Energy-conversion devices that generate electric energy from the movement of water have been actively studied. In case where ions play a major role in driving such devices, a clear causal relationship including the effect of ions on the operation of the device has not been clarified. Though, it is assumed that the electric signals generated when the devices are driven by a flowing water droplet or squeezed droplets are associated with the potential profile across the solid-liquid interface, the understanding of the cause and effect is unclear. The unclear understanding of the principle is a critical bottle neck for the enhancement of device performance and device application, such as ion type and concentration sensor. In this study, we investigated the correlation between the contact-area-dependent electric energy generated by ion dynamics and the electric potential across a solid-liquid interface. Further, the dependencies of the operational conditions of the device on output performance were faithfully investigated and the potential for a novel ion type or concentration sensor was introduced.