In this study, we developed a thermoelectric generator unit (TEGU) for sensors measuring river water levels and set the power generation target needed for sensing and radio transmission to 38 J/day. To generate power by utilizing one-day temperature changes, it is most effective to arrange the heat storage on the cooling side of the TEGU. We investigated heat storage materials that would ensure the necessary power. In our simulation, we found the average yearly amount of power generated using a sensible heat (SH) material, e.g., alcohol solution, to be nearly twice that when using a latent heat (LH) material, e.g., paraffin. This is because LH is efficient only within a limited period when the melting point of the LH material is approximately equal to the average temperature of the heat source. Experimental results show the difference between the simulated and experimental values to be within 20% with respect to both LH and SH storage. As a demonstration project, we installed the developed TEGU on the northeastern side of the steel frame of a bridge in the city of Yokohama, and then measured the generated power from September 8, 2017. We recorded an average power generation of around 58.5 J/day, which is much larger than our target value of 38 J/day, and which remained almost constant from September 2017 to January 2018. In this paper, we present our design of the TEGU, our simulation and experimental results, and the power generation results when utilizing SH.