This paper presents a comparative study of two energy system analysis models both designed for the purpose of analysing electricity systems with a substantial share of fluctuating renewable energy. The first model (EnergyPLAN) has been designed for national and regional analyses. It has been used in the design of strategies for integration of wind power and other fluctuating renewable energy sources into the future energy supply. The model has been used for investigating; new operation strategies and investments in flexibility in order to utilize wind power and avoid excess production. The other model (H2RES) has been designed for simulating the integration of renewable sources and hydrogen into island energy systems. The H2RES model can use wind, solar and hydro as renewable energy sources and diesel blocks as backup. The latest version of the H2RES model has an integrated grid connection with the mainland. The H2RES model was tested on the power system of Porto Santo Island, Madeira, Portugal, Corvo and Graciosa Islands, Azores Islands, Portugal and Sal Island, Cape Verde. This paper presents the results of using the two different models on the same case, the island of Mljet, Croatia. The paper compares methodologies and results with the purpose of identifying mutual benefits and improvements of both models. (c) 2006 Elsevier Ltd. All rights reserved.