The economic benefit and energy security brought by renewable energy have greatly promoted the development of microgrid (MG). However, the load demand varies from area to area, resulting in the unbalanced load distribution among MGs. With the perfection of charging facilities, electric vehicle (EV) has attracted more consumers in the automobile market. A large number of EVs connected to MGs without coordination may threaten the safe operation of the power grid. Moreover, considering the constraints of economic factors, the number of charging piles in the MG is limited, i.e., the service capacity of MG is limited. Based on this, the impact of service capacity on the load balancing and average user time is studied. The load balancing matching strategy (LBMS) is adopted to lead the EV charging behavior. We find that the larger service capacity means that the load distribution is more balanced and the average user time is shorter. Furthermore, we find the optimal service capacity of the power grid. When the service capacity is more than that, it will no longer improve the system performance and cause the waste of charging facilities.