The optimal turbine pressure drop ratio f(opt) for a solar chimney power plant (SCPP) is investigated using an analytical approach and 3D numerical simulations. Results indicate that the solar radiation and ambient temperature have obvious influences to the optimal turbine pressure drop ratio f(opt) and an improved performance of the SCPP system leads to a high f(opt). The performance comparison between the cases with different collector shapes, a circular collector and a square collector with the same area, is conducted for the first time. It is found that the m-th power law assumption can also be applied to the SCPP system with a square collector; the values of f(opt) for the cases with a square collector are close to those for the cases with a circular collector. A fitting equation with variables of solar radiation and ambient temperature is obtained using the simulation results of the Spanish prototype; the results reveal that the f(opt) of the Spanish prototype ranges from 0.90 to 0.94 under normal climate conditions. This paper provides an approach to the preliminary estimation of plant performance, and reference data for an optimal pressure drop through solar chimney turbines. (C) 2015 Elsevier Ltd. All rights reserved.