mFRR — where Estonian battery profits come from
Author
Volton Editorial Team
Date Published
mFRR (manual Frequency Restoration Reserve) is the third-tier balancing product on the European grid — manually activated by a TSO operator, with full delivery required within 12.5 minutes. It is the slowest of the three frequency reserves but the most flexible to bid into, and currently the largest single revenue line for grid-scale batteries in Estonia. The Baltic TSOs connected to the EU MARI mFRR platform on 10 October 2024.
At 17:42 on a Tuesday, an operator in the Elering control room watches a 200 MW deficit build for the 18:00 hour. FCR has caught the frequency dip. aFRR is holding the line, but it has burned through most of its band and needs to be released before the next imbalance arrives. The operator opens the merit-order list, picks the cheapest combination of standing mFRR offers that covers the gap, and clicks dispatch. Twelve and a half minutes later, those assets are at full output and the automatic reserves are free again.
That is mFRR in one paragraph. It is the slowest of the three frequency reserves, the only one a human activates by hand, and the tool TSOs use to restore the system to its normal operating state after the faster reserves have absorbed the initial shock.
The mechanics are unglamorous and important. Providers submit offers into a merit-order list, sorted by price. When the TSO needs energy, the operator dispatches from the top of the list down until the imbalance is covered. Full delivery is required within 12.5 minutes of the activation order in most European markets, with 15 minutes used in a few. Procurement runs through daily or intraday auctions; some markets pay separately for capacity (the right to be called) and activation (the energy actually delivered), and others run a voluntary, activation-only model. The European cross-border platform for all of this is MARI, the Manually Activated Reserves Initiative, which lets TSOs dispatch the cheapest mFRR offer in the connected area regardless of which country it physically sits in, subject to available transmission capacity. Baltic TSOs joined MARI on 10 October 2024, and Estonian mFRR has cleared through it since.
The asset profile is permissive. Anything that can ramp up or down inside 12.5 minutes and hold the output for the duration of the activation qualifies: batteries, hydro, gas turbines, demand response, aggregated portfolios of small loads. The 12.5-minute window is what separates mFRR from the truly fast reserves. By the time mFRR is moving, FCR has already been reacting for 30 seconds and aFRR for several minutes.
The opinion: mFRR is where you can hear the human in the loop. FCR and aFRR are pure control engineering, droops and PI controllers reacting to a signal. mFRR is a person looking at a list and making a decision. That makes it slower, but it also makes it the place where price formation is most visible and most volatile.
That volatility is the whole story for storage in the Baltics right now. Activation prices on Estonian mFRR have been structurally elevated since 2024, and the spread between mFRR activation and the day-ahead spot is currently the core revenue source for most battery business cases in the region. It is more lucrative than spot arbitrage by a wide margin. The economics are fragile, they depend on those elevated prices continuing, but as of 2026 the slowest of the three reserves is doing more for storage profitability than any other market.
Volton is a certified BSP at Elering and trades mFRR daily for the assets in our portfolio. The merit-order list is one of the most underrated mechanisms in European power markets, and at the moment, it is also the one paying for batteries in Estonia.
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