A complete power outage in Spain and Portugal on Monday plunged trains into tunnels, office workers stuck in lifts and cut the biggest outage in two decades in Europe’s largest power outage.

As governments compete to launch renewable energy and reduce carbon emissions in their power systems, the catastrophic failure of electricity supply has raised pressing questions about the resilience of infrastructure across Europe’s power grid.

The Spanish power grid collapsed shortly after 12.30 pm local time and was comparable to Portugal. But officials and engineers are still trying to figure out why the collapse means and affects the energy infrastructure.

What causes the power outage?

Electricity supply depends on a balanced system. Demand and supply must occasionally match, and the frequency of the grid (rate of current alternating currents) must be kept stable to avoid damage to electrical equipment and in the worst case failure.

Consulting company Aurora Energy Research said that at 12.33pm local time, the frequency on the Spanish grid suddenly dropped, starting at the 50 Hz level, and grid operators tried to maintain it to 49 Hz.

Movement greater than 0.1 Hz forces many power stations to automatically shut down for safety reasons. Any loss of power in Spain has an immediate ripple effect in Portugal, which largely depends on the power supply of its neighbors.

The frequency at which the frequency of triggering the frequency drop is not yet known. On Tuesday, Eduardo Prieto, director of operational services at Spanish grid operator Red Eléctrica, blamed an unexpected loss in southwestern Spain, home to many solar power plants. Other theories include cable damage.

Frequency fluctuations are not uncommon, but grid operators often overcome them by requiring generators to increase or decrease their output or use batteries. However, in this case, it is not possible to bring the additional power generation capacity online to a speed fast enough. Red Eléctrica ruled out cyber attacks.

Is renewable energy part of the problem?

If you don’t know the exact reason for the frequency drop, it’s impossible to say it. However, power systems that rely on renewable energy such as wind and solar are more complex than traditional coal-fired power plants and gas-fired power plants.

Renewable energy depends on weather dependence, but solar panels lack large turbines, which can help the system run if there is a power failure somewhere along the line. This process is called "inertia".

One-fifth of Spain's annual electricity supply is solar, but the proportion is much higher than 55% on Monday lunch hours. Aurora said the lack of inertia “causes unstable contributions.” Even so, nuclear power plants and other sources online at the time should have provided enough inertia, said Adam Bell of the British consulting firm Stonehaven.

However, due to mechanical and technical faults, power outages also often occur in power systems dominated by traditional power plants such as natural gas, coal or nuclear power plants.

Are the power grids in other countries more flexible?

The resilience of the grid is increasing worldwide as countries rely more on electricity to support a growing population and power heat pumps in electric vehicles, houses, data centers and air conditioners.

The Paris-based International Energy Agency provided advice on energy policy, warning last week that cyberattacks and climate change could pose an increasing threat. While renewable energy reduces reliance on the volatile fossil fuel market, the conversion of power systems “has brought new challenges”, the IEA said.

The UK has deployed 200-ton "flywheels" that mimic turbines in traditional power plants to avoid grid instability issues. Grid operators also use technology to try to measure inertia, so they can step in when needed.

More use of batteries and cables that go in and out of power to other countries can also help balance intermittent supplies. Spain's relatively poor connection with France has long been a complaint for Madrid.

“Sometimes, in decision making, we focus on adding more wind and solar, which is great. But you also need to add backups,” said Javier Cavada, CEO of UEFA, Mitsubishi Power. However, there is always a balance between the high cost and the risk of installing new cables.

Is there a possibility of energy transition?

The power outage in Spain is at a critical moment in an effort to get rid of fossil fuels, some countries’ commitments to reduce emissions and some efforts to scale up the technology.

Although the cause of the blackout has not been determined yet, opponents of renewable energy may seize the story of Spain as a warning.

Romanian Energy Minister Cristian Bușoi said the EU should reconsider its plan to end gas by 2050. But EU Energy Commissioner Dan Jørgensen tried to resist this claim. "One thing is clear: Energy security should be our top priority. Connectivity, solidarity and clean indigenous energy are key to making our energy systems more resilient," he said on Tuesday.

More certainly, cuts can help stimulate a much-needed investment in power transmission networks to incorporate more renewable energy sources and focus more on resilience. “Grids are considered to be something nerds and engineers, but that’s not the case,” said Javier Pamos Serrano of Aurora. “We have to have safe and reliable grids moving forward.”

What will happen next?

In addition to the investigation at the national level, European Commission officials also said an independent investigation into the reasons for the cuts in Spain and Portugal was likely to be conducted, led by experts from unaffected member states. The official added that its recommendations will be implemented by Brussels.