An EL panel is a lossy parallel-plate capacitor.  In other words, it’s an electronic device that is electrically charged and then loses its energy in the form of light.  To do this, an EL lamp requires an AC current, which is applied to both plates.  The current allows energy to build up within a phosphor layer between the plates, and the energy is released as light during each half cycle of the AC current (when the electrical field or charges applied to the plates alternate). The number of times that the field changes within a given time period (the frequency of the current) influences the brightness and color of the light emitted by the phosphor.