The annual energy potential of a wind site cannot be estimated with the commonly available average wind speed data. It rather involves a detailed knowledge of the probability distribution and a complex computation of its cubic power integral over the year. A site with lower average wind speed can have higher energy potential if the speed distribution is flatter. The wind speed variation with time can be accurately represented by the Weibull probability distribution function, which can be approximated by the Raleigh distribution. Since wind power has the cubic relation with the speed, the concept of the root-mean-cube (RMC) speed is defined and developed in the paper. All sites having the same RMC speed would have the same annual energy potential. An analytical expression has been derived that relates the RMC speed with the average speed. It allows a quick and easy determination of the annual energy potential by using the commonly available average wind speed data. It can also be used in a large scale computer model to estimate the annual energy capture potential of a region with known average speed contours.