Published by Global Banking and Finance Review
Posted on June 17, 2025
3 min readLast updated: January 23, 2026
A miscalculation by REE led to a massive blackout in Spain and Portugal, caused by a voltage surge and insufficient voltage control.
By Pietro Lombardi and David Latona
MADRID (Reuters) -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 also blames power generators for the worst-ever blackout to 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," Spain's Energy Minister Sara Aagesen told a news briefing in Madrid on Tuesday.
"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.
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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, declined to comment. REE did not immediately reply to requests for comment.
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.
(Reporting by Inti Landauro, David Latona and Pietro Lombardi; Additional reporting by Corina Ponswriting by Charlie Devereux; editing by David Evans and Tomasz Janowski)
The blackout was primarily caused by a surge in voltage that the grid was unable to absorb, leading to a cascade of disconnections of generation units.
REE's failure to calculate the correct mix of energy and not having enough thermal power stations operational contributed significantly to the blackout.
Energy Minister Sara Aagesen stated that the system lacked sufficient voltage control capabilities, which was a critical factor in the blackout.
The government plans to propose measures to strengthen the grid and improve its voltage control capabilities, as well as to further integrate the Iberian peninsula's electricity system.
The blackout caused massive gridlock in cities and left thousands stranded on trains and in elevators across the Iberian peninsula.
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