Contamination and erosion at the leading edge of blade tips adversely affect the annual energy production (AEP) of wind turbines. Obtaining pertinent quantitative data would help in the efficient planning of operations and maintenance of these turbines. In this study, to quantitatively analyze the effects of contamination and erosion on the aerodynamic performance of a blade tip airfoil (NACA 64-618) and in turn on the AEP loss of wind turbines, transient computational fluid dynamics simulations and AEP calculations were performed for a 5-MW National Renewable Energy Laboratory wind turbine. The simulations indicated that depending on the severity of the conditions, contamination and erosion conditions reduced and increased the lift and drag coefficients, respectively, of the blade by up to 53% and 314%. Contamination and erosion conditions reduced AEP by 2%-3.7%. (C) 2017 Elsevier Ltd. All rights reserved.