The precise cause of the blackout that affected nearly the entire Iberian Peninsula remains under investigation. Initial reports from Portugal’s grid operator suggested a rare atmospheric phenomenon-specifically, extreme temperature variations in Spain-caused anomalous oscillations in high-voltage lines, leading to synchronization failures across the interconnected European grid. However, Spain’s meteorological agency disputed the presence of unusual weather at the time, and authorities have not ruled out other technical or operational factors. There is currently no evidence of a cyberattack or sabotage.

A key technical event was a sudden loss of 15 gigawatts of power – about 60% of Spain’s national demand – in just five seconds, which fractured the interconnection with the French grid and triggered a cascading collapse of the Spanish system. Portugal, heavily reliant on imported electricity from Spain during daylight hours, was also severely affected.

Impacts on water supply

Based on available reports, there have not been confirmed large-scale water shortages directly attributed to the blackout. According to a detailed analysis of Spain’s water supply systems, a prolonged blackout would not immediately affect drinking water availability, as operating entities have contingency measures for the first hours of an outage.
In Almeria, authorities and operators were concerned about failures at the Carbonera desalination plant that supplies water to 140,000 people and to agriculture for irrigation. Both the government delegate in Andalusia, Pedro Fernández, and the Regional Minister of the Presidency, Antonio Sanz, have highlighted this problem as one of the most urgent issues to be resolved after the total blackout that affected Spain on Monday due to its impact on a very significant percentage of the population of the province of Almeria, elCorreo de Analucía reported.

Furthermore, there were disruptions in water supply to some critical facilities – such as hospitals. For example, a hospital in Setúbal, Portugal, stated that while backup generators kept critical departments operational for 8-12 hours, there was no water supply during the blackout, and no guidance was available on restoration or next steps. Furthermore, residents in affected urban areas also queued for bottled water, indicating localized interruptions and concerns about potable water access.

Impacts on wastewater treatment

Although there were no explicit reports of large-scale wastewater treatment failures, the loss of grid power likely impacted the operation of sewage pumps, blowers for biological wastewater cleaning, if they cannot use energy produced on site, such as from biogas or by decentralized generators.

By early Tuesday, April 29, power had been restored to nearly all of Spain and Portugal, though some transport and public services continued to face delays. The event has prompted urgent reviews of grid stability, particularly given the high penetration of renewable energy sources in Spain and the increasing complexity of managing supply and demand.