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Glossary · Energy industry & energy transition

Capacity Credit

Capacity credit Firm capacity security of supply resource adequacy renewable energies

Capacity Credit refers to the contribution of a power plant, a technology or a demand-side measure to the firm capacity of an electricity system. It means the capacity that can be counted on with a certain degree of reliability in situations of tight supply. The term therefore does not answer the question of how much electricity a plant generates per year. It shows how strongly the plant improves the system's ability to cope with demand peaks and outages without interrupting supply. In English, the terms capacity value and equivalent firm capacity are also used. In capacity markets, one speaks of accredited capacity or of derating.

Capacity Credit is usually expressed in megawatts or as a percentage of installed capacity. An example: a wind farm with a rated capacity of 100 megawatts has a Capacity Credit of 10 megawatts or 10 percent if, statistically, only this contribution can be reliably counted on in the hours that are critical for security of supply. This figure is not a fixed property of the technology. It depends on the electricity system, the location, the weather profile, the load structure and the power plant fleet. Storage, grid restrictions, import possibilities and the chosen reliability metric also play a role.

Distinction from capacity utilisation, availability and firm capacity

Capacity Credit is often confused with the capacity factor. The capacity factor indicates how much energy a plant delivers over a period relative to its maximum possible generation. It therefore describes a quantity of energy over time. A solar farm can have a decent annual capacity factor and still contribute hardly anything to covering load during a winter evening peak. Conversely, a rarely used peaking power plant can have a low capacity factor but a high Capacity Credit if it is reliably available in a scarcity event. US studies explicitly point out that the two metrics are often confused.

Technical availability is also something different. A plant can be technically ready for operation and still deliver no electricity because primary energy is lacking: a run-of-river hydropower plant has too little water, a wind turbine faces calm conditions. For thermal power plants, availability depends mainly on outage probabilities, maintenance and fuel supply. For Capacity Credit, what counts is availability in the critical hours of the overall system. If a plant fails or cannot deliver precisely when the system is tight, it contributes less to security of supply than a plant of equal rated capacity whose availability coincides better with scarcity situations.

Closely related is firm capacity. It refers to the capacity that can be counted on in the system. Capacity Credit is the metric used to describe the contribution of individual resources to this firm capacity. In practice, the two terms are sometimes used interchangeably. It is more precise to distinguish between the system's need for firm capacity and the Capacity Credit of individual resources.

Why the critical moment matters

For power system operation, it is not enough to generate sufficient kilowatt-hours over the year. At every moment, generation, consumption, storage operation, imports, exports and grid operation must match. Security of supply does not fail on the annual average. It fails in individual hours or multi-day periods in which high load, low feed-in, power plant outages and limited import possibilities coincide. The decisive factor is therefore often the residual load, i.e. load minus weather-dependent feed-in.

Capacity Credit makes this time dimension visible. For photovoltaics, the contribution depends strongly on when the scarce hours occur. In a system with summer midday peaks, solar power can have a significantly higher Capacity Credit than in a system whose critical hours lie mainly on dark winter evenings. For wind energy, the assessment depends more on weather patterns and spatial correlation. Many wind turbines at similar locations do increase installed capacity, but during large-scale lulls they often deliver little at the same time. Moreover, as a technology's share grows, the critical hours shift: a lot of solar capacity moves scarcity into the evening. The additional Capacity Credit of further plants of the same kind therefore generally falls the more of them are already in the system. Assessment studies explicitly show this effect as a dependence on the penetration level.

For storage, Capacity Credit depends on two variables: power and energy content. A battery with high discharge power can cover a short evening peak well. If scarcity lasts several hours or days, storage capacity limits the contribution. In addition, the storage must be charged before the scarcity. Its contribution therefore also depends on market rules, forecasts, operating strategy and available surplus electricity. Storage does not generate additional energy but shifts it in time. Its Capacity Credit arises from this shift, not from additional annual energy. In capacity mechanisms, this contribution is therefore usually tiered by discharge duration: a storage unit that can deliver for 10 hours receives a higher accreditation factor than one with 2 hours.

Measurement and assessment logic

In technical analyses, Capacity Credit is usually determined probabilistically, i.e. using probability calculations. The basis is reliability metrics. The Loss of Load Expectation (LOLE) is the expected number of hours per year, in North America often also days per year, in which load cannot be fully met under the assumptions made. A planning benchmark widespread in the USA is one day in ten years. The Expected Energy Not Served (EENS) is the expected amount of energy that cannot be delivered. A resource's Capacity Credit results from how strongly it improves these metrics.

The most widespread is the Effective Load Carrying Capability (ELCC). It indicates by how much load may increase when a resource is added without the defined reliability level deteriorating. The procedure runs in several steps: first, the system without the resource is brought to a target level, for example a certain LOLE value. Then the resource is added. Afterwards, load is added uniformly until the original reliability level is reached again. The added load corresponds to the ELCC. The concept goes back to a 1966 paper by L. L. Garver and is today regarded in North America as the industry standard for assessing wind and solar capacity. Related methods such as equivalent firm capacity compare the resource with a perfectly available reference plant and arrive at similar conclusions.

The difference between average and marginal Capacity Credit is important. The average value distributes the contribution of an entire technology class across all plants of that class. The marginal value describes what the next additional plant contributes. With high build-out, the marginal value can lie significantly below the average. Which variant a set of rules uses considerably influences investment incentives.

Such calculations are model-dependent. They require assumptions about weather years, power plant outages, load development, grids, imports, storage fill levels and demand response. This does not make the term arbitrary, but the figures must always be read together with the method. A single percentage value without specification of system boundary, time period, reliability standard and assessment method (average or marginal) says little.

Typical misinterpretations

  • "Wind and solar contribute nothing to firm capacity." In most systems this is not true. Wind and solar energy reduce the load that other resources must cover in many hours, and they are also available with a certain probability in scarcity situations. However, their Capacity Credit is usually significantly below installed capacity and declines with increasing build-out.

  • "Installed capacity equals firm capacity." It does not follow from 100 gigawatts of photovoltaics that 100 gigawatts are available in every critical hour. For resource adequacy planning, this equation would be unsuitable. It overestimates the contribution of weather-dependent capacity and underestimates the need for flexibility, storage, dispatchable generation, grids or load shifting.

  • "Conventional power plants count at full rated capacity." Thermal plants also have unplanned outages, maintenance periods, fuel risks, cooling water restrictions and regulatory limits. Several plants may moreover be affected simultaneously for similar reasons, for example through heat, low water levels or bottlenecks in gas supply. Therefore, even dispatchable power plants receive accreditation factors below 100 percent in capacity markets.

Significance for the market, planning and regulation

The term is important for electricity markets because revenues from energy sales are not always sufficient to finance reliable capacity. In a pure energy-only market, plants are paid mainly for generated kilowatt-hours. A power plant, a storage unit or a load curtailment that is rarely needed can nevertheless have a high value for security of supply. Capacity mechanisms, strategic reserves or tenders for firm capacity are intended to reflect this value institutionally. For this, it must be defined which Capacity Credit is granted to which resources.

In the EU, Electricity Market Regulation (EU) 2019/943 sets the framework. It defines a capacity mechanism as a measure that remunerates resources for their availability in order to achieve the necessary level of resource adequacy. Member States applying such a mechanism must set a reliability standard that transparently states the targeted level of security of supply. It is based, among other things, on the value of lost load, i.e. the willingness to pay for maintaining electricity supply. The regulation does not set in percentages how strongly an individual plant is counted. That is regulated by national mechanisms through their own derating factors.

An example from North America is the grid operator PJM. It determines an ELCC class rating for technology classes. The accredited capacity of a plant results, put simply, from its installed capacity multiplied by this class factor and a plant-specific performance factor. In Germany, a capacity mechanism with centrally defined derating factors was being prepared in 2026. According to information from the associations' consultation, a ministerial draft bill from spring 2026 provided a factor of 58 percent for storage with a 10-hour discharge duration. Associations criticised this value as low compared with other countries. The final values and the status of entry into force must be checked in the version currently applicable.

This accreditation is not a technical side issue. It determines investment incentives, cost allocation and the question of which resources are considered equivalent. If storage, load management, flexible electrolysers or industrial curtailment agreements make a real contribution, the rules must capture their Capacity Credit in a comprehensible way. If they are valued too optimistically, a false sense of security arises. If they are valued too cautiously, an unnecessarily large amount of expensive reserves is procured.

With electrification, the term becomes even more important. Heat pumps, electromobility and electric industrial processes increase electricity demand and at the same time change load profiles and flexibility potentials. A heat pump not only increases annual electricity consumption but, if operated inflexibly, can intensify winter peak load. An electric car can create additional load or relieve the grid through controlled charging. Capacity Credit links such developments to the question of what capacity is really needed and available at the critical moment.

Capacity Credit is thus a metric for the contribution to security of supply under specific system conditions. It replaces neither energy generation nor grid planning or market design. But it prevents megawatts of different quality from being treated equally. Anyone who wants to assess the performance of an electricity system must distinguish four things: installed capacity, generated energy, technical availability and the contribution in the hours in which scarcity can actually occur.

Sources

Related glossary entries

Edited by eCMS on 11.11.2026 16:47

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