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The purpose of this study is to investigate how much of the estimated annual electric energy production from Swedish wind power that actually was produced, as well as the history of this agreement over time. The aim is to also examine if there are properties that have caused some group of wind power plants to produce more or less, compared to their estimated production. Operational data on wind turbines collected by the Swedish Energy Agency was analyzed. A survey was made among owners of wind power plants with a nominal power of at least 2 MW. Two different methods for normal year correction of production data were used and compared. The result shows that wind power plants in the data from the Swedish Energy Agency on average had a normal year corrected production that amounted to 87 % of their estimated production during the years 2008 - 2010. The agreement between estimated and actual downtime appears to have a strong correlation to the agreement between actual and estimated production in all parts of the study. The larger wind power plants and the power plants taken into operation during the last ten years tend to have a normal year corrected production closer to their estimated production.