Photovoltaic (PV) energy-yield loss due to solar module soiling has become increasingly important as solar module deployment is now at the hundreds of gigawatts scale and continues to grow rapidly. The electrostatic attraction and adhesion force (Fes) of dust particles by the high voltages from solar panels have been reported to be 1 to 2 orders of magnitude stronger than the van der Waals and water capillary forces, which has been corroborated by observing the increase in system voltage-induced soiling rate by setting up an outdoor test. Here, we report another characteristic of Fes on soiling—long-lasting or slow decay after turning off the high voltage applied to solar panels. The Fes decay time varies across a wide time range of 1 to 10 h, depending on two factors: 1) whether the cell, the particle, or both were charged with high voltage before the voltages were turned off; and 2) how the cell was connected to the ground after the voltage was turned off—either connected through the power-supply electronics, directly connected to the ground, or electrically floated. The Fes decay is understood in terms of 1) net electrical charge dissipations in both particle and cell, 2) thermal disordering of dipole polarization in the module glass dielectrics, and 3) charge redistribution by the electrostatic interaction of particle and module glass. This long-lasting Fes for hours can affect the solar panel soiling after sunset, and it can have an even greater effect when combined with water condensation at night. •Designed direct measurement of electrostatic adhesion and attraction forces (Fes) on dust particles.•Found that Fes is 1-2 orders of magnitude larger than other physical adsorption forces, dominating the adhesion forces.•Found that the most important factor for long-lasting Fes after sunset is charges in either dust particle or solar module.•Found how the connection of solar array to ground is a dominant factor for lasting and decay of Fes.•The Fes and its decay are understood in terms of net charge interactions, dipole-dipole, and charge-dipole interactions.