Evaporation-driven hydrovoltaics (EDHV) harness the flow of water and interfacial electrokinetic phenomena to convert heat and light into electrical energy, offering a sustainable route to decentralized power generation. Despite promising device demonstrations, progress has been constrained by an incomplete understanding of nanoscale interfacial phenomena and by the lack of predictive design tools. Here, we present a unified experimental, spectroscopic, and theoretical framework that elucidates the coupled physical and chemical processes underlying evaporation-driven, thermally and light-assisted HV conversion across diverse material platforms. Using ordered silicon nanopillar and nanodisk architectures, together with a decoupled device layout incorporating an ion-conducting interlayer, we spatially and functionally separate the evaporative top interface from the nanostructured electrode, thereby independently controlling evaporation, ion transport, and interfacial chemical equilibria. This enables us to construct a general physical framework for understanding how light and heat drive energy conversion in hydrovoltaic systems, beyond the specifics of any one material. We uncover that the thermal and photo-induced charge carriers can dynamically modulate the surface chemical equilibrium in a coupled manner, which is governed by the bulk properties of the solid and liquid. Our multiphysics simulations, along with an analytically derived equivalent-circuit model validated by experiments, identify capacitive photocharging and chemical equilibrium-controlled surface potentials as the dominant energy-conversion pathways. This analysis predicts non-monotonic, concentration-dependent voltage responses at intermediate concentrations and charge-inversion phenomena at high salinities. In-situ, interface-sensitive second-harmonic generation (SHG) spectroscopy provides a direct optical voltmeter for locally monitoring the solidâ liquid interface. Nanostructured arrays exhibit dramatic (>200 fold) SHG enhancements relative to planar films, owing to the synergistic coupling of amplified local electromagnetic fields with electrical double-layer fields, enabling the quantitative tracking of ion adsorption and geometry-dependent surface potential. Under different light irradiation, operando SHG directly tracks reversible shifts in interfacial equilibrium that manifest as photovoltages and temporally complex potential dynamics; these shifts scale with geometric confinement and local field enhancement, establishing a clear optical fingerprint of capacitive photocharging and light-driven modulation of surface chemical equilibrium. Complementary studies on graphene and 2D materials reveal three microscopic photoresponsive mechanismsâ capacitive photogating, contact-dependent photovoltages, and trap-assisted carrier dynamicsâ that modulate surface charge under illumination, enabling novel pathways for ion-electron coupling. These findings show that EDHV devices can function across a wide salinity range and under combined thermal and optical stimuli, achieving high voltages and competitive power densities in optimized geometries and adequately engineered interfaces. By uniting spectroscopy, modelling, materials engineering, and device architecture, we deliver mechanistic insight and practical design rulesâ spanning surface chemistry, bulk properties of solid, and nanoscale geometryâ that enable targeted optimization of performance in more practical scenarios.