Initiatives to combat climate change imply the decarbonization of the economy. Transport and energy are the most promising sectors in terms of decarbonization’s technological feasibility. However, the decarbonization of these sectors may cause several problems in terms of the sustainability of the energy system. This is because electric transport creates additional atypical demand in the energy system, while non-carbon energy sources such as nuclear power plants and renewable energy sources have generation limited by maneuverability characteristics or weather conditions. To avoid significant consequences from imbalances between demand and supply, before implementing decarbonization projects, it is necessary to conduct modeling of the energy system’s flexibility. Solving such a task requires specialized modeling tools - optimization models of intra-annual commercial dispatching of generating and storage capacities. This article addresses critical requirements for a modeling tool, various applicable modeling approaches, and software options. The article also presents the results of modeling the conditions for balancing supply and demand in the UES of Russia at the level of 2050, considering a high level of transportation electrification and covering additional demand through various generation technologies, including carbon-neutral ones.