Based on data from 2083 wind turbines installed in Sweden since 1988, the annual energy production (AEP) predictions considered at the project planning phases of the wind turbines in Sweden have been compared to the wind-index-corrected production data. The production data and the predicted AEP data are taken from Vindstat, a database that collects information directly from wind turbine owners in Sweden. The mean error for all analyzed wind turbines was 11.9%,which means that, overall, the predicted AEP has been overestimated. There has been improved accuracy with time and error in prediction decreasing from 12% to 6.3% for wind turbines installed in the 2000s and 2010s, respectively. However, the overall improvement in accuracy seems to have stagnated around 2005 despite the refinement of forecasting methods and better data availability. From the results analyzed for effects of terrain, the error is smaller for wind turbines in forest areas than in open terrain, indicating that the complexity of forest terrain is not the reason behind the error. Also, there is no apparent increase of error with wind farm size, which could have been expected if the wind farm blockage effect was a primary reason for the overestimations. Comparison between significant wind turbine manufacturers Vestas and Enercon in the Swedish context, the error was more prominent for Enercon.