The development of thin-film solar cells has led the researches to evaluate different materials to improve the conversion efficiency, avoid the use of non toxic raw materials or reduce the manufacturing costs. One option is to use simulation tools, which allow the variation of solar cell materials, parameters, dimensions and other variables in order to evaluate performance indicators of the cell without wasting material. This paper introduces a mathematical model for kesterite based thin-film solar cells with both theoretical and experimental solar irradiance spectra. The model is implemented in Matlab and it is used to evaluate four performance indicators: open-circuit voltage, short-circuit current density, conversion efficiency and fill factor. Those indicators are evaluated for a ZnO/CZTSSe/buffer solar cell with buffer layers implemented with CdS and ZnS considering different thickness. The results show that all performance indicators are significantly improved with the ZnS buffer layer; moreover, the results also evidence that the irradiance model affects the performance indicators. Particularly, experimental based irradiance model improves the performance indicators for both evaluated layers. Finally, the thickness does not affect considerably the performance of the cell with ZnS buffer layer, while a higher thickness reduces the performance of the cell with CdS buffer layer.