Optimised quantity of nano-aluminium (Al) filler mixed with epoxy resin using percolation threshold criteria was adapted to fabricate nanocomposite with desired space-charge and charge-trap properties. Surface potential variation due to charge deposition on to the insulating material under transient voltages was investigated, and it showed the improved decay rate with reduced trap depth for the nanocomposites with nanofillers addition in the range between 0.5 and 5 wt%. Threshold electric field calculated based on space-charge density variation by pulsed electroacoustic measurement was found to be between 7-9 kV/mm and maximum electric field calculated through polarity reversal test is about 9 kV/mm, for 5 wt% (for the optimised value of nanofiller) epoxy nano-Al composites. © The Institution of Engineering and Technology 2019