The sweeping power outage that hit Spain and Portugal this week has raised questions about the electricity grid in a region not normally known for blackouts.

BUDAPEST, Hungary (AP) — An unprecedented blackout across Spain and Portugal upended the lives of millions across the Iberian Peninsula, a stark reminder of modern society’s reliance on electricity, and how quickly life can descend into chaos when the grid goes down.

Another source with direct knowledge of the sector said that leading up to the outage the Spanish grid was running with very little "inertia", which is energy stored in a large rotating mass like a generator or in some industrial motors that acts as a buffer as it can quickly be used to compensate for sudden changes in demand or supply.