Numerous nanoscale studies that are related to harnessing photon energy focus on quantum effects. Thermodynamics analyses indicate the occurrence of a paradox for the standard model of the photocell with the power generated by a decay process. In order to measure the power accurately, a light-harvesting system connecting to Fermi contacts is proposed. Results show that the interference effect between different transition channels plays a decisive role in enhancing the power output of a photocell. The proposed model may provide a foundation for the future development of photoelectric converters.