Assumption-based offline analysis tools may not be able to provide sufficient and accurate information for the corrective decision making to mitigate solar photovoltaic (PV) impacts in the future distribution grids. This is mainly due to the increasing penetration level of intermittent power generation resources and also the fluctuating behavior of consumer demand. Online assessment tools can assist to manage PV impacts and aid to mitigate those on a real-time basis. This paper proposes an approach for online assessment of rooftop PV impacts on low-voltage (LV) networks using real-time network data. A variable-width sliding window will be used to provide the analysis an outcome-based online data. The width of the sliding window can be varied according to user input so that the changes in network behavior caused by PV integration can be investigated conveniently. Several numerical indices are proposed in this paper to assess solar PV impacts on the LV networks. This approach also uses the online data to develop real-time distribution network models for a dynamic "what-if" analysis. The usefulness of the proposed online assessment approach is verified using an Australian LV distribution feeder.