The simulation model created in the environment of EnergyPro software is used to evaluate the influence of an accumulator tank used with 215 MW power unit in the Balti Power Plant, which can operate in cogeneration mode. The plant supplies heat to the city district heating system. Production of heat reduces the electricity generation in the power unit and the highest electricity capacity can be reached without heat being produced. The electricity sale prices are variable, changing hourly. After installation of the heat accumulator tank, the power unit can operate in cogeneration mode with charging the accumulator tank when electricity prices are lower and delivering district heat during the period when the unit operates in condensation mode (electricity only). The real input data on the operation of district heating system; heat and electricity generation and ambient temperature have been used for simulation.