Abstract: An electrical device (10, 13), in particular a power transformer (13), a shunt reactor, or an on-load tap changer (10), comprises a housing (101, 131) that is filled or fillable with insulation oil (11, 11?); and a sensor (12) that comprises a piezo element (16) in the housing (101, 131) on a level with a minimum fill level (19, 19?) of the insulation oil (11, 11?); and an evaluation device (18) that is connected to the piezo element (16); wherein the evaluation device (18) is formed such that it can induce an oscillation of the piezo element (16); and it can determine the fill level from a measurement of a characteristic of the oscillation, in particular the half-life period of the amplitude of the oscillation.