This article examines the legal frameworks surrounding the EU energy policy and its associated institutional architecture. Particular focus is on the Agency for Cooperation of Energy Regulators (ACER). Taking stock of ongoing debates concerning the Energy Union and recently proposed reforms, the purpose of this analysis is twofold. First, from a theoretical perspective, this article explores the limits of the principal/agent model for European integration after the Treaty of Lisbon. It argues that a rule of law analysis should supplement this model in order to improve the encapsulation of the complexities of delegation in policy areas such as energy. Second, this will enable a reflection on the principle of energy justice and the way in which it is articulated by policy and governance changes in the EU.