The decommissioning of the Fessenheim nuclear power plant is an opportunity to re-revisit energy strategies at the local, regional and national scales. The major part of the energy consumed locally (mostly fossil fuels, which are entirely imported) has never been produced locally, the electricity generated by the former nuclear power plant primarily fed the national grid, as well as the German and Swiss grids (32.5%) (Thiébaut, 2021). So, as elsewhere in France and around the world, a number of questions are rising: How can an energy system transformation be managed at local scale? What are the most efficient ways for regional energy-climate planning? How can strategies rely only on local renewable energy production, at what costs and benefits, at what time? What specific territorial features need to be considered? What difficulties must be overcome in implementing these strategies, and what infrastructures (production, storage, conversion, distribution facilities for energy carriers; multi-energy network coupling) and technologies must be implemented to achieve energy-climate targets? The Smart Renewable Energy System (SRES) concept (de Larochelambert, 2023) is used in this paper to design and model the future energy system of the Fessenheim region and to draw up different scenarios for its energy-climate and socio-economic transition (OHM Fessenheim ESTEES project). The concept is derived from the Smart Energy System (SES) concept developed by the Aalborg University, Denmark (Mathiesen et al., 2015). The SERI concept is based on the integral and optimized coupling of all energy sectors, all renewable energy sources, carriers and storages, all mobility technologies, from the local scale (building, district, city) to the regional and national scale. It involves systemic transformations, in a holistic and sustainable way, in order to eliminate fossil and nuclear energies by maximizing efficiency and minimizing energy needs and investments, external dependencies and emissions of pollutants and greenhouse gases (GHG). The SERI implements only efficient technologies and avoids the misuse of certain energy sources and inappropriate or environmentally unfriendly technologies. The paper will present the SRES concept and the numerical results of its implementation and simulation in the Fessenheim region. It will also discuss the urgent need to support an efficient energy-climate planning.