'A spirit of solidarity' in the EU energy policy (Article 194 TFEU) has raised questions about its nature and scope. It was initially treated as an abstract, purely political concept, not as a legal criterion for assessing the validity of European Union acts. Due to the Court of Justice of the European Union (CJEU), energy solidarity now constitutes a specific expression of the principle of solidarity, which is itself one of the fundamental principles of EU law. Whilst confirming the legal nature of the energy solidarity, the CJEU did not indicate how it should be applied. A certain solution seems necessary to ensure legal transparency. The multi-directional development of the EU energy policy is a challenge, as it includes – in the process of developing the internal market – not only energy security, but competitiveness, sustainable development, and in particular climate change. The energy solidarity standard is an attempt to find an adequate regulatory solution.

Due to the growing mix of different types of dispersed generation, energy storage, energy efficiency and new promising technologies, in combination with traditional large scale fossil fuel electricity generation, ensuring reliability of electricity systems is increasingly challenging. The electricity demand is also in constant change because of the increased frequency of extreme weather events. In this regulatory environment, the European Union has proposed a single regulatory model to assess the security of the electricity system, in which a key role is played by long-term resource adequacy assessment. The aim of the article is to show this model, indicating what the policy implications are for the development of renewable energy sources and decarbonisation targets.