We rarely think about how essential and reliable electricity grids are until they fail.

Now, millions of people across Spain, Portugal and parts of France are likely thinking of little else.

While local power cuts are fairly common, what's happened across the Iberian peninsula is something far more extreme.

Much of Spain and Portugal's electricity transmission system collapsed in seconds including in major cities Lisbon, Madrid, Barcelona and Seville.

Blackouts latest: 'Rare atmospheric phenomenon' behind outages

It's likely the outage will surpass Europe's largest blackout to date when 56 million people in Italy and Switzerland lost power for up to 12 hours in 2003.

The cause of the outage is unclear. Portugal's grid operator has blamed a "rare atmospheric phenomenon" that caused "anomalous oscillations" in high voltage power lines in Spain.

Spain's grid operator has yet to respond to that or provide an update on the cause. But it's unlikely whatever caused the outage was a single, localised event.

A major power line going down can cause a large outage - as it did in 2021, when an interconnector between France and Spain failed leaving a million people without power for a few hours.

But it's unlikely to cause a system-wide failure of the kind we're seeing now.

However, when things do start to fail on a power grid, they can cascade uncontrollably.

Keeping a grid running is a constant and highly complex balancing act.

Spain's mains AC electricity supply grid, like ours in the UK, runs at 50Hz. That frequency is based on the speed at which generating hardware such as gas and nuclear turbines spin.

If there are sudden fluctuations in power supply or demand - a power station failing or a high voltage power line going down, for example - the frequency of AC power in the transmission lines changes and circuit breakers trip to protect either the transmission network, or power plant hardware from burning out.

To prevent such failures, grid engineers constantly measure and forecast supply and demand to keep the grid balanced.

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To protect the system in emergencies, they occasionally have to "shed load" by cutting power to parts of the grid - the reason we've all experienced the occasional short-lived power cut.

But if balance is lost, a grid can fail in a domino effect with sections of the grid tripping, then power plants shutting down to protect themselves from the drop in demand, one after another.

The challenge now, and it's a nightmare for Spain and Portugal's power engineers, is to gradually restore the grid section by section while maintaining the balance of supply and demand.

Act too fast, and the grid can trip again. Take too long and some power plants or substations might struggle to restart - especially if they rely on battery power to do so.

While some regions of Spain have already had power restored, and Portugal says its power will be back to normal within hours, it could take much longer for the system to be fully restored.

(c) Sky News 2025: How electricity grids fail - and why restoring Spain and Portugal's power will be a nightmare