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Glossary · Energy industry & energy transition

aFRR, automatic Frequency Restoration Reserve

aFRR automatic Frequency Restoration Reserve balancing reserve frequency maintenance balancing energy power system

aFRR: automatic frequency restoration reserve

aFRR stands for automatic Frequency Restoration Reserve: an automatically activated frequency restoration reserve, also called secondary control reserve in Germany. It helps return the frequency to its setpoint and offsets unplanned power exchanges between control areas. The transmission system operators are responsible for it.

In the Continental European synchronous area, the nominal frequency is 50 hertz. A power deficit initially causes the frequency to fall, while a surplus causes it to rise. Positive aFRR increases generation or reduces consumption; negative aFRR reduces generation or increases consumption.

The sign therefore describes the effect on the system balance, not the type of facility. A hypothetical industrial plant that reduces its power consumption from 12 to 10 MW provides 2 MW of positive control relative to its starting operating point. If it increases consumption from 12 to 14 MW, that is 2 MW of negative control. It does not need to feed any electricity back into the grid to do so.

How automatic control works

The load-frequency controller combines the frequency deviation with the deviation of the actual power exchange from the scheduled power exchange of a control area. This continuously produces changing activation setpoints, which are transmitted automatically to the providers. What matters is therefore not the measured frequency alone, but also the exchange balance.

For German aFRR, a first response within 30 seconds and full activation within five minutes are required. This does not mean that aFRR only begins after five minutes. When setpoints change, delivery is assessed against a permissible acceptance corridor.

The difference between the start of the response and full activation can be illustrated by a control action: the provider begins to change its output and then reaches the requested value. The five-minute figure is neither a prescribed waiting time nor the maximum duration of a deployment. Nor does it mean that every facility always needs five minutes.

The European platform PICASSO optimises cross-border aFRR deployment. Imbalance netting, including via IGCC, avoids opposing activations. Available transmission capacity limits the exchange; the providers continue to be activated via the responsible controllers.

Capacity, energy and remuneration

Reserved control capacity refers to the available change in power in megawatts (MW). Control energy actually activated is a quantity of energy in megawatt-hours (MWh). Reservation and activation are remunerated separately.

A simplified calculation example: if an additional 10 MW is delivered constantly for a quarter of an hour, this produces 10 MW × 0.25 hours = 2.5 MWh of control energy. For a variable activation, by contrast, the time profile is decisive. If the additional power rises evenly from zero to 10 MW within five minutes, this ramp produces about 0.417 MWh. Simply multiplying the peak power by the entire duration of deployment would be wrong.

In Germany, under the market model published on 11 November 2026, aFRR capacity products are procured daily in four-hour blocks and separately for the positive and negative directions. Control energy bids refer to quarter-hours. The minimum bid size is 1 MW. A capacity award obliges the provider to submit corresponding energy bids; additional energy bids are also possible without a capacity award. Reservation is remunerated at the awarded bid price, and activated energy under the marginal pricing method.

The following follows for assessing an offer: a high energy price alone says little about annual revenue. Without activation, no corresponding energy revenue arises. Conversely, binding reservation can be economically significant even if activations are rare. A revenue calculation must also take into account availability, forgone alternative marketing opportunities, energy procurement, losses and, where applicable, wear. A single market price is therefore not a reliable investment calculation.

Distinction from FCR, mFRR and redispatch

FCR stabilises the frequency through a decentralised response to frequency deviations. aFRR is requested automatically via central controllers. mFRR can take over longer-lasting deviations and release aFRR again. The processes overlap; they are not stages switched on strictly one after another.

The term manual in mFRR must not be misunderstood as describing mandatory telephone operation of every facility. Its technical communication and implementation can also be automated. The essential contrast with aFRR is that it is requested through a separate activation call instead of continuous automatic load-frequency control.

Redispatch serves a different purpose: it changes the spatial distribution of generation in order to relieve overloaded lines. For example, less is generated on one side of a bottleneck and more on the other. aFRR, by contrast, corrects a deviation in the power balance.

This can be seen in two different questions: is the system under consideration currently short of 100 MW, or should existing 100 MW be fed in at a different location? The first question concerns balancing the power balance, the second the transport task. An overall balanced power balance does not rule out an overloaded line. Conversely, a free line does not eliminate a power deficit. The processes are therefore neither interchangeable nor to be lumped together arbitrarily under the term grid reserve.

Which facilities can participate

Flexibility alone does not yet entitle a facility to participate. In Germany, providers and the units or groups included must meet the prequalification requirements. Among other things, control behaviour, metering, data transmission and reliable setpoint implementation must be demonstrated. The product-specific conditions are decisive; the version reviewed here is dated 7 October 2026.

Generation facilities, consumption units and aggregated groups can be included. For battery storage systems and other facilities with limited energy capacity, usable energy content and storage management are additionally decisive. A fast change in power alone is not sufficient to demonstrate secure delivery.

A hypothetical storage system with 5 MW of power and 5 MWh of usable energy could, arithmetically, discharge for one hour at a constant 5 MW. This simple division is, however, not yet proof of permissible aFRR marketing: the initial state of charge, control direction and further obligations are not included. The idea that a short activation time requires only a correspondingly small energy content also confuses speed with endurance.

Significance and limits

Control reserve offsets the remaining balance of the system, not every forecast error individually. This must be distinguished from the settlement of deviations of individual balancing groups via balancing energy.

For the energy transition, what matters above all is therefore the quality of available control options: how reliably can they respond to changing requirements, and for how long can they meet them? Neither a fixed aFRR volume nor an inevitable cost increase can be derived from the expansion of wind and solar power alone. Likewise, it would be wrong to treat frequency maintenance as a problem that only arose through renewable energy.

aFRR is not a long-term safeguard against missing generation capacity and not an energy reserve for a multi-day period of low wind and solar output (Dunkelflaute). It corrects short-term deviations during ongoing operation. High prices of individual activations therefore prove neither a general electricity shortage in themselves nor the total costs of the energy transition. Only the product, period, activated quantity and the respective technical and economic conditions are meaningful.

The core of the term remains the automatic restoration of the power balance: available capacity is to be distinguished from delivered energy, balancing from congestion management, and short-term control from long-term security of supply.

Sources

Related glossary entries

Edited by eCMS on 11.11.2026 13:50

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