Spacecraft using solar electric propulsion must be able to continue the mission in the event the spacecraft enters safe mode and thrust is unexpectedly stopped. To help meet this requirement, a Monte Carlo method is proposed to estimate the propellant and schedule margin a mission should carry to account for missed thrust scenarios, including multiple thrust outages. This method is used in conjunction with a statistical model of safe mode events based on data collected from over 20 past and current planetary missions and is applied to an example trajectory from Earth to Mars.