For the purpose of this Thesis, the simulation of the operation of large-scale electric boilers in the District heating (DH) of the city of Zagreb, on the example of EL-TO Zagreb (Cogeneration Plant in Zagreb). Using the PLEXOS Integrated Energy Model software, a simulation-optimization model for electric boiler fitting in EL-TO Zagreb has been developed. The model has been created on the basis of a large amount of input data, including technical and economic parameters of modern electric boilers and all existing and planned units at the EL-TO, data of expected heat consumption and market data (the prices of electricity, natural gas and emission units). The main goal of the model is to minimize the total operating costs of EL-TO Zagreb.Three scenarios have been calculated using the model. The first scenario does not consider electric boilers usage, the second envisages the fitting of one hot water (30MW) and one steam electric boiler (30MW), while the third envisages fitting two steam electric boilers (2X30MW) and one hot water boiler. All the scenarios have been calculated for years 2020, 2025 and 2030. The obtained results show that electric boilers do not participate significantly in total heat energy production (according to second and third scenarios). For example, according to the second scenario, it has been calculated that the boilers produce 0.08% of the total heat energy in the year 2020 and 0.33% in the year 2025. Also, according to the third scenario, it has been calculated that the boilers produce 0.025% of the total heat energy in the year 2020 and 1.18% in the year 2025. The payback period for electric boilers is 181 years according to the second scenario and 91 according to third.