Abstract: Based on the energy index,which is an important indicator of the voltage sag problem, reference benchmarks for grading voltage sag severity for the system side and equipment side are given respectively in this paper, and the pros and cons of the system power supply quality and the response of the equipment to the voltage sag are visually shown. The grading evaluation criterion of system voltage sag severity is determined based on 484 measured energy index values of sag events as the sample data. Then the maximum entropy principle is used to establish the most reasonable energy index value distribution of the system, and division of P₉₅, P₇₅, P₅₀ and P₂₅ are taken as the grading evaluation reference value. For the grading evaluation criterion of system voltage sag severity at equipment side, the fault rate model of integrated load is established based on the energy index, and the reference levels for division of P₉₅, P₇₅, P₅₀ and P₂₅ are obtained respectively. In the end, the comprehensive severity index based on energy index is put forward, which comprehensively considers the seriousness of voltage sag at the system side and the abnormal operation degree at the equipment side. Both are respectively reflected by the energy index and the equipment failure membership function, which can be used for comprehensive assessment of actual voltage sags.