An appropriate design of solar thermal power plant (STPP) is important before construction. This present study attempted to deal with some fundamental questions in design of STPP. What is the relationship between the optimal design direct normal irradiance (DNI) and weather conditions? What is the optimal solar multiple for a STPP at a specific weather condition? How does the thermal energy storage (TES) affect the electricity output of STPP, and how does dispatch fraction affect the usage of stored thermal energy? To help answer these questions, a general model was built and a critical design DNI was proposed from the perspective of effective use of solar energy. There will be solar energy wasting if the design DNI is below the critical design DNI, while the power block will not be effectively used if the design DNI is above the critical design DNI. This critical design DNI can be used to determine the optimal solar multiple for a given design DNI. The effects of TES size and dispatch fraction on the critical design DNI were also discussed.