ACER Decision No. 08/2026, adopted on 22 June 2026, approves the first amendment to the Regional Coordination Centre (RCC) Regional Sizing of Reserve Capacity Methodology. The update sets a framework for how reserve capacity, balancing resources, system flexibility and cross-border deliverability are assessed across the region. It is designed for electricity systems that are increasingly reliant on variable renewable generation, with operational security highlighted as a key driver of market value.
ACER’s most prominent signal is quantified at 99.99%. The agency approved the use of a one-year historical imbalance record for all system operation regions and set the South-East Europe System Operation Region (SEE SOR) reliability parameters at 99.99% for both positive and negative imbalances. Under the methodology, reserve-capacity calculations must be built to cover almost all historically observed imbalance events. ACER’s approach places the region among the more conservatively treated areas in Europe.
The decision also notes that the methodology has not yet been fully implemented and that limited operational experience supports the use of highly conservative parameters. ACER links this conservatism to circumstances that can make balancing more difficult, including transmission constraints, reserve geography and rapid system changes. This context is reflected in how reliability requirements are set for the SEE SOR.
Reliability parameters and links to cross-zonal flexibility
The regulatory update follows ACER’s separate 2026 monitoring report on cross-zonal capacity and flexibility. That report pointed to severe price spikes during summer 2024, when electricity prices in some evening hours approached €1,000/MWh. ACER attributed those events to heatwave-driven demand, solar generation decline after sunset, reduced hydropower availability, limited storage resources, constrained cross-border transmission capacity and shortages of flexible generation.
ACER’s monitoring findings describe flexibility as a component of system security rather than a secondary feature of market design. The decision therefore connects reserve sizing rules with broader expectations for balancing performance during periods when variable generation output changes quickly.
RCC coordination scope and reserve-sharing implementation timelines
The RCC structure referenced in the decision is centred on SEleNe CC, with participation from ESO EAD, IPTO and Terna. Coordination arrangements also involve Transelectrica on key regional interfaces. While ACER’s decision operates within this RCC framework, its practical effects extend beyond it across a wider trading and balancing corridor.
The corridor includes Hungary, Romania, Bulgaria, Greece, Croatia and Slovenia, as well as the Western Balkans and Ukraine, together with links to Central Europe. Within this context, one of the key amendments concerns reserve-sharing arrangements between transmission system operators. Under the new framework, short-term assessments of reserve-sharing availability must be implemented within 24 months of notification to the relevant RCC.
Participating TSOs are required to notify the RCC within one week of establishing reserve-sharing agreements. The change is intended to introduce a more structured and transparent approach to regional cooperation by moving reserve-sharing beyond theoretical calculations into operationally validated resources.
Granular geography for deliverable reserves
The decision also approves using more granular geographical delineation when determining minimum reserve requirements. This is intended to allow RCCs to reflect internal transmission constraints and cross-border bottlenecks in their assessments. It also supports accounting for different load-frequency control configurations and regional network limitations.
The outcome is a stronger focus on reserves that can be physically delivered rather than capacity that exists only in calculations. In practice, this ties reserve sizing more directly to congestion patterns and operational feasibility across connected networks.
Implications for flexibility assets and transparency requirements
The methodology update affects how renewable projects are evaluated for market participation and balancing exposure. For solar assets, ACER’s decision points to value drivers including forecast accuracy, imbalance exposure, curtailment risk, storage integration, firming arrangements and access to balancing markets. For wind projects, it highlights dependence on grid access, balancing responsibilities and the ability to contribute to reserve adequacy.
The decision strengthens the investment case for battery energy storage systems (BESS), pumped-storage hydropower, flexible hydro assets, demand-response programs and fast-start thermal generation. ACER states that system operation regions may rely on measures such as demand response, renewable curtailment, load shedding and balancing energy bids submitted through European balancing platforms when reserve requirements are exceeded.
A further element is transparency: RCCs must publish key information related to reserve-sharing agreements. This includes participating TSOs, reserve types, reserve volumes, affected load-frequency control blocks and assessment results. The publication requirement is positioned as a way to make reserve cooperation visible across different parts of the regional system.
Western Balkans connectivity and operational lessons from 21 June 2024
Although direct legal impact on Serbia, Montenegro, Bosnia and Herzegovina, Albania and North Macedonia is limited because the decision addresses ENTSO-E within the EU RCC framework, practical significance remains described as substantial. The Western Balkans are characterised as closely connected to EU electricity markets through cross-border trading and transmission interconnections alongside future market-coupling initiatives.
As integration deepens, non-EU TSOs may be assessed based on real-time system visibility, high-quality operational data, reserve coordination capabilities and congestion-management practices. The decision links these expectations to lessons from a regional grid disturbance on 21 June 2024, which affected Albania, Bosnia and Herzegovina, Montenegro and Croatia.
An ENTSO-E investigation into that disturbance identified shortcomings related to regional observability and limited availability of corrective measures during system stress. These findings are used in support of increased emphasis on operational coordination and data exchange as systems become more interconnected.
Project pipeline references across Serbia and Montenegro
The decision’s implications are described as relevant for Serbia’s growing solar-plus-storage pipeline. It also references Montenegro’s Gvozd wind project expansion alongside proposed pumped-storage developments such as Bistrica and Đerdap 3. Emerging battery-storage projects across the region are also cited in connection with how reserves may be supported by flexible resources.
ACER’s framework indicates that storage and flexible assets can be evaluated based on their ability to reduce imbalance costs, improve operational stability, satisfy dispatch requirements and participate in balancing and ancillary-service markets. It also notes that projects relying only on optimistic merchant-price assumptions may face challenges under conditions where solar output depresses daytime prices while evening scarcity drives premium pricing.
The decision frames its central message around reserve sizing as a market signal through adoption of the 99.99% reliability threshold for both positive and negative imbalances within SEE SOR. It also raises questions about cost allocation alongside reliability outcomes by highlighting that higher standards require greater investment in reserves, flexibility resources and operational coordination across transmission operators, generators, storage providers, traders and end consumers.
