A power monitoring system with associated signal processing and storage capabilities. Historical data for an electrical signal is retained and displayed by dividing the electrical signal in the time domain into segments herein referred to as events. A detected disturbance of the electrical signal forms a boundary between two events. For each such boundary, the steady-state value of a signal parameter such as root-mean-square (rms) voltage is stored for the two events delimited by the boundary. Samples of the electrical signal immediately surrounding the boundary are stored so that disturbances of interest can be closely inspected.