To find the kinetic energy of ions leaving a charged plate, one can start by calculating the electric potential energy using the voltage of the plate and the charge of the ions. The power supplied to the plate can help determine the energy available for the ions, but additional information about the voltage is necessary. Ions leave the plate due to the electric field created by the charge, which accelerates them away from the surface. The electric potential energy is typically measured relative to a reference point, often ground or another charged surface. Understanding these concepts is crucial for accurately determining the kinetic energy of the ions.