Spain’s power company, Red Eléctrica, proudly declared on April 16 that enough renewable energy had been generated to cover demand. “The ecological transition is moving forward,” it said.

Less than two weeks later, Spain and Portugal experienced an 18-hour blackout that disrupted daily life, shutting down businesses and schools and crippling trains and mobile networks.

Officials have given few details on the cause of the outage. But the incident exposed how Spain and Portugal, promoted as success stories in Europe’s renewable energy transition, are also uniquely vulnerable to outages, given their relative isolation from the rest of the continent’s energy supply.

“This disruption serves as a clear warning,” wrote Pratheeksha Ramdas, an analyst at Rystad Energy, a consulting firm. “Future grid failures could have even more severe consequences,” she added.

What role may reliance on renewable energy have played?

The widespread outage raises questions about the resilience of the power infrastructure in Spain and Portugal — and to an extent, Europe. The two countries have invested heavily in building renewable energy sources like wind turbines and solar farms.

More than half of Spain’s electricity came from renewable energy as of last year, up from about a quarter 15 years ago. That rapid increase has put Spain at the forefront of Europe’s transition to renewable energy and led to much lower electricity prices and less reliance on fossil fuels.