In the 21st century, electricity storage systems are essential for the transition to sustainable energy. The existing model of the domestic electric power industry, shaped in the 2000s, today does not fully enable economic entities to adopt new technical solutions without legal complications. This article examines both the historical experience of the development of the electric power industry and its current state, with particular focus on the legislative incorporation of the concept of “electric energy storage system” and the use of such systems by natural monopolies, taking into account the prohibition in the electric power industry on combining competitive and natural monopoly activities.