Harvesting clean energy from water has emerged as a promising approach to tackle sustainability challenges. Triboelectric nanogenerators (TENG) offer exceptional advantages, such as low-cost, low-frequency driving, and high output, making them a promising avenue for capturing energy from water droplets. However, there remains a dearth of research on the impact of water droplet spreading area on TENG electrical output and the underlying energy conversion mechanisms. Through a series of comprehensive experiments, we identified the optimal parameters associated with water droplet spreading area and validated the reliability of the experimental results through multi-physics field coupling. The findings underscore a direct correlation between the spreading area of water droplets and the electrical output of interdigital electrode-based triboelectric nanogenerators (IDE-based TENG). Moreover, the introduction of parameters characterizing droplet motion state and their influence on electrical output unveils the intricate energy conversion mechanisms at playing when water droplets interact with the surface of IDE-based TENG. The practical application of this research materialized in a raindrop energy harvesting device, successfully illuminating approximately 80 LED bulbs and powering a clock. This work provides a theoretical reference for analyzing the energy conversion mechanism between water droplet spreading area and TENG electrical output and can guide the design of micro-power generation devices. This work holds significant promise in advancing clean energy technologies and addressing the challenges of sustainable development.