A solar-fuel generator is a complex device involving different species in multiple phases distributed under varying temperature and pressure over a broad range of scales of dimensions from nanometers to 100s of centimeters. Due to the coupling of a multitude of processes, any change in one property or component may affect the solar-fuel production reaction and consequently overall device efficiency and operation. It is difficult to comprehend intuitively these typically nonlinear effects and consequences on device operation. In this aspect, mathematical modeling is ideally suited to play a significant role in the research and development of devices, cells, and their components.