Quantitative analysis of additional losses in low-voltage distribution networks is one of the necessary ways to achieve energy conservation and loss reduction in power systems, in response to the serious increase in additional losses caused by the disorderly access of household photovoltaics. This paper focuses on the quantitative relationship between the additional losses of household photovoltaic power generation and distribution lines after being connected to the power grid. Firstly, a theoretical analysis is conducted on the changes in characterization of additional losses for household photovoltaic (PV) access. An equivalent circuit and mathematical model are established for active low-voltage distribution networks with three-phase four-wire systems. Secondly, the study investigates the disturbance of multiple types of power quality under different capacities and phase sequences of household PV and establishes a composite power quality disturbance additional loss model. Finally, based on the typical daily PV power output curve and additional loss model, the accuracy of the proposed quantification evaluation method for household PV's additional losses in distribution networks is analyzed and verified through case studies.