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 power performance data for each of the monitored wind turbines (2, 5), during the training period, - for each of the monitored wind turbines (2, 5), generating a transfer function establishing a relationship between the locally measured wind speeds at each of the reference wind turbines (2, 3, 4, 6) and the power performance data for the monitored wind turbine (2, 5), based on locally measured wind speeds and power performance data obtained during the training period, - 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, - obtaining power performance data for each of the monitored wind turbines (2, 5), during the test period(s), - for each of the monitored wind turbines (2, 5), comparing the power performance data obtained during the test period(s) to predicted power performance data obtained by means of the locally measured wind speeds at the corresponding reference wind turbines (2, 3, 4, 6) during the test period(s) and the previously generated transfer function for the monitored wind turbine (2, 5), and - assessing power performance changes for each of the monitored wind turbines (2, 5), based on 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 power performance data for each of the monitored wind turbines (2, 5), during the training period, - for each of the monitored wind turbines (2, 5), generating a transfer function establishing a relationship between the locally measured wind speeds at each of the reference wind turbines (2, 3, 4, 6) and the power performance data for the monitored wind turbine (2, 5), based on locally measured wind speeds and power performance data obtained during the training period, - 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, - obtaining power performance data for each of the monitored wind turbines (2, 5), during the test period(s), - for each of the monitored wind turbines (2, 5), comparing the power performance data obtained during the test period(s) to predicted power performance data obtained by means of the locally measured wind speeds at the corresponding reference wind turbines (2, 3, 4, 6) during the test period(s) and the previously generated transfer function for the monitored wind turbine (2, 5), and - assessing power performance changes for each of the monitored wind turbines (2, 5), based on the comparison.