Abstract: Utilizing V2G technology, plug-in electric vehicles(PEVs) can behave as mobile energy storage units to participate in power grid operation. Based on the traffic behavior characteristics, the V2G response capability boundaries of PEV considering time, energy and battery restraint is constructed, and the V2G response capacity models for individual PEV and PEV groups are built respectively. Besides, the value-at-risk(VaR) method is introduced to establish a reliability-at-risk(RaR) model to calculate the conventional storage capacity under a certain confidence level of power system reliability .