A method for monitoring and assessing power performance changes of one or more wind turbines (2, 3, 4, 5, 6, 7, 8) of a wind farm, the method comprising the steps of: - for each wind turbine (2, 5) to be monitored, defining a group of reference wind turbines (2, 3, 4, 6), the group of reference wind turbines (2, 3, 4, 6) comprising two or more wind turbines, - operating the wind turbines (2, 3, 4, 5, 6, 7, 8) of the wind farm, while obtaining locally measured wind speeds at each of the wind turbines (2, 3, 4, 5, 6, 7, 8), during a training period, - obtaining the power performance data in relation to the locally measured wind speed for each of the monitored wind turbines (2, 5), during the training period, - for each of the monitored wind turbines (2, 5), generating a wind speed transfer function establishing a relationship between the locally measured wind speeds at each of the reference wind turbines (2, 3, 4, 6) and the locally measured wind speed at the monitored wind turbine (2, 5), based on the locally measured wind speeds obtained during the test periods, - operating the wind turbines (2, 3, 4, 5, 6, 7, 8) of the wind farm, while obtaining locally measured wind speeds, at least at the reference wind turbines (2, 3, 4, 6) during one or more test periods following the training period, - estimate the wind speed for the monitored wind turbines (2, 5) during the test periods based on the measured wind speeds at the reference wind turbines (2, 3, 4, 6) during the test period(s), and the transfer function generated during the training period for the monitored wind turbine (2, 5), - obtain, during each of the test period(s), power performance data for each of the monitored wind turbines (2, 5) in relation to the estimated wind speed, - compare, for each of the monitored wind turbines (2, 5), the power performance data obtained during the test periods with the power performance data obtained during the training period, and - assessing power performance for each of the monitored wind turbines, based the comparison. - for each wind turbine (2, 5) to be monitored, defining a group of reference wind turbines (2, 3, 4, 6), the group of reference wind turbines (2, 3, 4, 6) comprising two or more wind turbines, - operating the wind turbines (2, 3, 4, 5, 6, 7, 8) of the wind farm, while obtaining locally measured wind speeds at each of the wind turbines (2, 3, 4, 5, 6, 7, 8), during a training period, - obtaining the power performance data in relation to the locally measured wind speed for each of the monitored wind turbines (2, 5), during the training period, - for each of the monitored wind turbines (2, 5), generating a wind speed transfer function establishing a relationship between the locally measured wind speeds at each of the reference wind turbines (2, 3, 4, 6) and the locally measured wind speed at the monitored wind turbine (2, 5), based on the locally measured wind speeds obtained during the test periods, - operating the wind turbines (2, 3, 4, 5, 6, 7, 8) of the wind farm, while obtaining locally measured wind speeds, at least at the reference wind turbines (2, 3, 4, 6) during one or more test periods following the training period, - estimate the wind speed for the monitored wind turbines (2, 5) during the test periods based on the measured wind speeds at the reference wind turbines (2, 3, 4, 6) during the test period(s), and the transfer function generated during the training period for the monitored wind turbine (2, 5), - obtain, during each of the test period(s), power performance data for each of the monitored wind turbines (2, 5) in relation to the estimated wind speed, - compare, for each of the monitored wind turbines (2, 5), the power performance data obtained during the test periods with the power performance data obtained during the training period, and - assessing power performance for each of the monitored wind turbines, based the comparison.