Triboelectric Nanogenerators (TENGs) are innovative energy-harvesting devices that convert mechanical motion into electrical energy through repeated surface contact and separation. The working principle relies on the processes of triboelectric charging and electrostatic induction. At the same time, microscale surface roughness and the contact load strongly influence the generation and distribution of surface charges in TENG. A measurable increase in performance can be achieved if the actual contact area exceeds the apparent one when a soft layer is pressed against a rough, rigid surface [1]. However, experimentally measuring the actual contact area in such a scenario proves challenging. Furthermore, despite extensive research, most analytical approximations of TENG overlook fringing effects in three dimensions, i.e., the effect of electric field lines bending towards the charged surface edges [2]. This study addresses the above limitations by developing an accurate finite element model of TENG focusing on coupling between contact mechanics and electrostatics and considering the surface roughness. To calculate and observe the contact area evolution, the states of small to large slopes of surface roughness have been considered for the representative surface elements under a given contact load and material properties. The numerical results show agreement with existing analytical approximations and experimental outcomes. Moreover, the study extends to explore the output characteristics at various TENG configurations, focusing on design optimisation. These developments are undertaken using the open source parallel finite element library MoFEM [3] and contribute to developing a unified multi-physical framework of TENG, aimed to reduce experimental efforts and accelerate the design and material optimisation. REFERENCES [1] C. Kumar, J. Perris, S. Bairagi, G. Min, Y. Xu, N. Gadegaard, and D. M. Mulvihill, Multiscale in-situ quantification of the role of surface roughness and contact area using a novel Mica-PVS triboelectric nanogenerator, Nano Energy (2023). [2] Y. Xu, G. Min, N. Gadegaard, R. Dahiya, and D. M. Mulvihill, A unified contact force-dependent model for triboelectric nanogenerators accounting for surface roughness, Nano Energy 76 (2020). [3] L. Kaczmarczyk, Z. Ullah, K. Lewandowski, X. Meng, X. Zhou, I. Athanasiadis, H. Nguyen, C. Mouriesse, E. Richardson, E. Miur, A. Shvarts, C. Pearce, MoFEM: An open-source, parallel finite element library (2020).