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Glossary · Energy industry & energy transition

Blackout

Blackout power outage security of supply resilience black start capability

A blackout is a large-scale collapse of the power supply. The transmission grid can no longer be operated stably, either entirely or in large parts, and a very large number of consumers are without power at the same time. A single local outage caused by a damaged cable, a brief interruption in one city district or the announced switching off of individual loads is not what is meant. A blackout concerns the functioning of the power system itself. Frequency, voltage, protection systems, generation and load must afterwards be brought back together in an orderly restoration.

EU law contains a precise definition. The Commission Regulation establishing a guideline on electricity transmission system operation, Regulation (EU) 2017/1485, refers to the "blackout state" (in German also "Schwarzfall-Zustand"). This means a system state in which the operation of the transmission system is partially or totally ceased. Under Article 18(4), this state exists if at least one of two conditions is met. Either more than 50 percent of the load in the control area of the transmission system operator concerned is lost. Or the control area has had no voltage for at least three minutes, so that system restoration plans are activated. The legal definition therefore does not require a "long" duration of hours or days. It is based on the extent and the state of the transmission system of a control area. For the synchronous areas of Great Britain and Ireland/Northern Ireland, the Regulation allows separate proposals on the load threshold. In everyday language and in civil protection, by contrast, the term is often understood more broadly, for example as a supra-regional and longer-lasting power outage.

In continental Europe, power grids operate at a nominal frequency of 50 hertz. The frequency shows whether feed-in and withdrawal are in balance. If more electricity is consumed than generated, it falls. If more is fed in than consumed, it rises. Deviations are balanced by balancing energy, automatic protection mechanisms, power plant control, storage, flexible consumers and measures taken by grid operators. Not every deviation leads to a blackout. A blackout arises from a chain of disturbances in which protective reactions, grid bottlenecks, missing reserves, problems with voltage control and organisational processes interact in such a way that supply can no longer be maintained in a larger area.

Distinction from power outage, brownout, load shedding and Dunkelflaute

In everyday language, any interruption of the power supply is often called a "blackout". Technically, that is too imprecise. A local power outage can be caused by construction work, cable damage, a defective transformer or a storm. For those affected it is significant. However, the overall power system usually does not collapse as a result. Grid operators can locate the fault, switch over grid sections and often restore supply step by step.

A brownout is a deliberate or unintended reduction in voltage. Consumers continue to receive electricity, but of poorer quality or only usable to a limited extent. In European interconnected grids, a brownout is not a planned operating tool. The term appears mainly in risk and crisis discussions.

Load shedding is something different from a blackout. In it, certain consumers or grid areas are deliberately switched off in order to prevent a larger collapse. If the frequency falls sharply, automatic under-frequency load shedding disconnects load from the grid in stages. In continental Europe it begins at about 49 hertz. This allows the remaining grid to stabilise again. The EU requirements on the system defence plan set a binding frequency value at which load shedding must start at the latest. Load shedding is an intervention to avoid a blackout. It does not mean the same thing as a blackout.

Dunkelflaute and blackout must also not be equated. A Dunkelflaute is a weather situation in which only little wind and solar power is generated over a certain period. It can strain the supply situation if firm capacity, storage, imports, demand flexibility or reserves are not sufficient. But it is not a blackout. Between a tight electricity balance and a grid collapse lie market processes, dispatch planning, grid operation, reserves and protection concepts.

Technical conditions of a blackout

For a stable supply, the grid needs at every moment sufficient power, stable voltage, controllable equipment and manageable power flows. The amount of electricity generated or consumed annually in kilowatt hours says little about the blackout risk. What is decisive is something else: whether in a given second enough available capacity is present at the right grid points, and whether lines, transformers and protection systems can carry the power flows.

Power grids are operated according to security principles. These include the n-1 criterion: if an important element of equipment fails, such as a line or a transformer, the grid must not enter an impermissible state. Grid operators therefore work with reserves, redispatch, voltage control, reactive power management and continuous monitoring. EU law distinguishes several system states: normal state, alert state, emergency state, blackout state and restoration state. If several disturbances occur simultaneously or spread faster than countermeasures take effect, a cascade can arise. Protection systems then disconnect lines, power plants or parts of the grid in order to protect the installations. Each of these reactions makes sense in itself. In an unfavourable situation, however, they can trigger further overloads.

Frequency is only one visible variable. Equally important are voltage control, short-circuit power, inertia (instantaneous reserve), controllability and functioning communication. More and more installations are connected to the grid via power electronics, such as photovoltaics, wind power and battery storage with inverters. This changes certain stability properties of the system. That does not automatically mean a higher risk of blackout. However, ancillary services, grid connection rules, the behaviour of the inverters and operational management must be adapted to the new generation structure.

Example: Iberian Peninsula on 28 April 2025

On 28 April 2025, the power supply on the Spanish and Portuguese mainland collapsed completely. The European Network of Transmission System Operators for Electricity, ENTSO-E, classifies the event as the most serious disturbance in the European power system in more than two decades. An expert panel of transmission system operators and regulatory authorities investigated the incident. It published a factual report in October 2025 and a final report on 20 March 2026.

According to the final report, the blackout did not have a single cause. Several factors acted together: oscillations in the grid, insufficient voltage control and reactive power provision, and the disconnection behaviour of generation installations. The voltage rose rapidly. Generation installations disconnected from the grid in a cascade, and within seconds the grids of both countries collapsed, even though the system defence plans had been triggered. Restoration began immediately. In Portugal it took about 12 hours, in Spain about 16 hours. The panel made 22 recommendations, including on active voltage control, the coordination of protection settings and grid restoration. The example shows that a blackout can also occur without a shortage of generation capacity if voltage control and protection behaviour do not fit together.

Why the term is politically and economically relevant

Blackout risks concern security of supply, but are not the same thing. Security of supply means reliably supplying consumers with electricity over seconds, hours, seasons and investment cycles. A blackout is an extreme event in this field. Anyone who discusses security of supply only in terms of blackouts overlooks many everyday questions. These include sufficient grid capacity, maintenance, reserve planning, grid restoration, IT security, skilled personnel, spare parts, permits and clear responsibilities.

Economically, the term is important because power outages cause high consequential costs. Production stops, communication networks fail, traffic is disrupted, and cooling and water supply can be impaired. These damages are not fully reflected in electricity prices. Security standards are therefore not organised solely through short-term market prices. Transmission system operators, distribution system operators, regulatory authorities, power plant and storage operators, balance responsible parties and public bodies each have their own tasks. EU Regulation 2017/2196, for example, governs the obligations of these parties in the emergency and blackout state.

Electricity markets provide price signals for generation and consumption, but do not replace grid operation. A high exchange price can indicate scarcity. But it does not create grid stability if equipment is missing or certain power flows are physically impossible. Conversely, a low market price does not mean that all technical requirements are met. Even when the market is balanced in accounting terms, grid operators must manage frequency, voltage and bottlenecks.

Typical misinterpretations

A common oversimplification is that a blackout arises simply from too little electricity. This formulation mixes up energy quantity, capacity, grid capacity and operational stability. A country can generate enough electricity over the year and still have problems with capacity or the grid in individual hours. In a given hour there can be enough generation in accounting terms and yet local bottlenecks. And enough power plant capacity can exist that is nevertheless not available, is poorly controlled or cannot be deployed because of disrupted communication. The Iberian example also shows that excessively high voltage can trigger a collapse.

A second misinterpretation links blackouts directly with renewable energies. Wind and solar power change operations because their feed-in depends on the weather and many installations are connected to the grid via power electronics. This results in a need for flexibility, storage, grid expansion, reserves, forecasts and adapted technical rules. A high share of renewable energies does not, however, necessarily lead to blackouts. What is decisive is whether the power system provides the necessary functions: frequency support, voltage control, congestion management, firm capacity, restoration concepts and controllable responses to disturbances. This also includes inverters actively contributing to voltage control instead of working only with fixed settings.

A third oversimplification treats blackout prevention as purely a question of additional power plants. Firm capacity is important but does not explain all risks. Distribution grids must be able to absorb rising loads from heat pumps, electromobility and electric process heat. Digital control can help but creates new dependencies on communication, software and data security. Storage can provide capacity, support the frequency and smooth load peaks. But it does not automatically replace grid capacity or seasonal reserve. Flexibility only works if connection conditions, metering, tariffs and controllability fit together.

Black start capability and restoration

After a large-scale blackout, supply cannot simply be switched back on. Many power plants themselves need electricity to start their auxiliary systems. Installations with black start capability can start up without external power and are the starting points for grid restoration. From them, grid islands are formed and energised. Then further generators are connected, loads are gradually supplied again and the sub-grids are synchronised with one another. Alternatively, a grid area can be re-energised from outside via connections to neighbouring grids. EU Regulation 2017/2196 obliges every transmission system operator to have a system defence plan and a restoration plan. It also governs coordination with distribution system operators, significant grid users and providers of restoration services, as well as tests and the exchange of information in an emergency.

Black start capability is therefore a value in its own right in the power system. It concerns not only power plants but also grid control, communication, protection systems, fuel supply, available personnel and priority for critical infrastructure. A system with many decentralised installations needs different concepts for this than one consisting mainly of large central power plants. Decentralised generation can help with restoration if it can operate in a grid-forming manner and is integrated into operational management. Without suitable rules, however, it can switch off automatically when the grid situation is disrupted or feed into the grid in an uncontrolled way when reconnecting.

Key takeaway

A blackout is not a general word for unease about the power system. It denotes a large-scale failure of the power supply because the grid can no longer be operated stably. In EU law it is defined as the blackout state of the transmission system: more than 50 percent load loss in a control area or at least three minutes without voltage. The term only becomes precise when one distinguishes between energy quantity, instantaneous capacity, grid operation, protection mechanisms, market rules and restoration. Blackout prevention therefore consists of more than reserve capacity. It includes stable grids, active voltage control, firm capacity, flexible loads, resilient communication, clear responsibilities and technical capabilities such as balancing energy and black start.

Sources

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Edited by eCMS on 11.11.2026 16:03

© 2026 Erik Schmidt. All rights reserved. Quotations with a link to the source are welcome.