The amount of energy generated by a wind energy conversion system (WECS) depends very much on the characteristics of the wind regime of the location of the site, where wind turbine is installed. In order to estimate the energy gain from a WECS an accurate model for WECS needs to be developed and used. This allows us to easy evaluate the performance of different wind turbines and to determine the amount of energy injected to the grid in the case of the grid-connected systems. This paper presents results of a study conducted to compare performance evaluation and energy gain of different wind turbines using the Monte Carlo operational model. The power curve of the wind turbines provided by turbine manufacturer is also used to estimate the power gain from wind turbine.