Electric spacecraft performance depends critically on the mass of the electric-propulsion system. The electric powerplant often is assumed to be the major component of such systems. It is shown, however, that when power conversion, power conditioning, and thrusters are included in mission analysis, the total effective specific mass aps*(kg/kwj) of the electric-propulsion system may be much higher than that of the electric powerplant alone, resulting in significant loss of payload or increase in trip time. A method of system analysis is presented which clearly evaluates the contribution of each subsystem to the total propulsion system mass.