ACER has adopted a Decision amending the electricity redispatching and countertrading cost sharing methodology for the Core capacity calculation region, concluding the review process initiated in October 2025.

What is a capacity calculation region?

A capacity calculation region (CCR) is a set of electrically interdependent bidding zone borders, where capacity calculation, regional operational security, redispatching and countertrading costs sharing tasks are coordinated by that region’s transmission system operators (TSOs).

The Core region is the EU’s largest electricity CCR, covering 19 bidding zone borders and 16 TSOs across 13 Member States: Austria, Belgium, Croatia, Czech Republic, France, Germany, Hungary, Luxembourg, the Netherlands, Poland, Romania, Slovakia and Slovenia.

What is the methodology about? 

The cost sharing methodology allocates the costs of redispatching and countertrading remedial actions within a CCR. These actions are triggered to address network congestions occurring within the region. 

Specifically, this methodology tracks how each remedial action affects congested network elements and assigns the related costs to the responsible TSOs, based on the order of flow types (loop, internal, allocated and power-shifting transformer flows). Loop flows are charged first, however, the portion below a given threshold is to be exempted.

What did ACER decide?

After reviewing the methodology and consulting Core TSOs and regulatory authorities, ACER introduced the following changes:

• requiring Core TSOs to analyse and provide the loop flow threshold;

• confirming the Power Flow Colouring (PFC) method for decomposing physical flows, providing a detailed explanation in the decision.

Next steps

The Core TSOs must implement the amended long-term capacity calculation methodology as soon as possible, along with the Core Regional Operational Security Coordination (ROSC) methodology.

On 7 July 2026, the national regulatory authorities of the Continental Europe Synchronous Area asked ACER to decide on the minimum activation period that frequency containment reserves providers with limited energy reservoirs must ensure to remain available during the alert state.

The Continental Europe Synchronous Area includes Austria, Belgium, Bulgaria, Croatia, Czechia, Denmark, Estonia, France, Germany, Greece, Hungary, Italy, Latvia, Lithuania, Luxembourg, the Netherlands, Poland, Portugal, Romania, Slovakia, Slovenia and Spain.

Why is the minimum activation period important?

EU’s electric power systems operate at the frequency of 50 Hz. Any imbalance between electricity supply and demand causes the frequency to deviate. If not corrected quickly, this can lead to blackouts.

Frequency containment reserves are the first line of automatic response to such deviations. They are activated within seconds to stabilise the frequency and are provided continuously by power-generating or consuming assets (like batteries, hydro plants, or demand response systems). 

Some of these providers, especially batteries or demand-side resources, have limited energy reservoirs and can only respond for a short duration. The System Operation Regulation therefore requires a minimum activation time to ensure these resources remain available during the alert state. 

What are the next steps? 

ACER intends to decide on the minimum activation period within 6 months. 

ACER has adopted two decisions amending the day-ahead and long-term capacity calculation methodologies for the Core capacity calculation region (CCR).

The Core CCR is the EU’s largest electricity capacity calculation region, covering 13 Member States: Austria, Belgium, Croatia, Czech Republic, France, Germany, Hungary, Luxembourg, the Netherlands, Poland, Romania, Slovakia and Slovenia.

ACER Decisions follow referrals by the regional national regulatory authorities under the Capacity Calculation and Congestion Management and the Forward Capacity Allocation Regulations.

What are the methodologies about?

Capacity calculation methodologies determine how much cross-border transmission capacity can be made available to electricity markets.

The day-ahead methodology determines how much electricity capacity is available for cross-border trading one day before delivery. The long-term methodology covers yearly and monthly capacity calculations, enabling long-term planning and hedging opportunities.

In the Core region, both methodologies apply a flow-based approach, meaning capacities are calculated by accounting for actual transmission network constraints. This improves efficiency compared to simpler methods.

Why amend them?

Currently, long-term capacity allocations are included in the day-ahead capacity calculation process. The Core transmission system operators (TSOs) proposed separating these two timeframes so that operational security, which is central to day-ahead capacity calculation, is no longer linked to the volume of capacity allocated in the long-term timeframe. 

This separation aims to better harmonise the region’s market timeframes and facilitate the transition towards long-term flow-based capacity calculation, planned for 2027.

What did ACER decide?

After reviewing the TSOs’ proposals and consulting with stakeholders, ACER approved the amendments, introducing changes to strengthen legal clarity and ensure compliance with EU electricity market rules.

Key amendments include:

Long-term methodology:

• introducing benchmark available transfer capacity (ATC) values as a transitional measure to support flow-based capacity calculation, better reflecting historical cross-border transmission capacities;
• establishing formal amendment procedures for any future changes to these values; and
• integrating the Ireland and Northern Ireland single electricity market–France bidding zone border into the Core region, following the commissioning of the Celtic Interconnector.

Day-ahead methodology:

• removing long-term allocations from the capacity calculation process; and
• updating the methodology to reflect the transmission system’s expansion and new infrastructure (including the Celtic Interconnector).

ACER’s Decisions support consistent capacity calculation in the Core region and facilitate the implementation of long-term flow-based capacity calculation.

Next steps

The Core TSOs must implement both amended methodologies for the 2027 capacity calculation processes.

Today, ACER published its Opinion on Romania’s National Resource Adequacy Assessment (NRAA), which complements the European Resource Adequacy Assessment (ERAA) 2025 edition.

What is a resource adequacy assessment?

The ERAA evaluates electricity resource adequacy across the EU and provides a consistent framework to assess whether additional national measures may be needed to ensure security of supply. It is carried out annually by the European Network of Transmission System Operators for Electricity (ENTSO-E) and reviewed by ACER.

Member States can complement the European analysis through national assessments (NRAAs) to reflect local conditions. When a national assessment identifies different adequacy concerns from the European assessment, ACER issues an opinion.

What did the Romanian NRAA find?

Romania’s NRAA identifies resource adequacy concerns for both 2027 and 2035 under its central reference scenario, indicating a higher risk of electricity supply shortages than the ERAA 2025 edition.

The assessment points to a slower-than-expected energy transition, driven by delays in renewable energy deployment and in the commissioning of new thermal generation capacity, along with a conservative rollout of non-fossil flexibility resources. As a result, the assessment finds that Romania’s electricity system may face resource shortages, potentially affecting the planned phase-out of coal-fired generation.

What does ACER recommend?

ACER acknowledges the rationale for adjusting capacity projections, originally based on the National Energy and Climate Plans, to better reflect recent market developments. However, long-term assumptions on generation capacity (for 2030 and 2035) should remain consistent with the projected pace of electrification.

To reinforce the assessment and provide a more robust evaluation of Romania’s resource adequacy risks, ACER recommends:

• Using the latest information on thermal generation projects. Coal phase-out assumptions should reflect current delays in the commissioning of new gas-fired power plants and the slower deployment of renewable energy. This is particularly important in the short term, as the electricity system still relies on thermal generation.
• Better reflecting the growing role of non-fossil flexibility. The assessment should capture the continued expansion of battery storage and demand response, including flexibility from data centres, across all target years up to 2035. These resources are expected to play an increasingly important role in maintaining security of supply during Romania’s energy transition, supported by emerging national policies, such as draft regulations on demand-side flexibility and the removal of double charging for electricity storage.
• Assessing the economic viability of new investments. The assessment should consider whether projected investments in flexible resources, such as demand response and storage, are commercially viable and therefore likely to be delivered. This would ensure that future flexibility needs can be met at the anticipated pace of electrification and renewables deployment.

Example: Historical and projected installed wind capacity

The Romanian assessment reflects a slower energy transition than the ERAA, with lower projected wind capacity in the first target year in line with recent trends, while still assuming continued growth toward national renewable energy targets.

Source: ACER based on Transparency Platform, Summer Outlook, the NRAA and ERAA 2025.

What are the next steps?

ACER encourages the Romanian authorities to:

• Consider its feedback to improve the assessment of the country’s resource adequacy risks.
• Regularly submit updated projections for realistic resource capacity and electricity demand in future ERAAs, ensuring they reflect latest market developments. This will support more realistic planning at EU level and align with the updated ERAA methodology, which includes a new ‘Trends and Projections’ scenario to capture potential deviations from National Energy and Climate Plans trajectories.