The thermodynamic limit of photovoltaic efficiency for a single-junction solar cell can be readily predicted using the bandgap of the active light absorbing material. Such an approach overlooks the energy loss due to non-radiative electron-hole processes. We propose a practical ab initio procedure to determine the maximum efficiency of a thin-film solar cell that takes into account both radiative and non-radiative recombination. The required input includes the frequency-dependent optical absorption coefficient, as well as the capture cross sections and equilibrium populations of point defects. For kesterite-structured Cu2ZnSnS4, the radiative limit is reached for a film thickness of around 2.6 μm, where the efficiency gain due to light absorption is counterbalanced by losses due to the increase in recombination current.