As the integration of renewable energy sources (RES) such as wind and solar power into the power grid increases, the primary challenge lies in the high integration costs and the complexity of quantifying the dynamic capabilities of these resources. This paper proposes the formation of Aggregated Generation Units (AGUs) that combine RES and energy storage systems (ESS) to address these challenges. Utilizing coalition game theory, this paper outlines the member selection and aggregation processes within AGUs, establishing a two-level comprehensive evaluation index system to quantify the aggregation degree of RES and ESS. The proposed index system includes resource complementarity, utilization, response flexibility, and economic efficiency. A case study based on a RES aggregation project in Lishui, China demonstrates the effectiveness of the proposed index system. The results indicate that well-balanced and diverse resource aggregation significantly improves the performance and reliability of power supply.