Monitoring the operation of a wind turbine

A method of monitoring an operation of a wind turbine, wherein the wind turbine is an upgraded wind turbine that comprises at least one performance enhancing upgrade that increases the energy production of the wind turbine compared to the wind turbine without the performance enhancing upgrade,

• wherein the method monitors an additional active power output of the upgraded wind turbine that is caused by the performance enhancing upgrade,
• wherein the monitoring compares an active power output of the upgraded wind turbine to an estimated active power output of a reference wind turbine that corresponds to the upgraded wind turbine without the at least one performance enhancing upgrade,
• the method comprising:
• obtaining at least two operating parameters of the upgraded wind turbine during operation, wherein the at least two operating parameters include at least the active power output of the upgraded wind turbine;
• deriving, from the at least two operating parameters of the upgraded wind turbine obtained during operation, an estimate of the active power output of the reference wind turbine;
• and
• estimating the additional active power output caused by the performance enhancing upgrade from the active power output of the upgraded wind turbine and the estimated active power output of the reference wind turbine.