In accordance with the goal of reducing greenhouse gasses emissions by 55% until 2030 compared to 1990 values and achieving climate neutrality by 2050, future energy systems will rely on renewable energy sources. In such systems, demand-side flexibility technologies will be necessary to ensure required consumption and safe supply in conditions of variable production and times when it is not available. Accordingly, the task of energy planning requires special attention focused on modeling the system that will meet the goals of the energy transition and ensure stable operation, all while achieving the lowest possible system costs. Therefore, this paper uses a computer software for long-term energy planning, H2RES, developed at the Faculty of Mechanical Engineering and Naval Architecture, University of Zagreb within the INTERENERGY project. The program has the ability to model flexible technologies and changes in the availability of renewable resources over the years, which is an essential element in the development of the system in accordance with changing climate conditions in the future. Different scenarios of climate parameter changes for Republic of Croatia, which consequently affect the production from renewable energy sources, will be studied and the behavior of the system will be compared in scenarios of different availability while respecting the goals of energy independence from fossil fuels and the desired climate neutrality of Europe by 2050. Simulations are carried out for each year in the observed period, which is a departure from the usual open source tools and therefore it approaches the capabilities of commercially available tools.