The performance prediction of straight bladed vertical axis wind turbines is always of interest in the wind power generation industry. Computational Fluid Dynamic (CFD) is performed to explore the geometric design space, such as a turbine diameter, a blade length, and a wing section. The objective is to relate these design parameters to the maximum power coefficient of the H-type Darrieus vertical axis wind turbine. This analysis is based on data already available in the literature complemented by additional CFD runs to enhance the number of turbines investigated. By analyzing these results, two main geometric parameters, the blade thickness and large aspect ratio, are identified as the main parameters that affect the turbine’s power coefficient. This confirms the importance of the blade geometry in the performance of vertical axis wind turbines. Furthermore, some model equations are proposed to predict a power coefficient without any large simulations. The suggested model equations can roughly predict the power coefficients; moreover, it can quantify the effect of the blade thickness and the blade aspect ratio.