Wind speed estimation is an important issue when addressing the control or the monitoring of a wind turbine. A realistic value of wind speed could be very useful to improve performance of wind turbine controllers, either for scheduling or as an extra feed forward term. In addition, it could also be useful for fault diagnosis and fault-tolerant control of the wind turbine. The objective of this work is to design and implement a wind speed estimator based on an Extended Kalman Filter (EKF) that uses a simplified nonlinear model and to compare the performance with experimental wind speed measurements. Even with such a simple model, the agreement with experimental data is significant, and shows great potential to be used by an advanced wind turbine controller.