Existing hydropower plants in the Republic of Croatia are an important factor in the share of renewable sources in the structure of electricity production and the provision of ancillary services for the management of the electricity system, which enables competitive positioning of Hrvatska elektroprivreda in the regional market of ancillary services. Reconstructions of existing hydropower plants are therefore necessary in order to preserve production and ensure sufficient regulatory capacity for the proper management of the electricity system. The economic effects and cost-effectiveness of reconstructions are reflected in economic and financial indicators. Reconstruction of hydropower plants, by extension of their lifespan, also realizes the interests of the Republic of Croatia in the field of energy: long-term reduction of dependence on energy imports, reduction of greenhouse gas emissions and efficient use of energy, reduction of environmental impact of fossil fuels. Hydropower reconstruction projects are characterized by good economic indicators that guarantee good profitability and a quick return on investment, which is shown by internal rates of return, short payback periods and positive net present values of projects. Based on the conducted economic and financial analyzes, taking into account the planned and changing business conditions, all the obtained indicators indicate that the reconstruction projects are acceptable and necessary for implementation.