Usage of thermal energy storage together with cogeneration technology provides an attractive solution by allowing the production of electricity in the periods, when heat load is low and later consumption of heat, when load is high. The purpose of the paper was to investigate the perspectives of using heat storage in the form of accumulator tank with cogeneration plants in Estonia. Two alternatives were compared: the first alternative being an energy system, which included small-scale cogeneration plant, thermal energy storage equipment in the form of accumulator tank and peak boilers, was compared with the second alternative: the traditional energy system including only cogeneration plant and peak-load boilers. The methodology offered for evaluation of these two alternatives included the following steps: defining of the optimal capacity by heat produced in cogeneration maximisation method; simulation of district heating system operating; construction of heat load duration curve taking into account thermal energy storage; comparing of alternatives by technical and environmental parameters.