h i g h l i g h t sA hybrid synthesis method for renewable energy systems is presented. Heuristic equipment preselection is combined with superstructure-based optimization. A real world case study is analyzed (Mljet Island, Croatia). Investment cost savings of up to 59% compared to classical simulation are achieved. The computational effort required for optimization is low. a r t i c l e i n f o t r a c tAn efficient synthesis method for renewable energy systems is presented that exploits synergies between heuristic-and optimization-based approaches. For this purpose, the RenewIslands method has been integrated into a superstructure-based optimization approach. The resulting hybrid approach consists of two steps: First, heuristic-based equipment preselection identifies a set of promising candidate technologies. Next, the preselected technologies are employed in superstructure-based optimization to determine the optimal renewable energy system. The heuristic preselection systematically avoids excessively large superstructures, while the subsequent optimization ensures that the optimal solution is selected. The proposed method is applied to the case of Mljet Island, Croatia. A renewable energy system is synthesized that requires 59% less investment costs compared to the solution derived by classical simulation. At the same time, the optimization problem is solved in less than 2 min. The proposed hybrid method is shown to provide an efficient route to the synthesis of renewable energy systems.