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.

ACER publishes today its Opinion on the draft TYNDP 2026 Scenarios Report prepared by the European Network of Transmission System Operators for Electricity and Gas (ENTSO-E and ENTSOG) under the TEN-E Regulation. 

These scenarios, produced every two years, provide the common foundation for EU-wide electricity, gas and hydrogen infrastructure planning, and feed into the Ten-Year Network Development Plans (TYNDPs).

ACER’s Opinion assesses whether the ENTSOs’ scenarios comply with ACER’s Framework Guidelines and provide a transparent and consistent basis for infrastructure planning.

What are the key findings?

ACER welcomes the significant work by ENTSO-E and ENTSOG to improve the scenarios’ accuracy, including:

• the continued delivery of joint scenarios combining electricity, gas and hydrogen; 
• the introduction of economic variants (high- and low-growth cases applied to the central scenario to assess the robustness of the underlying economic assumptions);
• the involvement of the Stakeholder Reference Group, which gives stakeholders structured scrutiny over the scenarios’ assumptions, methods and data; and
• the development of an Innovation Roadmap, which sets out planned improvements to the scenario-modelling tools and methods, and is updated every two years.

ACER also identifies two main areas for improvement:

• Alignment with EU climate and energy targets. The current methodology enables formal compliance with EU targets, but ACER considers that it does not adequately capture the structural changes needed to achieve those targets in practice.
• Stronger economic variants. These should function as meaningful alternative scenarios, with greater transparency and consultation on key scenario assumptions and infrastructure planning inputs, improved consistency with the European Resource Adequacy Assessment (ERAA) and a more timely scenario development process.

What are the next steps?

For the finalisation of the TYNDP 2026 Scenarios Report, ACER calls on ENTSOs to improve transparency on the remaining gap to EU climate and energy targets and the limitations of the current methodology.

For future cycles, ACER expects further improvements in EU targets alignment, economic variants, consistency with ERAA, stakeholder consultation and the overall timeliness of the process.

The European Commission will take ACER’s Opinion into account when assessing the draft TYNDP 2026 Scenarios Report.

In its Summer Supply Outlook 2026, the European Network of Transmission System Operators for Gas (ENTSOG) evaluates the evolution of gas supply and assesses whether the EU’s gas system can meet demand, support exports and storage injections over the summer under normal market conditions.

The Outlook also includes an analysis of supplies and storage inventories to assess security of supply for winter 2026/27 and evaluates the system’s resilience under different stress scenarios, including supply disruptions.

ACER acknowledges the storage-filling challenges identified by ENTSOG. As this year’s reference scenario does not introduce major methodological changes compared with previous editions (extensively reviewed by ACER), ACER is not issuing a formal opinion on the 2026 edition.

What are the main highlights of the 2026 Summer Supply Outlook?

ENTSOG's assessment identifies several developments that will shape the EU’s gas system over the summer season:

• Low storage levels: At the start of the gas summer season (1 April 2026), average EU gas storage stood at 28% capacity, below the levels recorded at the start of the previous three summer seasons.
• High LNG demand: Refilling storage to the 90% target before winter (by 1 November) will require greater use of the EU’s gas infrastructure and higher liquefied natural gas (LNG) imports than in previous years. At the same time, ENTSOG notes that EU Gas Storage Regulation provides some flexibility, allowing a lower filling target.
• Supply disruption risks: The ongoing conflict in the Middle East is pushing up gas prices and narrowing the usual summer-to-winter price gap. This weakens the economic incentive for gas storage injections, which could further slow refilling.
• A resilient system: The EU’s LNG regasification capacity is expected to help compensate for lower storage levels and support winter demand, provided adequate LNG supplies are secured.

What are ACER’s main considerations?

ACER highlights the challenges ahead, as identified in ENTSOG’s assessment:

• The EU will need higher LNG imports. ACER’s latest analysis of European gas wholesale markets (winter 2025/2026) confirms that the EU’s LNG imports will need to rise by around 13% over 2025 levels to meet summer demand and reach the 90% filling target before winter, while the 80% target remains achievable with 2025 LNG import levels. This reflects LNG’s growing role in the EU’s energy mix: it covers around half of total gas imports, making the EU the world’s largest LNG importer (see also ACER’s analysis of European LNG market developments in 2025).
• Market conditions are weakening storage-filling incentives. Storage refilling could become more challenging in the months ahead. Unfavourable winter–summer price spreads, the phase-out of short-term Russian LNG and pipeline gas contracts under the REPowerEU Gas Regulation (from 25 April and 17 June 2026, respectively) and continued volatility due to the Middle East conflict constrain the supply available for storage, increasing the pressure to accelerate injections before November.
• Current data point to tight storage conditions:
  • Storage injections are below both the 10-year summer average and 2025 levels, putting the refilling timeline at risk (see Figure 1). 

  • EU gas storage is currently at around 49% capacity, similar to 2021 levels (see Figure 2).

Given these risks, ACER encourages Member States and their competent authorities to closely monitor storage-filling trajectories in the months ahead and foster continued progress, actively managing risks in compliance with the Gas Storage Regulation.

Looking ahead

ACER welcomes ENTSOG’s ongoing monitoring of storage injection trajectories and LNG import availability, as geopolitical instability continues to affect global energy markets.

For future Outlooks, ACER underlines the importance of continuing to assess how planned and unplanned gas supply disruptions may affect the EU’s ability to meet its storage-filling targets.

Finally, ACER highlights the need for timely storage injections in the coming months.

See all ACER’s reactions to ENTSOG’s Supply Outlooks.