With the high penetration of renewable energy in modern power system,grid-side energy storage can support power system operation as a flexible resource. Evaluating the value of energy storage is a necessary preparation for its capacity sizing. Three value indexes that reflect the value on renewable energy utilization,power system flexibility,and dispatch economy are defined at first. Then,a value evaluation method based on multi-parametric mixed integer linear programming theory is proposed and applied to above indexes. As a result,each index is represented as an analytical piecewise linear function of the power and energy capacity of storage. Finally,the proposed models and method are verified on the Northeast Power Grid data. The simulation results show that proposed method can analytically describe how the value of energy storage is affected by its capacity,along with visualization. The analytical expression contains profound sensitivity information,which reveals the bottleneck of energy storage value and provides insightful reference for energy storage capacity sizing in different application scenarios.