The design of a solar cell should have into consideration a multiplicity of factors, including the incorporation or not of the BSF (back surface field), surface passivation and the contact grids. The cell series resistance plays an important role on efficiency determination. Thus, the cell structure and contact grids should be carefully designed taking into account the mentioned factors. The contacted fraction, defined as metal-semiconductor contacted area divided by total cell area, emerges as a key parameter to be optimized because it affects simultaneously the photogenerated current and the series resistance of the solar cell. Here we present a detailed analysis of the different contributions to the series resistance of a c-Si solar cell considering different scenarios. Using a proposed model to describe the cell behaviour based on a 1-D approach, the contacted fraction of the rear contact was optimized for cells with and without BSF. The results obtained show a larger series resistance for cells without BSF leading to higher values of optimum contacted fraction. The dependence of this optimum on cell thickness and base resistivity was also analyzed. In cells with BSF was found a weak dependence on both parameters, while in cells without BSF this dependence was more pronounced. Finally, the influence of metal-semiconductor contact resistance was considered. A higher limit of 0.01 Ω cm² was found to the metal-semiconductor resistance without affecting seriously the efficiency of the device.