On a leafy boulevard in central Madrid on the afternoon of 28 April, half a dozen residents stood in a loose semi-circle on the sidewalk, facing an apartment balcony. A man sat on the balcony with his battery-powered radio, the speaker oriented toward passersby whose mobile phones couldn’t get a signal due to a blackout that had swept Spain and Portugal. Everyone wanted to know the causes of the blackout, which had occurred at 12:33pm, local time. Some speculated it was a foreign attack, while others blamed unstable solar and wind generation, which together comprised 33 percent of Spain’s and 37 percent of Portugal’s electricity generation in 2024.

Almost two months after the Iberian blackout, the four official investigations into the cause haven’t yet released their conclusions, and people are still waiting to know the causes of the blackout. Yet academic researchers with access to voltage data, such as electrical engineer Antonio Gómez-Exposito, claim that there may have been sustained overvoltages, in which generating plants sent too high a voltage to the transmission grid, just before the grid’s frequency dropped, which implies a potential issue: poorly distributed reactive power sources. Such sources can help control voltages when renewables send power from the distribution level of the grid up to the transmission level, which is becoming more common as grids add more distributed renewables.

In the first days after the blackout, many outlets and experts focused on the frequency of the grid and the need for inertia, which refers to how spinning generators carry physical momentum that makes them slow to change the frequency of the alternating current (AC) they generate. Most equipment on an electrical grid must operate within fairly narrow range of a set frequency. Conventional power plants, such as combined-cycle natural gas or hydroelectric plants, can provide inertia, but newer sources such as photovoltaic solar power do not. So the inertia discourse was in part a discussion of how to incorporate direct current sources such as photovoltaics into an AC grid.

If overvoltages, rather than frequency drops, were larger contributors, then the discussion is still about renewables, but it depends more on reactive power than on frequency management. Reactive power is one component of any AC electrical distribution grid. It emerges from the phase shift between voltage and current as the grid stores and withdraws energy in electromagnetic fields. Reactive power helps to carry the active component of power along long-distance transmission lines, and grid operators must balance reactive power alongside active power, or they can get overloading or voltage fluctuations that force generators off the grid. “The problem is that the regulation of the grid doesn’t reward renewable plant operators for helping balance reactive power,” says electrical engineer José Daniel Lara at the National Renewable Energy Laboratory in Golden, Colorado.

The shift toward more decentralized power production means that the amount of reactive power absorbed by transmission lines is changing, and the direction of flows of reactive power are also changing, making its regulation more complex. For example, Austria, Germany, and Japan have all begun regulating reactive power management to account for the effect of more distributed production.

So, the fact that more renewable energy is often more distributed than conventional sources may have contributed to a different reactive power profile on the Iberian grid. Yet “other operators, such as in the U.S., require or reward grid participants for helping balance this reactive power,” Lara says. Spain could do that, too, given its commitment to expanding the role of renewable, and therefore distributed, power. Yet its reactive power rules pre-date the flood of solar and wind energy that has reshaped the country’s grid (the main rules are from 2000, with a 2014 partial update). Today’s rules also exempt renewable plants from helping to lower voltage peaks, Gómez says, which is a mistake: “Today’s grids, with their high renewable penetration, can’t be managed like grids of the twentieth century when everything was fossil fuels and hydroelectric plants.”

In early June, Beatriz Corredor, the president of Spain’s grid operator, Red Eléctrica de España (REE), blamed the blackout on up to five near-simultaneous failures of conventional generators with sub-standard voltage control. REE is not blaming the failure on transmission short circuits, lack of electrical inertia, an information technology hitch, a reserve shortage, or excess voltage, she said in an interview on Spanish television. Corredor didn’t give a specific explanation for the failures she mentioned or name the plants that may have failed, but the implication is that the responsibility is at the distribution level, below the transmission level that REE manages.

Most grid failures this big have multiple causes, and there are other ways to build resilience, in addition to better reactive power management. For example, Spain and Portugal have very little interconnection capacity with neighboring France and Morocco. The EU recommends its member states have 10 percent interconnection capacity, rising to 15 percent by 2030, but Spain and Portugal have only 2 percent interconnectivity with the rest of the EU, and very little connectivity with Morocco.

Another contributor to reliance is how operators respond to local failures. Energy engineer Ricardo Bessa of the Institute for Systems and Computer Engineering, Technology and Science in Porto, Portugal, is involved in a European research project called AI4RealNet that aims to provide grid operators with high-speed AI decision-making support when things go wrong, to avoid cascading events and blackouts. “It will mitigate, but it is not a silver bullet,” Bessa says. Just as importantly, it will help researchers to understand why a given failure or blackout occurred after the fact, but much faster than today’s methods.

Storage is another answer to preventing excess voltage from swamping a grid. Spain is building grid-scale storage, but so far has just over 3 GW in a grid with an installed capacity of around 129 GW. More storage capacity located near to generators would make it easier to handle reactive power when those generators produce too much of it.

It will be months before Spain’s official investigations release their conclusions, and meantime an European panel of grid operators is working on the problem, as are the hundredsof electrical operators on the peninsula who will want to avoid a likely decade-long legal fight over liability for the blackout.

In the meantime, the almost 60 million Spaniards and Portuguese affected by April’s blackout will have to turn their attention to researchers such as Gómez for insight into the blackout’s real cause. They are starting to patch together a mixture of public and private data and discuss openly the kinds of lessons regulators and industry may take from the blackout. “It’s going to force some changes,” Gómez says, “First will be operating procedures, which are always changing but they change slowly. Now it will be more agile.”