A probabilistic method for predicting the economic performance and reliability of autonomous energy systems, consisting of diesel generators and fixed pitch wind generators, is described. The method uses the duration curves of the load and the wind speed. Several constraints are applied among them the most significant are the following: load and wind speed are statistically independent and there is a limitation of wind power penetration due to load and minimum diesel plant output. The quantities that are computed are: wind and diesel energy production, diesel fuel consumption, loss of load probability (LOLP) and expected unserved energy (EUE). Results are presented for two Greek islands.