ACER Opinion 11-2025 on the Spanish National Resource Adequacy Assessment

No 11/2025

OPINION

on the National Resource Adequacy Assessment of Spain

7 November 2025

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Executive summary

Electricity Regulation (EU) 2019/943 requires the European Network of Transmission System Operators for Electricity (ENTSO-E) to carry out the European resource adequacy assessments (ERAAs) on an annual basis. To complement ERAA, Member States may also carry out a national resource adequacy assessment (NRAA). If an NRAA identifies concerns not reflected in the European assessment, the Member State must explain the differences and submit the national assessment to ACER for opinion. Following the ERAA 2024, Spain conducted its NRAA, prepared by the transmission system operator (TSO), Red Electrica, and submitted to ACER on 8 September 2025. The NRAA complements the ERAA by reflecting Spain-specific electricity system developments.

Key Findings of the Spanish NRAA

The outcomes for 2028 are broadly consistent with ERAA. However, for 2030, the national assessment shows an adequacy concern, expressed in the loss of load expectation (LOLE), rising to 2.4 hours – above the 1.5-hour reliability standard – identifying a new adequacy concern in 2030 not reflected in the ERAA.

ACER’s Assessment of Differences

Structural adjustments in the NRAA (geographic focus and implicit modelling of Balearic Islands and Ceuta) were deemed justified and improve national relevance. Methodological simplifications (reduced forced outage samples, exclusion of curtailment sharing) cut complexity, impacting modelling results. These simplifications are considered reasonable given limited impact on adequacy results in the Spanish peninsular electricity system. Input data updates reflect conservative assumptions: • Spanish NRAA projects storage growth considerably below both ERAA levels and the National Energy and Climate Plan (NECP) policies. The assumption that only currently secured projects will materialise is unjustified, given the rising renewables deployment and evolving policy frameworks. • Modelling of the gas turbine fleet availability is more conservative, with higher rate and longer duration of forced outages. These constrains are based on historical data and are, therefore, justified. • Demand profiles were updated, aligning with those of ERAA 2025 that better represent demand seasonality. This adjustment is, therefore, justified. • The fixed 2026 gas turbine fleet maintenance profile is applied to all target years modelled. Such approach is not consistent with the ERAA 2024 and is, therefore, not justified.

Conclusion

ACER acknowledges that in recent years, battery deployment in Spain has not seen an increase that would necessarily suggest the NECP targets are attainable without regulatory intervention. Nevertheless, taking battery projects already under financial support as the sole basis for future capacity expansion is deemed overly conservative. ACER also finds that fixed maintenance profiles fail to capture anticipated changes in the Spanish electricity system. However, ACER notes that the impact of these unjustified differences on adequacy results appears to be low. Most deviations from the ERAA 2024 are related to national specificities and local conditions and hence are deemed justified. Overall, ACER finds the assessment is clear and well-executed, complementing the European assessment.

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1. Background

1 Article 20 of Regulation (EU) 2019/943 (Electricity Regulation) requires Member States to monitor resource adequacy within their territory based on the European Resource Adequacy Assessment (ERAA). It also allows them to complement the ERAA by conducting a National Resource Adequacy Assessment (NRAA) according to Article 24 of the Electricity Regulation. Either of the two assessments can identify resource adequacy concerns, but both assessments must be based on the ERAA methodology. The adequacy concerns identified through these assessments should be in the first place addressed by eliminating regulatory distortions or market failures, and, where it does not suffice, by introducing capacity mechanisms.

2 According to Article 24(3) of the Electricity Regulation, where the NRAA identifies an adequacy concern that was not identified in the ERAA, the NRAA must include reasons for the divergence between the two assessments, including details of the sensitivities used and the underlying assumptions. The Member State must publish the NRAA and submit it to ACER for opinion. ACER’s opinion assesses, on a case-by-case basis, whether the differences between the two assessments are justified. ACER primarily considers the differences identified and reasoned by the Member State in the submission, but may also identify further differences, if they also have a material impact on the results.

3 The national body that is responsible for the NRAA, which can be a Transmission System Operator (TSO) or a different body designated by the Member State, must take due account the ACER's opinion, and, where necessary, amend its assessment. Where it decides not to take ACER's opinion fully into account, it must publish a report with detailed reasons.

4 The latest ERAA (ERAA 2024) was approved by the ACER Decision on 11 August 2025. To complement it, the Ministry for Ecological Transition and the Demographic Challenge of Spain (the Spanish Ministry) requested the Spanish TSO, Red Electrica, to conduct a National Resource Adequacy Assessment (Spanish NRAA or NRAA, published in September 2025).

5 On 8 September 2025, the Spanish Ministry submitted its NRAA to ACER. The submission includes reasons for its divergence from the ERAA 2024, as well as the details of the sensitivities used and the underlying assumptions.

6 This Opinion follows the submission of the Spanish NRAA and evaluates whether the differences between the Spanish NRAA and the ERAA 2024 are justified.

7 This Opinion is addressed to the Spanish Ministry and issued in accordance with Article 9(2) of Regulation (EU) 2019/942 and Article 24(3) of the Electricity Regulation.

2. Summary of the Spanish NRAA

8 The aim of the Spanish NRAA is to complement the ERAA 2024 by adopting different input assumptions and modelling approaches that more adequately reflect the technical performance of the Spanish electricity system – based on historical observations – and capture recent market trends in storage resource deployment in Spain.

9 The modelling results of the central reference scenario are shown in Table 1. The NRAA identifies adequacy concerns in Spain in both 2028 and 2030, as the modelled Loss of Load Expectation (LOLE) exceeds the reliability standard of 1.5 hours. While 2028 LOLE is broadly consistent with the ERAA results, the 2030 LOLE of 2.41 hours represents a new adequacy concern identified in the Spanish NRAA only. A C E R O P I N I O N N O 1 1 / 2 0 2 5

10 This divergence in the LOLE results is due to differences in the modelling assumptions between the Spanish NRAA and the ERAA 2024. These differences are discussed and assessed by ACER in the following section.

3. ACER’s assessment

11 ACER welcomes the Spanish NRAA and considers it a well-developed assessment of the Spanish electricity system, enriched with relevant country-specific details. The report provides a clear description of the methodological modelling assumptions, the differences to ERAA 2024 and the adequacy results. Also, the implementation of the Economic Viability Assessment (EVA) phase is well appreciated, as it allows to consider market-based resource development and thus provides a proper foundation for resource adequacy assessment.

12 The differences between the Spanish NRAA and the ERAA 2024 fall into three categories: (i) structural, (ii) methodological and (iii) input data. Each difference is listed in Table 2, along with its impact on LOLE. A detailed description of these differences is provided later in this section. A C E R O P I N I O N N O 1 1 / 2 0 2 5

3.1. Structure

Difference #1 Regional scope (see Section 4.2.2 in the NRAA)

13 Description: An NRAA is an assessment with a regional scope compared to the pan-European ERAA. In the Spanish NRAA, coverage is highlighted in Figure 1: Spain in red and the other modelled countries in pink. Power imports from non-modelled (white) bidding zones are only possible when electricity prices approach the price cap and are constrained by net transfer capacity.

14 Relevance: Reducing the geographical scope in electricity system modelling reduces complexity, enabling faster and more resource-efficient analyses while allowing for a deeper focus on a specific region.

15 Assessment: The reduced spatial coverage in the NRAA includes the relevant set of countries – Spain’s direct neighbours, Portugal and France – along with other bidding zones that may have a significant impact on Spanish power grid’s cross-border exchanges. Combined with simplified import assumptions from non-modelled bidding zones, this leads to only a minor impact on LOLE compared with the pan-European ERAA result and is, therefore, considered justified. Difference #2 Inclusion of Balearic Islands and Ceuta (see Section 4.2.4 in the NRAA)

16 Description: Balearic Islands and Ceuta are additional electricity systems with power connections to the Spanish peninsular electricity network, which are implicitly modelled in the Spanish NRAA.

17 Relevance: Improved representation of the Spanish electricity system bringing it closer to reality.

18 Assessment: The inclusion of the additional electricity systems connected to the peninsular grid enhances the representation of the Spanish system and is hence justified.

19 It is noted that in the NRAA, meeting demand in these newly added regions is given priority over the Spanish peninsular electricity system, which was not the case in the ERAA 2024. In the event of power shortage, electricity supply to these regions takes precedence. Under this assumption, the LOLE in the peninsular Spanish electricity system might increase. The largest absolute increase in LOLE this difference brings is 1.6 hours in 2028. A C E R O P I N I O N N O 1 1 / 2 0 2 5

3.2. Method

Difference #3 No curtailment sharing (see Section 4.2.1 in the NRAA)

20 Description: The curtailment sharing mechanism applied in the ERAA 2024 ensures that energy shortages are fairly distributed across bidding zones whose domestic capacity is not sufficient to meet local demand. This reflects the actual functioning of the spot markets. However, the curtailment sharing feature is not included in the Spanish NRAA.

21 Relevance: Curtailment sharing is embedded in the EUPHEMIA algorithm for day-ahead electricity trade among EU bidding zones. It spreads the impact of network constraints more evenly. When applied to future electricity system modelling, this feature is essential in multibidding-zone simulations to ensure accurate Energy Not Served (ENS) results in markets facing resource scarcity.

22 Assessment: Curtailment sharing significantly impacts the LOLE results. In ERAA 2024, its application led to more than doubling the average LOLE per bidding zone as explained in ACER’s ERAA 2024 Decision.

23 ACER observes that in the Spanish NRAA, curtailment sharing impact may be limited. The Spanish electricity system on the Iberian Peninsula may experience a lower curtailment sharing effect, as it is connected to only two other bidding zones – Portugal and France. Moreover, according to the ERAA 2024 , Spain shows a low correlation of scarcity events with its neighbours, which indeed could limit the curtailment sharing impact. Additionally, due to this difference the largest absolute LOLE decrease compared to the ERAA 2024 is 0.8 hours in 2028. This confirms the low impact of the curtailment sharing. Given this limited impact on LOLE, its omission in the NRAA is justified. Difference #4 Less forced outage samples (see Section 4.2.3 in the NRAA)

24 Description: In the ERAA 2024, each weather scenario is simulated with 15 different forced outage samples, whereas the Spanish NRAA reduces this to 5 samples per scenario.

25 Relevance: Repetitive forced outage runs (15 in ERAA) provide robust LOLE results accounting for uncertainty, capturing a wide range of potential outcomes and providing a more realistic analysis.

26 Assessment: Red Electrica ran its NRAA modules with 5 forces outage samples and compared the results with the ERAA, which uses 15 samples. The difference in LOLE was minor – the reduced sample count lowered LOLE by 0.1 hours in 2028. This limited impact on LOLE justifies the use of this simplified approach. The approach is also in line with ACER’s request to streamline the ERAA methodology by reducing computational complexity with a limited impact on the robustness of the results.

3.3. Input data: Storage

Difference #5 Lower capacity (see sections 5.2.1 and 5.2.2 in the NRAA)

27 Description: Initial assumptions for storage capacity, before the EVA phase, are much lower in the Spanish NRAA than in the ERAA 2024, as shown in Figure 2. The NRAA assumes that only the currently planned hydro pump and battery projects will be commissioned within the study horizon. All these projects are supported by State aid schemes. A C E R O P I N I O N N O 1 1 / 2 0 2 5

28 The NRAA accounts for 2.5 GW of storage capacity already procured under the Spanish State aid scheme . With this assumption, the storage capacity in the NRAA is around 8 GW lower in 2030 and 16 GW lower in 2035 compared with the ERAA, and compared with the Spanish National Energy and Climate Plan (NECP) targets in its storage assumptions.

29 This assumption in the Spanish NRAA is not in line with the projected growth trend of storage capacities in the EU. To illustrate this, the battery-to-RES (Renewable Energy Sources) capacity ratio is shown in Figure 3. At the EU level, this ratio gradually increases in the coming years, indicating that battery deployment speed in relative terms overtakes RES, providing additional flexibility to manage both RES surpluses and shortages. In contrast, the NRAA assumes that about 1.5 GW of battery capacity entering the market in 2028 would remain unchanged until 2035. With RES deployment rising rapidly over the same period, this results in a declining batteryto-RES capacity ratio in Spain, highlighting a growing gap compared to the European average.

30 The EVA modelling phase is expected to address the battery deployment shortfall. However, despite batteries being included among the capacity expansion options, the NRAA shows no new battery deployments. This outcome likely stems from the inadequate representation of the business case of batteries. Notably, balancing revenues, which represent a consequent share of current batteries revenues, are not explicitly modelled. ACER finds it unlikely that there will be no market-based development of batteries in the next 10 years in Spain. A C E R O P I N I O N N O 1 1 / 2 0 2 5

31 The need to better represent the business model of non-fossil flexible resources – an area where both the ERAA 2024 and the Spanish NRAA currently fall short – has been highlighted in ACER`s ERAA 2024 decision and also pointed out as an area for improvement in ACER’s request to streamline the ERAA methodology.

32 The Spanish NRAA is not asked to improve the EVA module in the current assessment; however, the combination of low initial storage capacity assumptions and the unchanged modelling approach is likely to result in an underestimated outlook for battery deployment (see Figure 3).

33 Relevance: A well-projected initial resource capacity, in particular for batteries, is essential as it forms the starting point for the EVA phase, where the business case for non-fossil flexibility is currently under-developed. Thus the EVA modelling phase likely underinvests in non-fossil flexible technologies and rather retains (does not de-invest) existing gas turbines longer in the system. As the NRAA decommissions fewer gas units than the ERAA, it offsets the impact of the differences on the adequacy results .

34 Even if the EVA in the Spanish NRAA smoothens the impact on the adequacy results, the initial assumptions on the expected storage development tilt the capacity mix towards fossil generators. This, in turn, may impact the assessment of the indicative objectives for non-fossil flexibility that Member States are expected to establish under the Electricity Market Design Regulation (EU) 2024/1747.

35 Assessment: Storage capacity expansion (pump hydro and battery) in the Spanish NRAA is projected far below the ERAA 2024 levels which broadly follows Spanish NECP – 8.6 GW in the NRAA compared to 16.6 GW in the ERAA by 2030. The storage expansion assumed in the NRAA is limited to currently planned projects, with no additional growth assumed over the next decade with or without State aid schemes.

36 According to the ERAA methodology, projected storage expansion should align with the NECPs and be updated to reflect recent market trends. While current situation provides a reasonable basis for assessing energy systems over the next few years, they may further evolve over a longer time horizon, where recent patterns may no longer hold.

37 The assumption that market conditions alone may not yet be sufficient to attract investment is supported by Spain’s relatively low level of battery penetration to date. Key underlying factors include: • Limited price arbitrage revenues: Due to the specificities of the Spanish electricity system, the wholesale price spreads are lower compared to countries such as Germany, which has a high level of battery deployment. In 2025 (till 3 October), the average daily maximumminimum day-ahead price spread in the Spanish wholesale electricity market is calculated by ACER at 103 €, compared with 138 € in Germany (ENTSO-E Transparency Platform data). A C E R O P I N I O N N O 1 1 / 2 0 2 5 • Regulatory conditions: The mandatory, non-remunerated contribution of generators to the Frequency Containment Reserve (FCR) constrain storage deployment (see ACER’s market monitoring report on regulatory barriers).

38 Considering the above factors, limited battery development may be reasonable in the near term, yet future market dynamics are likely to shift with rapid deployment of RES and improvements in market conditions. The Spanish implementation plan – particularly the Energy Storage Strategy and potential capacity mechanisms – aims to address these deficiencies. The strategy targets 20 GW of storage capacity (including solar thermal) by 2030 and sets out measures for effective deployment, including defining legal requirements for storage, clarifying the role of market actors, reducing administrative barriers, and facilitating research and development initiatives.

39 Moreover, the recent Iberian electricity system blackout – though unrelated to resource adequacy – has triggered policy developments that aim inter alia to facilitate storage deployment. In particular, the Spanish government Royal Decree-Law 7/2025 introduces measures aimed at enhancing the resilience of the Spanish electricity system by facilitating greater integration of storage and flexibility. This provides increased reassurance for meeting the policy targets.

40 Given that the current EVA implementation does not fully capture the potential of battery technologies, the input assumptions regarding initial battery capacity deployment in future years should be considered concurrently. A realistic battery planning should start from the evolving landscape rather than rely solely on the EVA outcomes. This is also in line with the planned “trends and projections scenario” for adequacy assessments. This scenario aims to capture the actual pace of the energy transition, which develops further from current levels but could remain below the trajectories envisioned in the NECPs . For these reasons, the low storage assumptions applied prior to the EVA phase, while acceptable for 2028, can no longer be justified from 2030 onwards.

41 Recommendation: The initial storage assumptions (prior to the EVA investment modelling), should consider the potential removal of barriers and the anticipated developments in the electricity system that could facilitate greater storage deployment in the coming years.

42 Recommendation: In future assessments, consider incorporating into EVA multiple revenue streams available to storage resources – already covered under the ERAA methodology – such as intraday market energy arbitrage and grid support services. This becomes particularly important when the initial assumptions on storage capacity deployment are low. As the energy transition accelerates, the performance of EVA should evolve accordingly, to better reflect the business models of non-fossil flexible resources. A C E R O P I N I O N N O 1 1 / 2 0 2 5 Difference #6 Lower hydro pump availability (see Section 5.2.1 in the NRAA)

43 Description: Introduction, of a weekly capacity availability constraint for hydro pump closed-loop technology, is based on historical records and is absent in ERAA.

44 Relevance: The availability constraint slightly reduces the maximum hydro pump generation power, limiting it to 80-100% of installed capacity depending on the week of the year.

45 Assessment: Aligning seasonal availability of the hydro pump capacity with actual observations in the last three years aligns the results with the real-world conditions and is, therefore, justified. Difference #7 Longer battery duration (see Section 5.2.2 in the NRAA)

46 Description: Compared with the ERAA 2024, around 40% of batteries in the Spanish NRAA are modelled with a doubled storage duration – rising from 2 to 4 hours.

47 Relevance: Accurately reflecting battery size – specifically, the duration it can sustain full power at full charge – is essential for modelling power scarcity events. This constraint directly affects the reliability of results, making battery discharge time a key parameter in adequacy assessments.

48 Assessment: This update in assumptions builds on the data from ongoing projects provided by promoters during the public support scheme tendering process. This is in line with the actual capabilities of the battery units in the currently planned projects, and thus it is justified. Capturing these details is crucial, as battery duration directly determines its contribution to covering scarcity periods. As shown in Figure 4, a significant share of ENS events in Spain – 47% in 2028 and 62% in 2030 – last three hours or more.

3.4. Input data: Gas turbines

Difference #8 Higher forced outage rate (see Section 5.2.3 in the NRAA)

49 Description: The performance of the Spain’s gas-fired Combined Cycle Gas Turbine (CCGT) fleet has declined, with historical records from the past four years showing increases in both outage frequency and duration. To reflect these trends, the NRAA updates the relevant parameters by raising the CCGT forced outage rate from 5% to 9% and extending the mean repair time from one day to three. These updates reflect the technical availability of the CCGT fleet as observed in 2024. As a result, on average, 9% of the installed CCGT capacity is unavailable due to longer forced outages. A C E R O P I N I O N N O 1 1 / 2 0 2 5

50 Relevance: Historical availability data is essential in power system modelling, as it enables more accurate estimates of future resource needs and system reliability.

51 Assessment: Incorporating recent historical availability data on CCGT forced outages into the module improves accuracy and is therefore justified.

52 ACER recommends that future assessments apply different CCGT forced outage rates per subtechnology as categorised in ERAA – specifically CCGT old 1, old 2, present 1, present 2, new and Carbon Capture and Storage (CCS). This distinction is particularly important for Spanish assessment, as a significant share of CCGT capacity is decommissioned during the EVA phase. Decommissioning begins with the least technically efficient and oldest power plants, which typically have the highest outage rates. Applying different outage rates per sub-technology ensures that the remaining generators after the EVA phase are not burdened by the poor performance characteristics of the decommissioned old units. Difference #9 Fixed planned maintenance for all years (see Section 5.2.3 in the NRAA) Description: ERAA 2024 assumes that the maintenance of CCGTs is optimised to avoid scheduling during periods of adequacy risk. However, in recent years, planned CCGT maintenance in Spain has also occasionally coincided with scarcity periods. Therefore, in the Spanish NRAA, the planned 2026 CCGT maintenance profile is used, and this profile is applied to all modelled target years. Relevance: Capturing the historical seasonal profile of planned maintenance – especially the unavoidable maintenance near scarcity periods – is crucial for accurate future electricity system modelling. It ensures realistic reliability assessment, as such maintenance can directly limit resource availability during critical times. Assessment: While applying the actual maintenance profile may be reasonable in the near term, it becomes less appropriate over longer horizons. The NRAA projections show that by 2030, periods of system stress in Spain increasingly shift toward December, driven by changes in electricity demand patterns and the evolving generation mix under the energy transition. Consequently, the actual 2026 maintenance schedule is arguably not aligned with expectations for 2030 and is therefore not justified. ERAA methodology requires planned outage schedules to be optimised to avoid maintenance during likely ENS periods, while still respecting the share of capacity that must undergo maintenance in winter.

53 ACER recommends that the NRAA moves away from a fixed maintenance profile and considers adopting an approach based on optimised planned maintenance, allowing for partial overlap with scarcity periods, as observed historically. This would better align maintenance profiles with the evolving Spanish electricity system.

3.5. Input data: Demand

Difference #10 Hourly profile from ERAA 2025 (see Section 5.2.4 in the NRAA)

54 Description: The hourly demand profile in the Spanish NRAA is sourced from the ongoing ERAA 2025 and then adjusted to match the annual demand volume of the ERAA 2024. Hence, in this process only the hourly demand shape is updated.

55 Relevance: In 2030, the annual Spanish demand in the ERAA 2025 (291 TWh) is higher than in the ERAA 2024 (270 TWh). The hourly demand profile from the ERAA 2025 is scaled down to align with the ERAA 2024 annual volume. This adjustment leads to reduction in annual peak demand across all weather scenarios – by -3 GW in 2028 and -2.2 GW in 2030. As a result, the demand-supply balance, previously affected by lower storage capacity, is partially restored.

56 Assessment: On the one hand, combining modelling inputs from different ERAA editions in the Spanish NRAA is both challenging and difficult to justify, as demand does not evolve in isolation. A C E R O P I N I O N N O 1 1 / 2 0 2 5 On the other hand, compared to the ERAA 2024, the updated NRAA profile provides a more accurate representation of actual demand seasonality in Spain with peak demands shifting from summer towards winter. Furthermore, the ERAA 2025 demand represents the second generation of the ENTSO-E’s weather scenario approach, which in theory should bring improvements to demand forecasting. Overall, these factors make the updated demand profile justified.

4. Conclusions

57 Table 3 presents ACER’s assessment of whether the differences between the Spanish NRAA and the ERAA 2024 are justified.

58 ACER considers most of the differences between the Spanish NRAA and the ERAA 2024 edition to be justified. However, ACER finds two differences – #5 and #9 – not justified. The previous section explains ACER’s position and offers recommendations for improvements. In particular to (i) review battery capacity assumptions in light of evolving market frameworks; and (ii) consider an optimised gas turbine maintenance schedule that could better reflect the evolution of the Spanish system.

59 The overall impact of these two differences on adequacy results (LOLE) is, however, limited. The lower battery capacity and constrained gas turbine availability under the fixed maintenance profile are ultimately offset by the reduced gas turbine decommissioning observed in the EVA modelling phase, thereby keeping the overall impact on LOLE low. However, the precise effect on system adequacy can only be accurately quantified through dedicated adequacy modelling using sensitivity scenarios.

60 Considering the above, overall, ACER is of the view that the Spanish NRAA provides robust adequacy findings. ACER also recognises that the extended geographical scope and EVA investment modelling is a good practice, on par with the ERAA 2024 module. A C E R O P I N I O N N O 1 1 / 2 0 2 5 This Opinion is addressed to the Ministry for Ecological Transition and the Demographic Challenge of Spain. Done at Ljubljana, on 7 November 2025. — SIGNED — V. ZULEGER, ACER Director ad interim