Loading experiments of four electret fibrous media having various properties, including fiber diameter, porosity, thickness, charging density and fibrous structure, with fine and nanosized particles were conducted for finding out the influencing factors on efficiency decline and particle holding capacity. The four media include a commercial HVAC media rated with the minimum efficiency reporting value of 13 (MERV13), and three home-made media, including a coarse fiber (CF) layer, a melt-blown (MB) layer, and a bead-on-string (BS) layer. A theoretical model for predicting the pressure drop growth along the loading was applied and compared with that of experiments. The experimental results revealed that the charges of electret media did enhance the holding capacity. The decline of efficiency was found to closely relate to the holding capacity of each media. Higher capacity resulted in a higher decline of efficiency. The theoretical calculation illustrated that the ideal holding capacity was significantly higher than the current electret media had achieved, which indicates that it leaves much improvement of media design more desirable. Comparing the theoretical and experimental results of pressure drop growth with respect to mass load, it was found the current electret layers performed different levels of ideal depth filtration, with an order of CF > MERV13 > BS > MB. Beyond the ideal filtration, pressure drop growth raised exponentially. By utilizing their individual ideal filtration stage, three composite media consisting of 3–4 layers of these electrets with a total thickness less than 1 mm were combined and tested. At 1 in-H2O of pressure drop, the composite electret CF + MERV13 + BS achieved the highest holding capacity with 14.1 g m−2 which was at least 5–10 times higher than the existing commercial HEPA filters. The bead-on-string fibrous structure of BS enhanced the holding capacity of the composite media significantly. The newly developed electret-based composite filters performed an energy efficient filtration for fine and nanosized particles.