The energy yield assessment is carried out during the development phase of a project and as part of the feasibility study, aiming to predict the energy production of the wind farm over its lifetime. Using mathematical models and wind flow simulation tools, the theoretical expected annual energy production is determined along with the uncertainty at various confidence levels. Important parameters such us long-term wind distribution, technical availability, energy losses are taken into account. A prerequisite of a low uncertainty energy yield assessment study is the performance of an adequate wind potential measurement campaign.