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