This paper represents an analysis of possible solarization of the city of Zadar. The mathematical calculation of investments in solar power plants was carried out, their cost-effectiveness was examined, and the electricity trading model was created. Within this work, great importance is added to investment energy cooperatives, to their establishment and to the benefits that their membership brings. In the introduction the trend of exploitation of renewable energy sources is analyzed and an overview of the European Commission's legislative package "Fit for 55" is given. There is also an overview of the new package of measures "REPowerEU" which came due to new geopolitical relations in eastern Europe. After the introductory part, energy cooperatives were defined and business models of energy cooperatives were presented. In the second part of this paper, assumptions and data used in the mathematical calculation were presented. Formulas for calculating the cost-effectiveness of the project and how electricity is traded are explained. Mathematical calculation was carried out for three different characteristic groups of consumers: household, entrepreneurship and public lighting. Finally, the results are presented, accompanied by appropriate diagrams. It has been shown that it is worth investing in the construction of a solar power plant for all three groups of consumers. Cash flows have been presented as savings, due to the reduced use of electricity from the grid, and as profits, due to the sale of surplus electricity generated into the grid. At the same time, the same mathematical calculation was carried out with the assumption of membership in the investment energy cooperative. It has shown that membership brings greater profits and greater cost-effectiveness of the project, that is, a shorter time of return on investment.