<p>Light can charge plasmonic nanoparticles by photoredox reactions, significantly modifying their optical and chemical properties. However, the charging process has been challenging to track experimentally, severely hindering its thorough evaluation. In this study, we investigate the charging of gold nanorods during a light-induced reaction in situ, utilizing the sensitivity of the rods’ longitudinal localized surface plasmon resonance to charge accumulation. Describing the particles as nanocapacitors, we present a model to quantify the number of charges on the particles and their connection to the illumination intensity. We find that the Fermi level, together with all other energy bands, is raised because of the repulsive potential of the additional charges. Experimental observations of the dependence on the solvent, the particle size, and ligand type further corroborate the proposed capacitor model. The results presented in this study lay the groundwork for the rational engineering of dynamic charge accumulation during plasmon-driven photoreactions.</p>