This final e-book of the 3-part series focuses on measuring change in power performance. There is a brief review of the Power Curve, Annual Energy Production and common methods of measuring the absolute level of power performance. The alternatives for measuring change in power performance for a specific site are narrowed down. An in-depth case study is performed, presenting the methodology and results of the Side-by-Side Testing method. Using only SCADA data for the analysis, the effect of a blade add-on upgrade is evaluated. The analysis is based on a relative power relation between a test and reference wind turbine.