In the midst of the energy transition process emphasis has been placed on citizens' initiatives, democratization of the energy sector and active end users. The thesis highlights the potential of solar photovoltaic power plants in Croatia. The categorization of the end users with renewable energy sources is presented. They are divided in Croatia into the users of self-sufficiency plants and the end customers with their own production. Existing tariff models and detailed displays of electricity billing for households and businesses are presented - both have a solar photovoltaic power plant installed. The introduction of the definitions of the citizen energy community and the renewable energy community created the foundation for the establishment of a legislative framework for a joint civic action. The basic definitions and regulations concerning energy communities are given which are found in the Directives of the European Union and in the laws of the Republic of Croatia. Modelling of active end users and energy communities was performed in five phases. In the first phase the profitability of household investment in a solar power plant and the payback period were examined. In the second phase the same was done for business. In the third phase the energy community of kindergarten and library (both members belong to the category of business) with local self-consumption was modelled, while in the fourth phase the energy community of households and kindergarten was modelled (members are not in the same category of electricity users). The cost-effectiveness of participation in the energy community, the impact of (non) payment of additional fees for shared energy and the impact of the large difference in the electricity prices of households and businesses were examined. In the fifth phase two energy communities of apartment buildings were modelled - in Senj and Kvatrić. Three different keys of fixed distribution of produced energy have been made for both communities, after which additional surpluses are distributed in proportion to consumption among community members. The most cost-effective way of sharing energy is examined, as well as the impact of paying for additional shared energy.