Most of the autonomous power systems in Greek islands are based almost entirely on imported expensive oil and experience high variation of demand between summer and winter due to tourism development. Local conventional power stations operate with low load factors and the systems are characterised by high electricity production cost. In parallel, wind energy is a mature, free of charge, local source, but wind penetration is restricted due to technical reasons related with the safe operation of the autonomous power systems. Combined use of wind energy with pumped storage systems (WHPS) is considered as a mean to exploit the abundant wind potential, increase the wind installed capacity and substitute conventional peak supply. In this study, the effect of wind energy on the reliability of the autonomous power systems is analysed. A definition of the WHPS capacity credit (CC) and a methodology for the calculation that is based on probability theory principles are proposed and applied in three representative Greek islands. The aim is to assess the prospects of wind energy with or without pumped storage systems to substitute the installation of conventional power plants. WHPS are evaluated through the improvement of the reliability of the system with reference to the required installed capacity. Results show that WHPS contributes beneficially to wind CC, besides the required large wind installed capacity.