Evaluation of power performance has been carried out of the Bonus 600 kW Mk IV situated on the Suorva dam. The surrounding mountainous terrain does not fulfil the requirements for power performance set up by the IEC. Site calibration was not implemented in the process described in this report. As a consequence, the uncertainty of the estimated annual energy production is large as the uncertainty of the measured wind speed has been estimated to 5%. The results from the analysis of measured data have been compared with another set of measurements carried out on an identical turbine on a more suitable place for the evaluation of power performance. Large uncertainties aside, the Suorva turbine exhibits a 7% lower power performance capability. Of particular interest has been the influence of ice on the power performance. A significantly lower energy production during icing conditions has been confirmed.