Abstract: The run-time on battery power available from an uninterruptible power system is accurately determined both during supply of power from the battery through an inverter to the load and during the time where power is supplied directly from an AC power system to the load. During power outages where power is supplied from the battery through the inverter to the load, the output voltage of the battery is directly measured as is the output current supplied to the load. The remaining run-time is determined utilizing the measured battery voltage, the measured output current, and system specifications including the full charge open circuit voltage of the battery, the allowable lower limit of battery voltage, and constants which are characteristic of the uninterruptible power supply and the battery.