With the soaring energy demands, an urge to explore the alternate and renewable energy resources has become the focal point of various active research fronts. The scientific community is revisiting the inkling to tap the wind resources in more rigorous and novel ways. Recent idea of net-zero buildings has prompted the realization of novel ideas such as employment of omni-directional vertical-axis wind turbine (VAWT) for roof-top application etc. Generally, owing to the high computational cost and time, different levels of geometric simplifications are considered in numerical studies. It becomes very important to quantify the effect of these approximations for realistic and logical conclusions. The detailed performance of a 2.5 m diameter VAWT is sequentially presented with various levels of approximations spanning from two-dimensional to complete three-dimensional geometry. The performance along with the flow physics with focus on tip effects, spanwise flow effects, effect of supporting arms and central hub is discussed. We conclude that two-dimensional approximation can over predict the performance by 32%. Similar trend is also observed for other geometric and flow approximations. (C) 2014 Elsevier Ltd. All rights reserved.