Accurate prediction of the performance of a vertical-axis wind turbine (VAWT) using Computational Fluid Dynamics (CFD) simulation requires a domain size that is large enough to minimize the effects of blockage and uncertainties in the boundary conditions on the results. It also requires the employment of a sufficiently fine azimuthal increment (d theta) combined with a grid size at which essential flow characteristics can be accurately resolved. The current study systematically investigates the effect of the domain size and azimuthal increment on the performance of a 2-bladed VAWT operating at a moderate tip speed ratio of 4.5 using 2-dimensional and 2.5-dimensional simulations with the unsteady Reynolds-averaged Navier-Stokes (URANS). The grid dependence of the results is studied Using three systematically refined grids. The turbine has a low solidity of 0.12 and a swept area of 1 m(2). Refining de from 10.0 degrees to 0.5 degrees results in a significant (approximate to 43%) increase in the predicted power coefficient (Cp) while the effect is negligible (approximate to 0.25%) with further refinement from 0.5 degrees to 0.05 degrees at the given lambda. Furthermore, a distance from the turbine center to the domain inlet and outlet of 10D (D: diameter of turbine) each, a domain width of 20D and a diameter of the rotating core of 1.5D are found to be safe choices to minimize the effects of blockage and uncertainty in the boundary conditions on the results. (C) 2017 The Authors. Published by Elsevier Ltd.