Harnessing natural evaporation offers a sustainable pathway for next-generation energy technologies. We present a unified physical and experimental framework for evaporation-driven hydrovoltaic (EDHV) systems that decouples and controls the key interfacial processes underlying electricity generation from heat and sunlight. An intermediate ion-conducting layer separates the evaporative top interface from the silicon–dielectric nanopillar array, enabling independent modulation of evaporation, ion transport, and interfacial chemical equilibrium. This strategy enhances performance and clarifies mechanisms governing thermal and photo-induced charge generation, improving ion migration and electricity output. We develop a predictive equivalent-circuit model that captures process coupling through an analytically derived transfer capacitance. Our results show that capacitive photocharging and thermally modulated surface equilibria—rather than faradaic or photothermal effects—dominate energy conversion. The device achieves 1 V open-circuit voltage and 0.25 W/m² power density, with silicon doping and dielectric choice further boosting performance. These findings inform EDHV optimization across environmental and material conditions.