As installed wind capacity of the world increases, more and more issues are revealed during operation. One of these issues is related to the stochastic nature of wind speed that results in low accuracy of wind power forecasts. That is why energy storage becomes increasingly important. In this paper, a simulation tool is introduced developed to simulate the cooperation of a grid-connected wind farm and a generic energy storage unit. The aim of the tool is to decrease the difference between forecasted and actual wind power production. After the introduction of today's forecasting methods, the rule-based simulation tool is detailed. The operation of the tool is demonstrated using data of the Hungarian wind farms. Rated power and capacity is then calculated for a generic energy storage unit that is able to keep the resulting output of Hungarian wind farms and the belonging energy storage inside the ±50% range of the forecasted power, 95% of the time.