Existing studies have shown that particle collection efficiency of a charged filter tends to decrease with particle loading to a certain extent, then increase with further loading. This contrasts with pressure drop which monotonically increases with particle loading. This trend in particle collection efficiency is due to a variety of factors including, but not limited to, neutralization and aerodynamic shielding of the fibers' electrostatic field. The current paper presents a semi-empirical macroscale simulation method to predict the instantaneous pressure drop and particle collection efficiency of an electrostatically charged filter during the early stages of particle loading. The simulations were performed by using ANSYS software enhanced with a series of in-house subroutines. The simulation results are compared with experimental data (for calibration and validation) obtained from testing a bipolarly-charged (55 μC m−2) polypropylene filter exposed to different levels of nanoparticle loadings. The filter media was loaded (both experimentally and computationally) with polydisperse NaCl nanoparticles (count median diameter of 75 nm) having charge values of ±1e. The loaded media were then tested (experimentally and computationally) with NaCl nanoparticles spanning 10 nm–500 nm in electrical mobility diameter (from TSI 3160 filter tester) having a Fuchs charge distribution. In addition, a high-porosity conditional factor was developed for the Kozeny-Carman permeability equation to expand its application to the case of nanoparticle deposits, where the dendrites’ porosity is very high, and aerodynamic slip is expected to occur.