This study develops a wind speed sensorless performance monitoring method for stand-alone vertical axis wind turbines, by means of a numerical analysis using a dynamic simulation model. The method focuses on improvements in the response speed of the rotor with deterioration in wind turbine performance, by decreasing the load torque level for a constant tip-speed ratio operation. The method is unique and original because it can detect the deterioration in the wind turbine performance without any anemometer. First, a numerical analysis is conducted on the operating behaviors of a stand-alone system using a straight-wing vertical axis wind turbine with performance deterioration under decreases in the load torque level. Then, a method is developed to detect the deterioration in the wind turbine performance. In this method, the response speeds of the rotor before and after decreases in the load torque level are compared using the wind speed estimated from the rate of change for the rotational speed. Finally, a performance monitoring algorithm is constructed, and its effectiveness and limitations for detecting the gradual deterioration in the wind turbine performance are discussed. (C) 2012 Elsevier Ltd. All rights reserved.