This paper demonstrates the capability of CFD to accurately simulate the effect of the flow turbulence intensity on the performance of vertical-axis wind turbines (VAWT). This effect is quite important as it increases the performance of small VAWTs. For this study, two and three dimensional CFD analysis has been performed on different straight-bladed Darrieus-type rotors. Both a small and a large H-Darrieus VAWT with diameters of 0.5 m and 35 m respectively are investigated using NACA0018 blades. Computational results based on a commercial CFD code (Star CCM+) are compared with experimental measurements. Several simulations based on full URANS calculations are proposed. Firstly, the sensitivity of time step, the number of iterations by time step, and discretization schemes are studied. Secondly, the effect of turbulence intensity on the VAWT performance is simulated and compared with experimental data. Finally, results reveal that the power coefficient of a small turbine is increasing for higher turbulence intensity, up to 20%, but stops increasing afterward. For a small turbine H-Darrieus turbine, the power coefficient is increased by 22% when the turbulence intensity is changed from 0.7% to 20%, however, there is no increase detected in the case of a large H-Darrieus wind turbine. The impact of the turbulence intensity was assessed and a range of behaviors was identified.