Evaporation induced electric potential in functional materials has been promoting the emergence of a new discipline, hydrovoltaics1,2, but the phenomena have been widely ascribed to streaming potential related classic electrokinetic effects3,4. Here we show that evaporation in ambient environment can directly generate a sustainable voltage of up to 1 V with a current density of around 20 nA cm-2 from a porous carbon black film without contribution from streaming potential. Multi-electrode measurements confirm that the evaporating potential is generated mainly within the precursor ahead of the main capillary front. Detailed theoretical and experimental analyses indicate that the evaporating potential following the quadratic distribution arises from molecule evaporation induced carrier transfer in the precursor film. This new finding should lay a solid foundation for the emerging hydrovoltaics.