In this paper, we present the design of an estimator for the assessment of the power coefficient of an offline wind turbine in a variable wind turbine generator system (WTGS) using a direct drive permanent magnet synchronous generator. It is shown that the estimator is supplying accurate estimates of the power coefficient. A further advantage of the estimator design presented is that it can be easily connected to WTGS where different types of generators and turbines are employed. The simulation results are presented using a graphic user interface and MATLAB Simulink.