The Government of Slovenia was in the process of developing the Energy Concept of Slovenia (ECS) for the next 40 years, delivering the fundamental development document in the field of energy. The Energy Concept of Slovenia replaced the National Energy Programme (NEP). The concept included a clear framework for potential investors in the energy sector, which involved long-term investments. In order to efficiently develop the Energy Concept, the Government of Slovenia requested the development of long-term energy balances for Slovenia until 2035 and indicative until 2055, accompanied with the cost–benefit analysis of possible different scenarios, and provision of expert support during the drafting, public discussion and the adoption of the ECS. In this framework, the required quantitative modeling provided the decision makers a quantitatively detailed set (projection of detailed energy balances into the future) of possible alternative policy options, and the way these options could be more effectively combined, in order to complied with Energy Union’s main objectives, from the perspective of Slovenia, but also enabling national energy policies to promote the objectives of the Energy Concept of the Government of Slovenia. An extensive sensitivity analysis and assessment of impacts on energy costs, emissions and security of supply, with regard to macroeconomic, sectorial, employment and social implications had been performed. The offered service also covered the requirements for supporting dissemination of project outcomes in Slovenia and the dialogue with stakeholders. The service envisaged delivering the study outcomes within the deadline of five months, and then support dissemination and dialogue over the remaining period of the project (total duration 17 months).