Balancing Services Explained: FCR, aFRR, and mFRR at a Glance

Why Balancing Services Are Needed

Electricity systems must maintain a continuous balance between power generation and consumption. Even small deviations can destabilise the system and cause the grid frequency to deviate from its target level of 50 Hz.

Frequency deviations can occur for many reasons: forecast errors in renewable generation, sudden changes in demand, unexpected outages, or deviations between planned and actual electricity schedules.

To address this, TSOs procure a range of ancillary services - from frequency response and voltage regulation to inertia and black-start capability.

This article explains the three frequency-based reserve products in Germany: FCR, aFRR, and mFRR. They build on each other - from the fastest, fully automated response to slower, manual activation.

Frequency Containment Reserve (FCR)

FCR is the fastest type of balancing service. It automatically reacts to changes in grid frequency and must be fully activated within 30 seconds. Participating assets continuously measure the system frequency and adjust their power output automatically. FCR must be provided symmetrically - both upward and downward regulation. The minimum bid size is 1 MW.

Automatic Frequency Restoration Reserve (aFRR)

Once the frequency has been stabilised by FCR, aFRR restores the system to its scheduled state. Unlike FCR, aFRR is activated centrally by the TSO's control system. Providers receive a control signal instructing them to increase or decrease power output and must be fully activated within 5 minutes. aFRR is procured separately for upward and downward regulation, so providers can bid asymmetrically. The minimum bid size is 1 MW. Capacity is currently procured in 4-hour blocks; under the TSOs' FlexRL project, optional 15-minute capacity products will be introduced alongside the existing 4-hour blocks from November 2026. This opens the door to more dynamic participation by flexible assets like BESS.

Manual Frequency Restoration Reserve (mFRR)

The third layer is mFRR, activated manually by system operators when imbalances last longer. Full activation takes up to 12.5 minutes. Like aFRR, it can be bid asymmetrically with a minimum bid size of 1 MW, and the optional 15-minute capacity products under FlexRL will apply here as well. mFRR is typically covered by assets that can sustain output for longer periods, which is why it plays only a minor role for battery storage today.

Balancing Services in a Changing Energy System

Historically, balancing reserves were primarily provided by large conventional power plants. Today, a much wider range of technologies can participate, including battery storage systems, demand response, and aggregated distributed energy resources.

At the same time, the increasing number of distributed assets introduces new operational complexity. Coordinating thousands of flexible resources requires reliable communication, real-time data flows, and advanced control systems. Balancing services are therefore becoming more than a technical system service — they are a central mechanism for integrating renewable energy and ensuring the stability of modern electricity systems.

Sources: Regelleistung.net, Bundesnetzagentur, FfE, ENTSO-E