Abstract In a solar chimney power plant, only a fraction of the available total pressure difference can be used to run the turbine to generate electrical power. The optimal ratio of the turbine pressure drop to the available total pressure difference in a solar chimney system is investigated using theoretical analysis and 3D numerical simulations. The values found in the literature for the optimal ratio vary between 2/3 and 0.97. In this study, however, the optimal ratio was found to vary with the intensity of solar radiation, and to be around 0.9 for the Spanish prototype. In addition, the optimal ratios obtained from the analytical approach are close to those from the numerical simulation and their differences are mainly caused by the neglect of aerodynamic losses associated with skin friction, flow separation, and secondary flow in the theoretical analysis. This study may be useful for the preliminary estimation of power plant performance and the power-regulating strategy option for solar chimney turbines.