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Glossary · Energy industry & energy transition

Adequacy Assessment

Adequacy assessment security of supply resource adequacy power system Capacity load generation risk analysis

An adequacy assessment systematically examines whether an electricity system has sufficient usable resources to meet expected electricity demand. It quantifies remaining shortfall risks and enables comparison with a defined reliability standard. What matters is not installed power plant capacity alone, but the interaction of generation, storage, flexible demand and cross-border exchange. The underlying concept of resource adequacy describes the adequacy of these resources; the assessment evaluates it.

In German, terms used for this include Ressourcenangemessenheitsbewertung and Kapazitätsbewertung. A security of supply assessment, by contrast, can have a broader scope: security of supply encompasses not only sufficient resources but also their secure delivery through the grid and the management of disruptions. An adequacy assessment is therefore neither a comprehensive security certificate nor a prediction of a specific power outage.

Why rated capacity alone is not enough

A gas-fired power plant with a rated capacity of 500 megawatts, a battery with a discharge capacity of 500 megawatts and a wind farm with an installed capacity of 500 megawatts make different contributions during a cold evening hour with little wind. The power plant may break down or be unavailable for lack of fuel. The wind farm's output depends on weather conditions. The battery can supply electricity only while enough stored energy remains. Equal capacity figures therefore do not provide equal assurance that demand will be met.

Power is expressed, for example, in megawatts and energy in megawatt-hours. A simplified calculation illustrates the difference: if a fully charged battery can deliver 1,000 megawatt-hours of usable energy and discharge at 500 megawatts, its energy will last for two hours at full discharge capacity. It must then be recharged. The calculation assumes constant power and an energy quantity already stated as usable. It shows why a shortfall lasting several hours must be assessed differently from a brief demand peak.

Firm capacity is not an immutable property of a technology either. Its contribution depends, among other things, on when the system experiences scarcity and what other resources are available. An adequacy assessment therefore considers not only individual contributions to meeting peak demand, but also developments over time, shared weather dependencies, outages and interactions across the entire resource portfolio.

Risk analysis rather than a simple capacity balance

A balance of power plant capacity and annual peak demand offers an initial indication, but captures the risk of a shortfall only incompletely. Combinations of factors are critical: high demand, low wind and solar generation, unplanned power plant outages, low storage levels and limited exchange opportunities. Modern assessments therefore use probabilistic methods. Monte Carlo simulations combine different weather or climate scenarios with random outage patterns and calculate numerous possible system states.

It is important to consider time periods in a connected sequence. Storage cannot be assumed to be full again in every critical hour. Electricity demand shifted to another time may need to be met later. Weather data for demand and renewable generation must also be consistent: a cold spell can simultaneously increase electricity demand for heating and affect several countries. If such relationships are disregarded, the support available may appear greater than it would actually be.

Interpreting LOLE and EENS correctly

LOLE, Loss of Load Expectation, denotes the expected duration of situations in which the modelled resources are insufficient to meet demand in full. In an annual assessment, it is usually expressed in hours per year. EENS, Expected Energy Not Served, by contrast, describes the expected amount of energy not supplied, for example in megawatt-hours per year. The two indicators complement each other: LOLE captures duration, while EENS additionally captures the extent of the shortfall.

A LOLE of three hours per year does not mean that electricity is lost for exactly three hours every year. It is an expected value across the possibilities considered. As a purely mathematical example, 90 equally weighted annual outcomes without a shortfall and ten annual outcomes with 30 shortfall hours each together produce this average. The example makes no claim about the actual distribution in a particular electricity system.

A modelled shortfall is also not synonymous with an uncontrolled blackout. Extraordinary measures can alter how it unfolds. The model boundary remains decisive, however: reserves, contractual load reductions or price responses already included must not subsequently be counted a second time as additional safeguards. A robust presentation of results must therefore explain which market resources and which measures outside the market are already included.

Weather dependence, storage and flexible demand

As wind and solar generation increase, residual load becomes more important: demand minus the weather-dependent generation considered. The most critical situation need not coincide with the highest total demand. A winter evening hour can be more challenging than a sunny midday peak, despite lower demand. Besides the size of such a gap, its duration matters: short-term power peaks and weather conditions with low energy availability lasting several days place different demands on the resource portfolio.

Electrification also changes demand profiles. Heat pumps require electricity particularly in cold weather; electric vehicles can charge at the same time or shift their charging to another time. Industrial processes each have their own scope for adjustment. Flexibility must therefore not be treated as an unlimited reserve. Activation duration, lead time, maximum frequency of activation, subsequent catch-up effects and actual controllability must be consistent with the assumed use. There is a significant difference between theoretically shiftable demand and reliably available load reduction.

A similar limitation applies to imports. Technical transmission capacity alone does not indicate how much electricity a neighbouring country can supply in a scarcity situation. Sufficient resources must be available there and exchange capacity must be usable. Joint regional modelling takes account of the possibility that several countries may experience scarcity simultaneously. Adequacy does not mean self-sufficiency: reliably available imports can help meet demand without every country having to maintain all resources itself.

Distinction from grid security

Grid security concerns, among other things, line loadings, voltages and the management of equipment outages. An adequacy assessment does not replace such studies. It would also be wrong, however, to exclude grid constraints from adequacy assessments as a matter of principle: limited transmission capacity can prevent available resources from reaching demand. European assessments therefore take exchange and grid constraints into account at their respective spatial resolution. Detailed local congestion analyses and dynamic stability assessments remain additional tasks.

A power plant behind a grid bottleneck can therefore contribute to the resource balance across a wider area while providing only limited help to a particular grid region. Conversely, technically manageable grid operation does not prove that sufficient generation and stored energy will be available in the long term. Both perspectives must be brought together. Accordingly, Germany's September 2025 security of supply monitoring examines both market-side resource issues and the task of transporting electricity through the grid.

Institutional and legal significance

In the European Union, Regulation (EU) 2019/943 governs the European resource adequacy assessment and complementary national assessments. ENTSO-E produces the European Resource Adequacy Assessment, or ERAA; ACER reviews and approves it or makes amendments. The statutory assessment horizon covers ten years. The assessment examines the electricity system with its different resource classes, not the legal permissibility or operational safety of an individual facility. The ERAA 2025 edition was amended and approved by ACER on 24 April 2026.

For EU capacity mechanisms, an identified adequacy concern is an important legal prerequisite, but does not result in automatic approval. Further requirements concern, among other things, market reforms, effects on neighbouring countries, consideration of a strategic reserve and state aid law. A reliability standard is required when a capacity mechanism is applied. The assessment of this standard links security of supply with the cost of additional safeguards; a statistical zero risk is not the general benchmark. The legal reference is the consolidated version of the regulation dated 16 July 2024.

What the results can and cannot show

Long-term assessments must consider economic developments as well as technical possibilities. A technically operational facility will not necessarily remain in the market; an announced project will not necessarily be built. Assumptions about revenues, investments and closures therefore influence the result. In its assessment of ERAA 2025, ACER explicitly identifies limitations in economic modelling and the need for further improvements concerning storage and demand flexibility. Results from different studies can be meaningfully compared only when their assumptions are comparable.

Readers of an adequacy assessment should therefore ask about the assessment year, scenario, weather data basis, import assumptions, storage operation and reserves included. Equally important is whether fuel risks and rare simultaneous outages were examined at all. High installed capacity does not prove that safeguards are sufficient; a positive risk value does not prove that a blackout is imminent. The strength of the method lies in making potential gaps visible early. Its limitation arises when model results conditional on assumptions are presented as certainties.

Sources

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Edited by eCMS on 11.11.2026 06:48

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