Droplets are ubiquitous in nature and the preferential control of droplet transport offers limitless potential for efficient mass and momentum transfer as well as energy conversion. In this work, we show that even without the need for any external energy input, the self-propelled motion of droplets driven by a surface wetting gradient can lead to reliable electricity generation. Simple analytical analysis demonstrates that the output voltage results from the modulation of the surface charge distribution on the dynamically changing solid/liquid interfaces, which can be programmed by tailoring the wetting gradient and the size of the droplet. We demonstrate that a self-propelled 25 μL droplet can generate a peak current of 93.5 nA and a maximum output power of 2.4 nW. This work provides a new angle for optimizing energy harvesting devices based on liquid-solid interfaces.