The grid operators needed more than a day to fully restore the power supply in Spain and Portugal. One of the circulating theories is that excessive solar power caused the blackout. Is that true?As soon as the lights went out in Spain and Portugal, the speculating about the cause of the blackout began. Was it a cyberattack? Atmospheric interference? Aliens? Also Read | India News | Chithirai Festival: Golden Palanquins of Deities Taken in Procession, Devotees Welcomed with Refreshments. Though the Spanish government is looking into the possibility of a cyberattack on the Spanish electricity grid, the head of the Spanish electricity transmission system operator Red Electrica (REE) , Eduardo Prieto, has ruled this out already. Also Read | Sports News | IPL 2025: GT Batting Duo Sudharsan, Buttler Shatter Records with Blistering Display Against Hyderabad. Extreme frequency fluctuation According to Prieto, two apparently separate incidents that happened within 1.5 seconds of each other in southwestern Spain threw the grid frequency off balance. He said that this caused the outage of power plants with a capacity of 15 gigawatts (GW) — almost half of the active power plant capacity at the time. Very large and sudden deviations can cause a cascading failure, whereby extreme frequency fluctuations trigger elements and mechanisms in other power plants (and large consumers), which then automatically take these plants off the grid. Prieto described it as quite possible that this domino effect may have been triggered by solar power plants. Does that mean critics of renewable energies are right when they say that an oversupply of solar power triggered the blackout? Claim: "Renewables, in this case solar, have just caused the first major #power outage in Spain, specifically due to an oversupply of solar," an alleged physicist with a German account, posted on X, formerly Twitter, garnering almost 100,000 views (as of 30.04.2025, 16:40). The user refered to Fritz Vahrenholt, a German chemistry professor and manager, who is critical of energy transition efforts and had explained in another post on X, which attracted more than 400,000 views, that before the grid failure in Spain, the production of renewable energies, especially solar, had increased more than demand. DW fact check: Unproven The cause of the power outage has not been conclusively clarified. It is correct that at the time of the blackout, solar energy provided around 60% (19.3 GW) of the power available in the Spanish grid. Moreover, it appears that solar power plants dominated electricity production in the southwest of Spain. Did all PV systems suddenly switch off? For energy transition critic Fritz Vahrenholt, there is no doubt that the surplus of photovoltaic (PV) power in the Spanish grid was the "primary cause," as he told DW by email. He said that Spain had been forced to export electricity to France as a result. The "secondary cause" was the loss of this transmission line and as a result "all" the PV systems had then "abruptly" disconnected from the grid, he said. However, PV systems are not supposed to shut down abruptly. because standardized regulations for electricity producers have been in force in the EU since 2016. These regulations stipulate that PV systems have to gradually reduce their feed-in if the grid frequency exceeds the limit value of 50.2 hertz (Hz) due to a power surplus. "In Spain, corresponding grid connection guidelines were introduced at the end of the 2000s to ensure that PV systems do not simply disconnect from the grid in the event of a sudden grid fault," said Sönke Rogalla, a researcher at the German Fraunhofer Institute ISE, Europe's largest solar research institute. He explained that these properties were verified prior to operation as part of certification procedures. Furthermore, data avaiable so far also contradict Vahrenholt's thesis. Even if all solar plants had been taken off the grid at the same time, there would not have been a shortfall of 15 GW, but at least of 20.4 GW as the PV output at the time of the blackout was at least 17 GW. However, also nuclear power plants, which supplied 3.4 GW, were taken off the grid as a precautionary measure. Is high PV power a problem for electricity grids? It is documented that Spain's power grid connections to France and Portugal were capped during the blackout. However, it is still unclear whether this was the cause of the blackout in Spain or one of the domino effects. What is certain is that Spain exported a net total of 3 GW shortly before the grid disconnection. As a result of the interruption to the power lines, this surplus was suddenly available in Spain. Such an incident would undoubtedly be a major burden for any electricity grid. Vahrenholt believes that the low number of coal and nuclear power plants in Spain was an additional problem, as the turbines of such power plants can serve to smooth frequencies. This stabilizing effect, known as instantaneous reserve, was probably lacking in Spain, wrote Enrique Garralaga, a managing director at a subsidiary of the German PV component manufacturer SMA, in a LinkedIn post. "Technical solutions are available" Fraunhofer researcher Rogalla confirmed that this could have contributed to the blackout but he had a different perspective: "The blackout in Spain was not a PV failure, but probably a system failure," he said, adding that "in this respect, I see it as an urgent reminder that the restructuring of the energy system poses major challenges." With the systems installed today, it is not possible to operate an electricity grid with solar and wind power alone, he said. He also rejected the claim that a high proportion of renewable energies was an unsolvable problem. "We are constantly learning. The technical solutions are now available, now we have to set about implementing them." The outgoing German government outlined a possible way forward in a roadmap. The regulatory authority, the European Union Agency for the Cooperation of Energy Regulators, or ACER, has already submitted a proposal for the implementation of new solutions at EU level. Conclusion: The theory that 'an oversupply of solar (power)' was the reason for the blackout cannot be substantiated. Although a high proportion of renewable electricity in the grid is one of the major challenges of the energy transition, it is not an unsolvable problem. This article was originally published in German.