A triple-objective design method was developed for a solar chimney power plant system that simultaneously optimizes the expenditure, total efficiency, and power output. A multi-objective genetic algorithm was used to obtain the best combination of geometric parameters of the power plant. The following design parameters were selected: collector radius, chimney height, and chimney diameter. Two different solar chimney power plant configurations were considered: the Kerman pilot power plant and Manzanares prototype power plant. A set of possible optimal solutions (Pareto optimal set) was obtained. Based on the optimal solutions, the best configuration for each power plant was selected. The performance and expenditure of the optimal solutions and the built power plants were compared. The results showed that the increment of the power output was higher than the increment of the expenditure in the optimal configuration. A parametric study was conducted to evaluate the effects of changing design parameters on different objective functions. This paper provides a very useful design and optimization methodology for solar chimney power plant systems.