Triboelectric nanogenerators (TENGs) have emerged as promising devices for converting mechanical energy into electrical energy through contact electrification and electrostatic induction. However, the generated energy, unlike instantaneous power, current and voltage, is rarely addressed in the vibrant research field of TENGs. In this study, we investigate Intrusion–Extrusion Triboelectric Nanogenerators (IE-TENGs) based on nanoporous silicon monoliths and non-wetting liquids (i.e., water and a 1   mg/mL polyethylenimine solution), addressing the energy generated during this process, conversion efficiency as well as the mechanism underlying the observed phenomena. Compared to powder-based IE-TENGs, the use of monolithic silicon structures enables more efficient and reproducible energy harvesting, with significant improvements in both instantaneous power density and energy per cycle. We also analyzed the impact of compression rate and liquid properties on electrical output, showing that higher compression rates improve power generation, while modifying the liquid medium significantly improves conversion efficiency, reaching up to 9%. Furthermore, through computational analysis, we identify the crucial role of grafting defects on the generated triboelectric output. This work introduces a novel approach to triboelectric energy harvesting by implementing a monolithic nanoporous architecture and offering an alternative pathway for enhancing contact electrification via confined solid–liquid interfaces. These findings provide new insights into the triboelectric behavior of porous systems and pave the way for next-generation high-performance IE-TENGs, with potential applications in wearable electronics, environmental energy harvesting, and self-powered sensing systems.