A method of monitoring the efficiency of a wind farm, the method comprising the following process steps: - measuring at least one wind speed and at least one wind direction during a specifiable time period, - measuring an output power of the wind farm in the specifiable time period, - associating the output power with the measured wind speed and the measured wind direction, - storing the output power as an output power value in a first power matrix, in which matrix elements are associated with various parameters comprising at least wind speeds and wind directions, as long as the output power represents a representative value for the wind farm, characterized in that the method comprises the following further process steps: - providing a first counting matrix having matrix elements which are associated with the various parameters comprising at least wind speeds and wind directions, - adding a counting value to the matrix element of the first counting matrix that corresponds to the measured wind speed and the measured wind direction, - wherein the preceding process steps are carried out for a plurality of specifiable time periods in a reference time period, - wherein the following process steps are carried out in a later monitoring time period for a plurality of specifiable time periods: - measuring at least the wind speed and the wind direction during the specifiable time period, - measuring an output power of the wind farm in the specifiable time period, - providing a second counting matrix having matrix elements that are associated with the various parameters comprising at least wind speeds and wind directions, - adding a counting value to the matrix element of the second counting matrix that corresponds to the measured wind speed and the measured wind direction, - wherein, in order to monitor the efficiency, a comparison is performed between the output power values of the first power matrix weighted with the second counting matrix, and the sum of the output power values measured during the monitoring time period. - measuring at least one wind speed and at least one wind direction during a specifiable time period, - measuring an output power of the wind farm in the specifiable time period, - associating the output power with the measured wind speed and the measured wind direction, - storing the output power as an output power value in a first power matrix, in which matrix elements are associated with various parameters comprising at least wind speeds and wind directions, as long as the output power represents a representative value for the wind farm, - providing a first counting matrix having matrix elements which are associated with the various parameters comprising at least wind speeds and wind directions, - adding a counting value to the matrix element of the first counting matrix that corresponds to the measured wind speed and the measured wind direction, - wherein the preceding process steps are carried out for a plurality of specifiable time periods in a reference time period, - wherein the following process steps are carried out in a later monitoring time period for a plurality of specifiable time periods: - measuring at least the wind speed and the wind direction during the specifiable time period, - measuring an output power of the wind farm in the specifiable time period, - providing a second counting matrix having matrix elements that are associated with the various parameters comprising at least wind speeds and wind directions, - adding a counting value to the matrix element of the second counting matrix that corresponds to the measured wind speed and the measured wind direction, - measuring at least the wind speed and the wind direction during the specifiable time period, - measuring an output power of the wind farm in the specifiable time period, - providing a second counting matrix having matrix elements that are associated with the various parameters comprising at least wind speeds and wind directions, - adding a counting value to the matrix element of the second counting matrix that corresponds to the measured wind speed and the measured wind direction, - wherein, in order to monitor the efficiency, a comparison is performed between the output power values of the first power matrix weighted with the second counting matrix, and the sum of the output power values measured during the monitoring time period.