Among the fundamental principles of European Union law is the principle of solidarity, even if its approach has been marginal. The economic, social and political realities of the last decade have, however, highlighted the importance of this principle for the Union as a whole, but also for the Member States. The principle of solidarity has been highlighted since the migrant crisis, the health crisis caused by the COVID pandemic and the energy crisis, and the Court of Justice has ruled on this principle. The present study will highlight the importance of the CJEU ruling in the OPAL case, but also how energy policy can be aborted in the broader context of economic competitiveness, taking into account the Letta and Draghi reports. The study highlights the Union’s energy strategy as well as regulatory developments in national law, both in terms of energy solidarity and in particular in the broad area of energy security. The study also shows how Romania has positioned itself in the various cases before the Court of Justice concerning energy security or the application of the principle of energy solidarity.