Energy conversion by water flowing over graphene is a promising mode of energy harvesting. However, the mechanism, energy-conversion efficiency, and quantification of power density for this mode of interfacial electricity generation remain unknown. Quantification of the output performance for the flow-induced electricity generation using graphene is presented in this work. The output performance per unit contact area between water and graphene is found to be proportional to the flow speed, with an electromotive force density of 0.0025 [μ V / mm 2 / mm / s] for the conditions used in this work. The ability to quantify output density will help to construct guidelines for future applications of this form of electricity generation.