A recent massive power outage is reportedly being attributed to the potential failure of two solar plants in southwestern Spain.

Spain’s electricity operator, Red Eléctrica, reported that over 99% of the country’s energy demand was restored by 7 a.m. local time on Tuesday. Meanwhile, Portuguese grid operator REN stated that all 89 power substations had been restored since late the previous night, bringing power back to all 6.4 million customers.

Red Eléctrica identified two power generation loss incidents in southwest Spain, likely involving solar plants, which caused instability in the Spanish power grid and led to a breakdown in its connection to France, .

According to investment bank RBC, the economic impact of Monday’s blackout across the Iberian Peninsula could range from $2.5 billion to over $5 billion.

Spanish Prime Minister Pedro Sánchez stated in a televised address on Monday night, “We have never had a complete collapse of the system.”

Emergency workers in Spain reported rescuing approximately 35,000 passengers on Monday who were stranded and in underground tunnels.

Spanish television showed footage of people evacuating metro stations in Madrid and empty stations with trains stopped in Barcelona. Spain’s parliament was also affected by the blackout, according to public broadcaster RTVE.

The ATP Tour announced that play at the Madrid Open tennis tournament was suspended for the day due to the power outage.

In Portugal, subway cars in Lisbon were evacuated, courts were disrupted, and ATMs and electronic payment systems were affected. Traffic lights in Lisbon also ceased functioning during the outage.

REN, Portugal’s grid operator, described the incident on Monday as a

attributed the outage to “extreme temperature variations in the interior of Spain” causing “anomalous oscillations in the very high-voltage lines, a phenomenon known as induced atmospheric vibration. These oscillations caused synchronization failures between the electrical systems, leading to successive disturbances across the interconnected European network.”

However, on Tuesday, Spain’s meteorological agency AEMET stated that it had not detected any “unusual meteorological or atmospheric phenomena” on Monday, and no sudden temperature fluctuations were recorded at their weather stations.

Eduardo Prieto, Red Eléctrica’s chief of operations, stated that the instability in the power grid caused the Spanish and French electricity interconnection through the Pyrenees mountains to split, leading to a failure on the Spanish side, according to . The news agency reported that some parts of France also experienced brief power outages on Monday.

Authorities were still investigating the cause of the outage on Tuesday.