Spain’s massive blackout on Monday was partly caused by its electricity grid’s inability to handle a surge in solar power, experts say.

Around 55% of supply came from solar when 15GW of generation dropped in five seconds, triggering widespread outages in Spain and Portugal.

Analysts blamed a lack of firm backup power from fossil fuels or nuclear sources and poor grid management.

Some suggested either a solar oversupply or sudden drop in solar output due to cloud cover could have caused the frequency crash, the Financial Times has reported.

Red Eléctrica, Spain’s grid operator, has not confirmed the cause but denied that renewables made the system more vulnerable.

Written by B.C. Begley