One of the important tasks in the short-term forecasting of the work of the united energy system (UES) of Ukraine is to ensure grid balance reliability, one of the key elements of which is the reliability of fuel supply. Today in Ukraine to solve this task the forecast of the electricity balance (FBE) is developed, within which balance of fuel for TPPs is formed, but the methodology of its development has certain shortcomings which are analyzed in article. The article proposes an approach to improving the development of FBE by the introduction into practice a specialized optimization mathematical model to calculate the required total change in fuel stocks at TPP. The developed mathematical model takes into account the modes of TPPs operation, requirements for limiting emissions to the air according to National Emission Reduction Plan, restrictions on the possibility of fuel supply. The article provides a detailed description of the developed mathematical model, and also presents the results of test calculations and comparison with the official data of the Ministry of Energy of Ukraine on the need for total fuel reserves for TPPs. The results show: 1. It is expedient that the methodology of FBE formation should be improved taking into account a number of factors, in particular, to take into account the restrictions on air pollutant emissions volumes while determining electricity generation capacity at different groups of power units, the opportunities for general fuel supply for all TPPs, more reliable forecast of electricity production at individual TPPs provided by FBE than according to retrospective data. Improving the methodology of FBE formation can be based on the developed mathematical model of optimal monthly national electricity balances calculation which operates with separate groups of TPP power units with similar technical and economic parameters. 2. Comparisons of the obtained results with the Ministry of Energy of Ukraine’s actual forecasted data about accumulation of fuel stock indicated significant difference. The latest forecast is unrealistic in terms of its feasibility - the need to ensure a sharp increase in fuel stocks in the short term against the background of growing electricity production at TPPs, while the previous forecast had an underestimation of the need for fuel reserves. 3. The results of calculations for FBE 2021 showed that the accumulation of fuel reserves should be carried out in April - June. 4. The introduction of the proposed mathematical model will ensure taking into account the number of important factors in the context of providing TPPs with fuel, which are not taken into account by the existing Procedure for its development. This will increase the reliability of electricity supply to consumers by minimizing the risks of lack of necessary fuel stocks at TPPs. Keywords: electric power system, thermal power plants, fuel balance, forecast electricity balance, fuel reserves