Abstract: A nonintrusive monitoring system is used to determine the onset and duration times of an electrical power outage-caused down condition of a computer without requiring a pre-power outage polling of a running component of the computer and thereby degrading computer system performance. In response to a supply power outage the monitoring system switches on a battery-powered counter device, representatively a digital counter or a real time clock, and then switches the counter device back off in response to resumption of power supply to the computer. A software portion of the monitoring system then detects the count value of the counter device, and utilizes the count value to compute the onset and duration times of the previous power outage.