Battery fleets respond to frequency quickly. Their response time is measured in milliseconds rather than seconds, allowing them to meet the technical requirements of frequency containment reserve and frequency restoration reserve. For fleet operators, the key question is whether frequency response revenue warrants reserving capacity that could otherwise pursue price arbitrage.
There is no universal answer. The result depends on the relevant country's ancillary service market, direct-participation capacity thresholds, and the fleet's expected revenue profile. This article explains FCR and FRR mechanics, their direct conflict with arbitrage, and a framework for assessing the tradeoff.
What FCR and FRR actually require from a battery
Frequency containment reserve is the grid's first response when frequency moves away from the nominal 50 Hz. Under the ENTSO-E framework, FCR must deploy fully within 30 seconds after a deviation exceeds the activation threshold. An FCR battery must remain available with enough SoC in both directions and respond symmetrically: absorb power, or charge, above 50 Hz, and inject power, or discharge, below 50 Hz.
To remain symmetric, the battery keeps SoC near a midpoint, typically 40 to 60% of usable capacity. At 90% SoC, it lacks enough upward headroom to absorb the full FCR response. At 10% SoC, it cannot deliver the full downward response. FCR therefore requires a floating mid-SoC throughout the commitment period.
Frequency restoration reserve is the secondary layer, activated once FCR has stabilized frequency and the system must return it to nominal while replenishing FCR reserves. FRR generally activates more slowly, typically within 5 to 15 minutes, depending on the market and product. It is a larger-volume product than FCR and uses a different auction structure: FRR is typically procured daily, while FCR is procured through weekly or monthly tenders in most European markets.
The capacity commitment problem
FCR and FRR both require battery capacity commitments for defined periods. Throughout a committed FCR period, that capacity cannot be freely dispatched for intraday arbitrage. The FCR obligation takes precedence over the arbitrage plan for every committed unit of capacity.
That is the central conflict. An operator assigning 5 kW of a 10 kW battery to FCR for a six-hour window must keep it at mid-SoC and ready for frequency deviations throughout those six hours. Sending that capacity into a MIBEL intraday price spike during the same window breaches the contract.
FCR pays for availability, not delivered energy. The provider receives its capacity payment whether frequency deviations occur or the battery responds. This is attractive when FCR capacity prices are high and the concurrent arbitrage spread is low. The result changes when arbitrage spreads are high but FCR capacity prices are modest.
Minimum participation thresholds and the aggregation problem
Most European FCR markets set a minimum committed capacity for direct participation. Under the FCR cooperation framework across Central Europe, the typical minimum bid is 1 MW. Portugal's REN ancillary service market has its own thresholds. A distributed fleet with 15 kWh at each of twenty sites totals 300 kWh, or roughly 150 to 200 kW of discharge power capacity. That falls below the direct-participation threshold in most FCR markets.
A VPP or aggregator can combine assets to clear the threshold. However, aggregation brings the revenue-sharing and dispatch-control issues discussed in our earlier article on VPP models. An aggregator delivering FCR for a distributed battery pool decides which batteries commit and when, their SoC management approach, and how FCR capacity revenue is divided among participants.
Operators should compare the FCR capacity revenue share received through an aggregator, after its fee and the opportunity cost of unavailable arbitrage capacity, with the direct arbitrage revenue possible under full dispatch control of that same capacity.
Modeling the tradeoff: a per-capacity-block framing
A clear approach to the FCR-vs-arbitrage tradeoff is to express each option in EUR per kWh per year for the same physical battery capacity block. For a committed block, FCR contribution equals: (FCR capacity price in EUR per MW per hour) x (committed MW-hours per year) x (revenue share fraction if aggregated). The same block's arbitrage contribution reflects market spread, dispatch efficiency, and its annual cycle count.
Neither figure is fully predictable. European FCR capacity prices change with the scarcity of available response capacity, which has risen as renewable penetration grows and traditional synchronous generation is displaced. Arbitrage spreads vary with the relevant market's generation mix and demand patterns, which change seasonally and year to year.
One historical reference is useful: during high-renewable periods in MIBEL, midday negative or near-zero prices have widened evening arbitrage spreads while also creating more demand-side grid stabilization needs, supporting FCR capacity prices. The streams have not consistently moved in opposite directions, and both can be elevated at the same time.
What Pollen tracks for operators with frequency response potential
For operators whose fleets are nearing economically plausible aggregated frequency response participation, Pollen records per-site SoC trajectories and dispatch histories to establish an opportunity-cost baseline. Assessing whether aggregator-based FCR participation raises or lowers total revenue requires actual historical arbitrage capture per site, by hour, across a representative period.
We are not saying frequency response should always be pursued or avoided. The answer varies by site and market. The decision needs a clear view of the opportunity cost of committing capacity, and that figure can only come from real dispatch data, not theoretical spreads.
