An analytical model has been developed and used to determine solar irradiation on flat collectors augmented with planar reflectors. The model uses measured insolation data from the NREL National Solar Radiation Data Base. In addition the model accounts for direct and reflected components of beam, diffuse and ground reflected insolation, considers finite length systems, accounts for shading of the collector by the reflector, and considers different configurations of the reflector relative to the collector. Thus the model represents an extension and refinement of many previous models. Computer simulations have been carried out over different time spans to assess the effects of including these features. In general the addition of reflectors enhances the radiation received by the collector. Reflected beam radiation is the major contributor to the enhancement, although reflected sky diffuse and ground reflected radiation can be significant in many instances. To optimize effectively the design of the system with respect to selecting tilt angles and specifying a reflector-above or below-collector configuration requires proper modeling of the energy received by the collector system. Finite length analysis used in the model enables the optimization of reflector size relative to that of the collector. The model presented can be the basis of an analytical design tool for optimizing the design of a variety of reflector-enhanced solar energy collection systems.