Solar thermal and coal are two major energy sources in solar aided power generation (SAPG) systems. Reasonably distinguishing the solar shares of the total electricity output is a problem that must be solved. This study modeled the energy balance, exergy balance and solar exergy balance of the key subsystems in an SAPG system. Meanwhile, this study also designed a new solar share evaluation method by tracing solar exergy flows and allocating exergy destruction. How and how much solar exergy was transferred from the sun to electricity could be theoretically estimated by analyzing the distributions of solar share and solar exergy destruction. The proposed method considered energy level differences between solar energy and coal without assuming plant efficiency or solar shares in each energy stream to be constant. The proposed method could not only be used to calculate solar-generated electricity on the point of exergy transfer and destruction, but also be further used for excavating the potential of solar share expansion.