Although solar photovoltaics (PV) are recognized as a promising source of clean energy production, researchers and policy makers need to know the optimum level of solar PV capacity penetration into the existing generation structure under the current fuel mix for the region. As the level of installed PV capacity increases, it is possible that the aggregated generation mix could produce electrical power exceeding electrical demand, thus requiring generator curtailment. Therefore, determining the optimum penetration of large-scale PV and aggregated technical and economic benefits is becoming an issue for both power utilities and policy makers. We report the development and validation of a new methodology for assessing the optimum capacity and benefits of state-wide grid-connected large scale solar PV systems in Illinois. The solar carve-out portion of the current renewable portfolio standard is also evaluated within the context of the state's sustainable energy plan for the near term future. Published by Elsevier Ltd.