One of the main problems when designing a solar thermal power plant is finding the optimal parabolic trough field size. Errors made in this context can easily lead to financial disaster. Simulation tools that handle all aspects of a power plant (technical as well as economical) treat such economical problems as a whole and can be very helpful during the design process. However, even the smartest simulation tool depends significantly on the input parameters, such as the solar irradiance. As a result of the given considerations, a new method for estimating the optimized solar field size as a function of the solar irradiance is proposed. Additionally, this paper demonstrates a path to simulate the complexity of a parabolic trough power plant.