Why did the lights go out in Spain and Portugal? 

A frantic search is on to figure out what, exactly, caused Spain and Portugal to lose power on Monday, leaving millions of people without access to basic modern necessities. 

The answer will have serious ramifications for the two countries — and for Europe as a whole. 

The EU has been pressing the Iberian Peninsula, which has long operated as an energy island within the bloc, to link up better to Europe’s grids. Already on Monday, specialists were debating whether such connections could have helped prevent the massive blackouts — or put more countries at risk of experiencing spillover effects. 

There was also hand-wringing throughout the day about whether the outages were the work of cyber-saboteurs launching a strike on Europe — even as no evidence emerged suggesting that was the case.

POLITICO walks you through what we know so far and what fallout we may see.

What went wrong?

On Monday afternoon, Eduardo Prieto, director of Spanish transmission system operator Red Eléctrica, said the blackout was caused by a “very strong oscillation in the electrical network” that led Spain’s power system to “disconnect from the European system, and the collapse of the Iberian electricity network at 12:38.”

A press conference by Spanish Prime Minister Pedro Sánchez late on Monday evening presented more questions than it answered.

Looking grave and warning that it may take longer to restore power than originally anticipated, the Socialist leader said: "This has never happened before."

Authorities in both countries declined to hypothesize as to what might have caused the imbalance in the power system.

"What caused it is something that the experts have not yet established — but they will," said Sánchez, adding that "no hypothesis has been rejected, and every possible cause is being investigated."

Why does it take so long to get the power back?

Restoring power to Spain and Portugal has been no easy task.

According to Leonardo Meeus, a professor specializing in electricity at the European University Institute, grid operators have to follow a “technically very complicated” process to get the energy system up and running again.

Both countries have to restore supplies “step by step,” first by relying on special generators, like hydroelectric pumps, he said.

Neighboring countries are also chipping in, with French grid operator RTE supplying 700 megawatts of electricity to Spain within hours of the blackout. The lights were back on in most of the Iberian Peninsula’s northernmost and southern regions by the late afternoon, thanks in part to power supplies from France and Morocco.

"The power systems are connected through 'interconnectors'," said Solomon Brown, a professor specializing in energy systems at the University of Sheffield. "This means that there is interdependency between the networks, but also that they will have to be restarted separately.

"As the two networks have gone down they will have to be re-powered, which means that the grid operator will slowly bring on key generators matched with users (so that production and consumption of electricity match) in regions of the network that slowly expand, until the whole system is back on and can then be reconnected to external networks."

Why are the power systems linked?

Spain and Portugal have tightly linked power systems with few cross-border links to France and the rest of the EU.

The European Commission, the EU’s executive, has long wanted to change that fact, strongly encouraging EU countries to build more interconnections. Officials argue the move would improve energy security and ease electricity flows across borders. Madrid, Lisbon and Paris have repeatedly signaled a willingness to move forward with that plan, but advances have been slow.

Although Spain and France are now finally forging ahead with a new link in the Basque country, the EU’s energy regulator association warned last year that not enough electricity transmission capacity has been freed up, despite legal requirements to do so. 

It remains unclear whether more connections to Europe would have ameliorated the blackout. 

On one hand, more cross-border links could have strengthened Spain’s ability to balance supply and demand and import electricity, said Pratheeksha Ramdas, a senior power analyst at the Rystad consultancy. Madrid’s current limited trading capacity with France “constrains rapid response capabilities during large-scale disturbances,” she said.

Conversely, “increased connectivity might risk propagating frequency instability into neighboring systems,” she added, creating a domino effect of cascading blackouts across other EU countries.

Could this have been a cyberattack?

There was palpable concern regarding the situation across Spain. One student exiting a university in Barcelona was heard asking, “Is this a cyberattack? A terrorist attack?”

Spain’s opposition to Israel’s war in Gaza and its support for Ukraine against Russia's aggression have made it a major target for cyberattacks, and throughout the day there was heightened speculation that the crisis could be the result of nefarious action.

But in Brussels, European Commission Executive Vice President Teresa Ribera appeared to dismiss that possibility, saying there was “nothing that allows us to say that there is any kind of sabotage or cyberattack.”

In 2015, Russian-linked hackers knocked down a chunk of Ukraine’s power grid. But to bring down the electricity grid of two EU countries would be a vastly different prospect, in terms of both the technical effort required and the bellicose implications of such an attack. 

Why are people talking about green energy?

Spain and Portugal are champions of green energy in the EU, and were sourcing over 80 percent of their electricity from renewables just before the outage hit on Monday. 

Experts have previously said that the rapid expansion of wind and solar power is putting increased pressure on the Spanish grid, which is in need of an upgrade to handle record volumes of intermittent renewables.

But green energy was not to blame for Monday’s blackout, Meeus said, since the EU has in recent years enforced several sets of rules, like updated grid connection codes from 2016, to prevent renewable power generators from disconnecting from the network in a way that endangers the system.

“The nature and scale of the outage makes it unlikely that the volume of renewables was the cause, with the Spanish network more often than not subject to very high volumes of such production,” added Daniel Muir, senior European power analyst at S&P Global. 

“There was sufficient conventional generation available, with nuclear, hydro, cogeneration and thermal technologies all on the system prior to the event and ... available to the operator,” he said.

This article has been updated. Gabriel Gavin reported from Tallinn. Aitor Hernández-Morales, Sam Clark and Victor Jack reported from Brussels. Max Griera reported from Barcelona. Joe Stanley-Smith contributed to this article from Florianópolis, Brazil.