Employees stand inside a supermarket without lights in Burgos on April 28, 2025, during a massive power cut affecting the entire Iberian peninsula and the south of France.
Cesar Manso | Afp | Getty Images
Spanish grid operator REE’s failure to calculate the correct mix of energy was one of the factors hindering the grid’s ability to cope with a surge in voltage that led to the massive blackout across Spain and Portugal on April 28, a government investigation concluded.
The report, made public on Tuesday, also blames power generators for the worst-ever blackout to have hit Spain and Portugal, since some conventional power plants, such as nuclear and gas-fired plants, failed to help maintain an appropriate voltage level in the power system that day.
“The system did not have sufficient voltage control capabilities,” Spanish Energy Minister Sara Aagesen told a news briefing in Madrid.
“Either because they were not sufficiently programmed, or because those that were programmed did not adequately provide what was required by the standard, or a combination of both,” she said.
While several factors played a role that day, Aagesen confirmed that the ultimate cause was a surge in voltage that the grid was unable to absorb. It triggered a cascade of disconnections of generation.
Voltage surges can be caused by multiple factors ranging from lightning strikes, faulty equipment, or grid instability. The probe pointed to grid instability earlier that morning.
Planning and plants’ shortcomings
REE, which is partly state-owned, did not have enough thermal power stations switched on when the voltage surge caused a chain reaction leading to the power outage, Aagesen said.
REE “told us that they made their calculations and estimated that (switching on more thermal plants) was not necessary at this time. They only set it for the early hours of the day, not the central hours.”
The blackout caused massive gridlock in cities and left thousands stranded on trains and in elevators across the Iberian peninsula.
Power plants “should have controlled voltage and, moreover, many of them were economically remunerated to do so. They did not absorb all the reactive power that was expected,” Aagesen said.
Utilities lobby Aelec, which represents Spain’s main electricity companies including Iberdrola and Endesa, said in a statement it agreed that voltage control was the main cause of the outage, adding that “the responsibility for ensuring such control lies with” REE as system operator.
It has evidence that the power plants controlled by the companies it represents “have complied with the regulatory requirements regarding voltage control” even “operating above the regulatory obligations” to help stabilise grid.
People queue at a bus stop at Cibeles Square in downtown Madrid as subway and trains are totally out of service due to a massive power outage in Spain, on April 28, 2025.
Thomas Coex | Afp | Getty Images
Despite having sufficient resources to guarantee voltage control, REE “opted to manage voltage with limited synchronous capacity and an unbalanced geographical distribution, which left the system in a vulnerable situation,” it said.
REE did not immediately reply to requests for comment. Its chairman will hold a press conference on Wednesday.
The government said on Tuesday it would propose measures to strengthen the grid and improve its ability to control voltage in the system. It would also push to further integrate the peninsula with the European grid, it said.
Pratheeksha Ramdas, Senior New Energies Analyst and Iberia power market expert at Rystad Energy said the incident highlighted the essential role thermal power plants, especially gas-fired plants continued to play.
“It appears that the blackout reflects a critical failure in the Spanish electricity system not due to lack of installed capacity but due to mismanagement of available energy resources and accountability in grid operations,” Ramdas said.