Highly efficient microfiber electret nonwovens, in which the fibers are electrostatically charged to complement the mechanical deposition mechanisms by electrostatic effects, are nowadays widely used in numerous applications for example to improve the indoor air quality or to reduce emissions. Simulations of the filtration efficiency of electret filters are typically based on a single fiber approach and thus imply strong simplifications. Here, a novel simulation method based on direct numerical solution of the Navier-Stokes equations and the Euler-Lagrange approach for particle tracking taking into account the real three-dimensional microscopic structure of the electret media is presented. Besides the conventional mechanical deposition mechanisms, the model also considers the Coulomb effect as well as dielectrophoresis. It furthermore allows to allocate different surface charge amounts and distributions of both polarities to the microfibers and thus to investigate their influence on the filtration efficiency. To validate the simulations, the results were compared to an empirical approach and measurements for an exemplary electret medium. In the investigated case, the results from the simulations agree well with the measurements for a bipolar charge distribution on the microfibers. The simulation results also point out that by the used empirical approach, for a given surface charge amount, the filtration efficiency is overestimated.