We conduct an axisymmetric, two-dimensional (r-z) magnetohydrodynamic numerical analysis of a large-scale, nonequilibrium disk-shaped MHD generator with a thermal input of 1 GW to examine the influence of a total pressure loss between the hot duct inlet and the supersonic nozzle outlet of the MHD generator on its isentropic efficiency. Numerical results show that the total pressure loss there substantially affects the isentropic efficiency of the MHD generator. Most of the total pressure loss there, which is mainly caused by a self-excited Joule heating, occurs in a localized area from the vicinity of the hot duct outlet to the radial position of the upstream edge of anode. The numerical results also indicate that the isentropic efficiency is improved by widening an anode width toward the upstream side of the supersonic nozzle. This is because the total pressure loss due to the self-excited Joule heating in the supersonic nozzle decreases by widening it.