gained better reliability from their grids. Batteries allow for instantaneous discharging of power and can offer grid services like voltage and frequency regulation. Had Spain invested more in battery storage alongside grid-forming inverters, the blackout might have never happened or might have stayed localized to a smaller area.
As technologies have developed and been introduced to energy systems, the grid has grown in its complexity. Intermittent resources and renewables added an extra layer of complexity to what is already a complex system. The structures and institutions that govern the grid were made when all the generation was made up of large fossil fuel plants and hydroelectric turbines; the specific cascading failure seen in Spain would have been unlikely in a more conventional grid. These institutions need to evolve with the technology; if they don’t, the grid will become increasingly unreliable.
Conclusion
Some are waiting expectantly for the results of official investigations into what caused the Iberian blackout; they want some person, policy, or technology to blame. But, electrical systems are not so simple as to care about your pet policies. We need a wide variety of generation sources and types: stable baseload power to always be on and provide generation in all hours of the day; quick, responsive power for when demand is changing rapidly; and emergency power for when there are outages. Grids are more reliable when there is diversity. Nuclear, natural gas, wind, hydroelectric dams, diesel, geothermal, and coal can all contribute to a resilient system.
The REE report does not blame wind and solar explicitly, but the operator does point to the Photovoltaic Plant A as being the source of the oscillations that began to knock behind-the-meter generation off the grid. As generation disconnected, the voltage on transmission lines increased. This knocked more wind and solar generation offline, including grid-scale resources, and voltage increased further until there was a cascading failure. When REE tried to bring gas and nuclear online to help, there wasn’t enough lead time for those resources to start up before the grid collapsed. No matter how one looks at it, a solar installation malfunctioned and started the chain reaction that caused the blackout.
In the United States, we are fortunate to have the expectation of reliable power. The average power loss across an entire year is only 6 hours. Not all of the world has the luxury of electricity powering their lives 24/7. For some, blackouts and brownouts are a daily occurrence. For others, there is no power to go out. Over a billion people, most of them in Africa, lack access to electricity at all.
Most people take the electrical grid for granted. As long as the electrons flow through their lights and devices, people don’t give a second thought to what it takes to create and deliver that power.
The electric grid is the largest complex system ever made by humanity. The grid is undertaking a giant balancing act; every second is a new equilibrium. Choices about the grid based on politics or preference may undermine the system. As technology develops and the world moves away from fossil fuel generators, the governance of these systems must also evolve.