Due to the challenge in measuring hot electron energy under reaction conditions, very few studies focus on experimental determination of hot carrier energy. Here, we adjust the energy state of free electrons in Au nanoparticles to quantify the hot electron energy in plasmonic photocatalysis. Reactant molecules with different reduction potentials such as 4-nitrothiophenol (4-NTP), 4-iodothiophenol (4-ITP), etc. are chosen as molecular probes to investigate the reducing ability of hot electrons. By comparing the voltage required to achieve the same conversion of photo- and electro-reaction pathways, we calibrate the maximum energy efficiency of hot electrons in 4-NTP reduction to be 0.32 eV, which is much lower than the excitation photon energy of 1.96 eV. Our work provides insight into the energy distribution of hot electrons and will be helpful for rational design of highly efficient plasmon-mediated chemical reactions.