Beyond Formal Competence: ACER's Unexpected Autonomy and the Paradox of Regional Disunity Driving EU Energy Integration

National discretion and regulators have historically dominated the integration of the European Union's energy market. However, the Agency for the Cooperation of Energy Regulators (ACER) has demonstrated unexpected autonomy since 2015 by drafting network codes, extending beyond its formal competencies and enabling significant harmonization in sensitive areas of national energy policy. This paper conceptualizes practical interdependence and goes beyond the analysis of formal independence by examining ACER's surprising divergence in preference from system operators and national regulators. This article argues that ACER's recent decisions are examples of practical independence because they significantly deviated from stakeholder expectations, leading to unprecedented legal actions from regulators against ACER. Given their high costs and complex procedures, these challenges are smoking gun evidence of ACER's de facto independence. This autonomy is particularly puzzling given ACER's structure, which includes a decision-making committee dominated by national regulators, leading many to view it as a mere forum rather than an autonomous actor. The article analyses two critical cases: First, implementing the fast-activation process marked the first instance of financial accountability for failing to prioritize market efficiency. Second, mandatory capacity sharing with neighboring countries for system security represents a crucial shift toward European-level security management. The article leverages a natural experiment comparing two regional energy regions with similar functional stakes. In the different regions, system operators submitted comparable drafts. In one region, the agreement was reached through national regulators, while in another, it required the intervention of ACER. The subsequent difference in outcome highlights ACER's role in promoting transparency and establishing stringent rules for system operators. Through process tracing and comparative analysis of non-papers, draft documents, and court proceedings, I demonstrate that ACER's autonomy emerges from the network code rulemaking structure. This occurs because fragmented regions are more likely to delegate decision-making to ACER, which implements more integrative solutions.