By JUSTIN SPIKE, Associated Press

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.

Power was nearly completely restored across the region on Tuesday, but the cause of the previous day’s blackout remains unclear. For those affected, the experience will likely leave a lasting impression and raise questions on how to respond if another crisis emerges.

While such widespread failures of power grids are rare, having a plan for how to react to such an event — and how to prepare ahead of time — can make a major difference in staying safe and managing the disruption if the lights go out.

What should I do if the power goes down?

A general power outage affects everyone whether they are at home, at work or on the move. As in Spain and Portugal, electric trains, trolleys and subway systems can stop in their tracks and strand passengers, while traffic lights can go down turning roads into a snarl of gridlock.

No matter where you are when the power goes down, your first priority should be to remain calm and tend to your most immediate needs like food, water and shelter, and develop a plan for how you can get to a safe place and connect with your loved ones.

According to the American Red Cross, unnecessary travel, especially by car, should be avoided during a power outage. Without traffic lights, roads may become congested or hazardous, increasing the risk of becoming stranded.

If you are at home, exercise caution when using candles or gas lanterns for light because of the risk of fire and noxious gases like carbon monoxide. Use battery-powered lanterns or flashlights if available, but conserve batteries to extend their working life.

Turn off any unnccesary electronics, and unplug appliances such as ovens or stoves that were in use when the power went out. If the power comes back on, surges can damage equipment and appliances can be hazardous if allowed to run unattended.

If the outage lasts for more than a few hours, you’ll want to ensure the food you have on hand doesn’t spoil. According to emergency guidelines published by the Estonian government, refrigerators and freezers should be opened only briefly to preserve the cold. Use perishable food from the fridge first, followed by items in the freezer, then nonperishables.

How do I prepare my home for a power outage?

Preparedness, “be it psychological or real preparedness in case of crisis,” is a key factor in successfully managing disruptive events, Argo Kerb from the Estonian Rescue Board, a government agency, told The Associated Press.

As concerns grow around the world over instability stemming from armed conflict, political discord and climate change, many governments are prioritizing crisis preparedness among their populations.

In March, the European Union urged all 450 million residents of the 27-nation bloc to stockpile food, water and other essentials to last at least 72 hours as war, cyberattacks, climate change and disease increase the chances of a crisis.

According to Kerb: “Preparedness is a shared responsibility. It’s a shared responsibility in a family, in a country, but it’s also a shared responsibility in Europe and the world.”

While long-lasting power outages such as the one in Spain and Portugal are relatively uncommon, having a few basic supplies on hand can make a substantial difference in how safely and comfortably they can be endured.

The EU recommends food, water, flashlights, ID papers, medicine and shortwave radios as things to have at home. Matches or lighters, chargers and a power bank for your devices are also recommended.

In a sign of how critical access to information can be in an emergency, battery-powered radios flew off the shelves in Spain and Portugal as internet and cellphone services went down. Keeping a supply of batteries, and a battery powered or hand-crank radio, will help keep you connected to vital information when internet and television services are down.

If you’re caught without these items during a crisis, you’ll need to buy them, but outages can leave electronic payments unavailable. You’ll want to make sure you keep a reasonable quantity of cash on hand.

As European commissioner for crisis management Hadja Lahbib said in a recent video urging preparedness: “In the middle of a crisis, cash is king, and your credit card might just be a piece of plastic.”

In a brochure provided to every household in Sweden, “In Case of Crisis or War,” the Swedish government says each person will need at least three liters (nearly a gallon) of water per day for drinking and cooking, which should be stored at home in bottles or other containers.

It also recommends keeping extra warm bedding like sleeping bags and blankets on hand in case services are interrupted during cold weather.

If you have a portable generator, strictly follow its instructions and never run it indoors or in enclosed spaces as it produces lethal carbon monoxide gas. Plug the devices you want to power directly into the generator, and never attempt to wire it to your home’s electrical system.

Emma Burrows contributed to this report from London.