The work considers partial discharge activity on the insulator surface that is partially covered with water (conductive zones). A discharge is characterized by the magnitude of the apparent charge passing through the system when the conductive zones short out with a spark. The present work shows how various configurations of conductive zones (their size, distance between them, location on the insulator) affect the mean interzone electric field and the apparent charge. Attention is paid not only to the onset of discharge activity when the surface of the insulator is still hydrophobic, but also to the case when the hydrophobicity is lost and discharges occur between conductive films covering a large area of the insulator. The research is carried out with the help of computer simulation using Comsol Multiphysics software package. The results show that the characteristic apparent-charge value indicates the state of the insulator surface: there are only droplets, isolated shed-sized conducting zones, or large conducting zones that cover most of the surface.