Spain and Portugal are still recovering from Monday’s region-wide blackout. The cause remains unknown, but already a debate has broken out over whether grids like Spain’s, which has a well-above-average proportion of renewables, are more at risk of large-scale disruptions.

At the time of the blackout, Spain’s grid had little “inertia,” which renewables opponents have seized on as a reason to blame carbon-free electricity for the breakdown. If the electricity system as a whole is a dance of electrons choreographed by the laws of electromagnetism, then inertia is the system’s brute force Newtonian backup. In a fossil fuel-powered grid, inertia comes from spinning metal — think a gas turbine — and it can give the whole system a little extra boost if another generator drops off the grid. Solar panels, however, don’t spin. Instead, they produce direct current that needs to be converted by an inverter into alternating current at the grid’s frequency.

“If a power plant goes out, that frequency starts to drop a little bit because there’s an imbalance in the power between supply and demand, and inertia provides a little bit of extra power,” Bri-Mathias Hodge, an electrical and energy engineering professor at the University of Colorado and a former chief scientist at the nearby National Renewable Energy Laboratory, explained to me. Inertia, he said, “just gives a little bit more wiggle room in the system, so that if there are big changes, you can sort of ride through them.”

Of course, blackouts happen on grids dominated by fossil fuels — the 2003 Northeast Blackout in the U.S and Canada, for example, which plunged several states and tens of millions of people into darkness. Even on renewable-heavy grids, blackouts can still come down to failures of fossil fuel systems, as with Texas’ Winter Storm Uri in 2021, when the natural gas distribution system froze up. Much of the state had no electricity for several days amidst freezing temperatures, and over 200 people died.

But Bloomberg’s Javier Blas was nevertheless fair to the Iberian blackout when he bestowed on it the sobriquet, “The first big blackout of the green electricity era.”

Spain has been especially aggressive in decarbonizing its power grid and there’s some initial evidence that the first generators to turn off were solar power. “We started to see oscillations between the Iberian Peninsula and the rest of the European power grid, and this generally means that there’s a power imbalance — somebody’s trying to export power that they can’t, or import power that they can’t because of the limits on the lines,” Hodge told me. “The reason why people have gone on to say that this is a solar issue is because where they’ve seen some of those oscillations and where they saw some of the events starting, there are a couple large solar plants in that part of southwestern Spain.”

While Spanish grid and government officials will likely take months to investigate the failure, we already know that Spain and Portugal are relatively isolated from the rest of the European grid and rely heavily on renewables, especially solar and wind. Portugal has in the past gone several days in a row generating 100% of its power from renewables;Spain, meanwhile, was boasting of its 100% renewable generation just weeks before the blackout.

Last week, Spanish solar produced over 20,000 megawatts of power, comprising more than 60% of the country’s resource mix.Spain’s seven remaining nuclear reactors — which still provide about a fifth of its electric power — are scheduled to shut down over the next decade (though officials have indicated they might be open to extending their life), while its minimal coal generation is scheduled to be retired this year.

“Spain and Portugal have been relatively early adopters of wind and solar power. The Iberian Peninsula is actually relatively weakly connected to the rest of Europe through France. And so that’s one of the tricky parts here — it’s not as well integrated just because of the geography,” Hodge said.

The disturbances on the grid started on the Spain-France interconnection, but a European power official told The New York Times that transmission issues typically don’t lead to cascading blackouts unless there’s some major disturbance in supply or demand as well, such as a power plant going offline.

Spain’s grid had issues before Monday’s blackout that can be fairly attributed to its reliance on renewables. It often has to curtail solar power production because the grid gets congested when particularly sunny parts of the country where there’s large amounts of solar generation are churning out power that can’t be transmitted to the rest of the country. Spain has also occasionally experienced negative prices for electricity, and is using European Investment Bank funds to help support the expansion of pumped-hydro storage in order to store power when prices go down.

On Monday afternoon, however, solar power dropped from around 18,000 megawatts to 8,000, Reuters reported. At the time the blackout began, the grid was overwhelmingly powered by renewables. Spanish grid operator Red Electrica said it was able to pinpoint two large-scale losses of solar power in the southwestern part of the country, according to Reuters.

That a renewables-heavy grid might struggle with maintaining reliability thanks to low inertia is no surprise. Researchers have been studying the issue for decades.

In Texas — which, like Spain, has a high level of renewable generation and is isolated from the greater continental grid — the energy market ERCOT has been monitoring inertia since 2013, when wind generation sometimes got to 30% of total generation, and in 2016 started real-time monitoring of inertia in its control room.

That real time monitoring is necessary because traditionally, grid inertia is just thought of as an inherent quality of the system, not something that has to be actively ensured and bolstered, Hodge said.

As renewables build up on grids, Hodge told me, operators should prepare by having their inverters be what’s known as “grid-forming” instead of “grid-following.”

“Right now, in the power system, almost all of the wind, solar, battery plants, all the inverter-based generation, they just look to the grid for a signal. If the grid is producing at 60 Hertz, then they want to produce 60 Hertz. If it’s producing at 59.9, then they try to match that,” Hodge said. This works when you have relatively low amounts of [renewable generation]. But when [renewables] start to become the majority of the generation, you need somebody else to provide that strong signal for everybody else to follow. And that’s sort of what grid-forming inverters do,” he said.

Grid-forming inverters could hold back some power from the grid to provide an inertia-like boost when needed. Right now, the only sizable grid outfitted with this technology, Hodge said, is the Hawaiian island of Kauai, which has a population of around 75,000. Spain, by contrast, is home to nearly 50 million.

The other key technology for grid-forming inverters to provide stability to a power system is batteries. “Batteries are actually the perfect solution for this because if you have a battery system there, you know most of the time it’s not producing or charging and totally full output or input. So the vast majority of time you’re going to have some room to sort of move on in either direction,” Hodge said.

But this requires both technology and market structures that incentivize and allow batteries to always be ready to provide that instantaneous response.

“The entire stability paradigm of the power grid was built around this idea of synchronous machines,” Hodge told me. “And we’re moving toward one that’s more based on the inverters, but we’re not there yet. We have to fix the car while we’re driving it. We can’t turn off the grid for a couple years and figure everything out.”