The aerodynamic performance of an upwind, three-bladed, small horizontal axis wind turbine (HAWT) rotor of small diameter will be investigated experimentally and theoretically to assess the applicability of the blade element momentum (BEM) theory for modeling the rotor performance for the case of small HAWTs. Theory of operation: The wind turbine will be tested in the low and high-speed sections of the Boundary Layer Wind Tunnel in order to determine the power curve over a wide range of wind speeds. Afterward, the BEM theory was implemented to evaluate the rotor performance and to investigate three-dimensionality effects on power prediction by the theory.