The technical and technological characteristics of the Stanari Power Plant block were designed based on the results of the analysis of coal samples from the Raskovac opencast mine. There have not been any notable variations in the coal quality during the Stanari thermal power plant’s present operating term from the project’s stated values. For the expected lifetime of mine, i.e., the thermal power plant operation until 2050, it is necessary to unearth the total remaining available coal reserves from the Stanari basin. The thermal power plant’s operation may be significantly impacted by variations in coal quality at the Ostruznja opencast mine. The use of coal with quality parameters lower than designed can cause problems in the operation of the boiler, stoppages in operation, and an increased volume of maintenance and overhaul of the boiler. Coal quality control in the process of sampling, mining, transportation, and depositing of coal must be at a level that enables smooth operation of the thermal power plant. This paper presents test results during the operation of the thermal power plant burning coal of different quality, (lower calorific value in range from 9100 kJ/kg to ≤ 7500 kJ/kg), a risk assessment of long-term planning, and the effectiveness and economy of the thermal power plant operation. The potential economic effects and reliability of the operation of thermal power plants due to the increased consumption of coal, limestone and self-consumption in operating modes with significantly lower coal quality than designed were analyzed in particular.