FCR, aFRR, and mFRR: which reserve service fits your flexibility?
FCR, aFRR, and mFRR all help Elia keep the electricity grid in balance, but they differ in response speed, control, and activation duration. Research Manager Stefaan Soenen explains how they work, which assets suit them, and where the economic opportunities lie.
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The short version
- FCR responds fastest and continuously helps steer the grid frequency.
- aFRR is activated automatically by Elia via a setpoint and requires tight follow-up.
- mFRR is slower, requires larger volumes, and often runs longer.
- For batteries, FCR or aFRR is usually more logical than mFRR.
- Small assets can often still participate in the market through aggregation.
What exactly do reserve services do?
Reserve services keep the grid in balance when production and consumption start to drift apart. “Elia uses several layers of flexibility for this, each with its own role in time and volume,” Stefaan explains. “In essence, FCR, aFRR, and mFRR are three different responses to the same problem: the electricity grid must stay in balance at every moment. So the question is not only what an asset can do, but also how fast, how long, and how accurately it responds.”
What is FCR, and why is it the fastest reserve?
FCR, or Frequency Containment Reserve, is the first line of defense. This reserve responds directly to deviations in the grid frequency and must therefore be available extremely fast.
“In concrete terms, this means an asset continuously measures whether the frequency deviates from 50 hertz and responds to that automatically,” Stefaan says. “In Europe, FCR carries very fast response requirements: an initial response within 15 seconds and full delivery within 30 seconds for larger frequency deviations. That’s especially useful for technologies that can steer quickly, accurately, and repeatably, such as batteries. Speed is the core of the product here.”
What is aFRR, and why does it demand more operational discipline?
aFRR, or automatic Frequency Restoration Reserve, comes after FCR. Elia activates this reserve automatically by sending a reference value, or setpoint, so that the frequency recovers and the Belgian control area returns to balance.
“With aFRR, an asset does not respond to a local frequency measurement, but to an external control signal from Elia, a new setpoint every 4 seconds, that must be followed correctly. That is precisely what sets it apart from FCR, which does respond autonomously to the local frequency,” Stefaan says. “That makes this product operationally heavier than FCR. Performance monitoring and possible penalties come into play when the requested activation is not delivered correctly. That matters for energy partners and managers. aFRR is attractive for assets with sufficient controllability and communication, but only when the entire chain is sound: reliable measurements, correct control, a good baseline, and tight follow-up.”
What is mFRR, and why does it more often suit larger installations?
mFRR, or manual Frequency Restoration Reserve, is the manual variant within the restoration reserves. This reserve is deployed for larger volumes and longer activations, which often makes it better suited to larger production units or industrial installations.
“With mFRR, the emphasis is less on split-second work and more on endurance,” Stefaan explains. “Activation is slower than with FCR and aFRR. In the current Elia documentation, a full activation time of 12.5 minutes applies. So it’s not always the best match for batteries. Not because of technical limitations, but because longer activations and greater energy demand sometimes yield less return than FCR or aFRR.”
A battery, a charging hub, or an industrial installation each have different strengths. The challenge lies in finding the reserve service in which each asset’s flexibility gains the most value.
What are you paid for: capacity and energy?
For all three services, you receive a capacity payment—you’re paid to be available. The difference lies in the energy you actually deliver.
- FCR: a capacity payment only. For the steering itself, the energy you deliver, you receive no separate payment.
- aFRR and mFRR: on top of the capacity payment, there is an energy bid for which you are indeed paid for the energy delivered.
Those who fail to follow the requested activations correctly also risk penalties.
When do the bids open and close?
Capacity is auctioned daily for the next delivery day (D-1). The FCR auction closes at 08:00; the capacity auctions for aFRR and mFRR close over the course of the morning. Contracted energy bids for aFRR and mFRR are submitted until 15:00 on D-1. On the energy market itself, the bidding gate closes 25 minutes before the delivery quarter-hour each time (T-25). FCR has no separate energy auction.
Exact opening and result times vary slightly by service; consult Elia’s current auction calendar.
How do the differences between FCR, aFRR, and mFRR show up in practice?
So not every flexible asset is automatically suited to every reserve service. The differences between FCR, aFRR, and mFRR only become truly clear when we look at practice. The overview below sets you on the path to the right match.
Reserve service | Activation | Role | Suitable assets |
FCR (Frequency Containment Reserve) | Automatic, based on frequency | Quickly absorb frequency deviations | Batteries and other fast flex assets |
aFRR (automatic Frequency Restoration Reserve) | Automatically via an Elia setpoint | Restore frequency and balance the control zone | Assets with strong control and communication |
mFRR (manual Frequency Restoration Reserve) | manual | Larger volumes and longer activation | Larger installations and production units |
Conclusion: the choice of a reserve service therefore doesn’t begin with the product’s name, but with the asset’s characteristics. A battery that excels in speed, for example, has different strengths than an industrial installation that can steer larger volumes for longer.
In practice, five questions play a role here:
- Response speed: how fast can the asset really ramp up and down?
- Duration: how long can that response be sustained technically and economically?
- Measurement and control quality: is the data reliable, and can the asset be controlled precisely enough?
- Minimum scale: : does the asset reach the required threshold on its own, or is aggregation needed?
- Operational discipline: can the organization deliver what the market and the contract require?
Can smaller assets connect too?
Reserve services used to be mainly the playing field of large installations. But the market has since broadened. Batteries, digitalization, and aggregation make it possible to bundle the flexibility of several installations. “Elia applies a minimum threshold of 1 MW for access to balancing services,” Stefaan explains. “But aggregation makes it possible to offer several smaller assets together as one flexible whole. For companies, this also changes the conversation with installers and energy partners. It’s no longer only about the type of hardware, but also about which flexibility is available and to which market it can be coupled.”
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