Accurate power usage for a portable information handling system is provided using input power measurement circuitry. A calibration step is performed when the portable information handling system is powered down and charging. The calibration step includes obtaining a battery charge measurement from a battery, and then using this measurement to calibrate an input power measurement made with respect to power being provided by the AC/DC adapter. Known losses from other sources within the system can also be considered in the auto-calibration of the input power measurement. Subsequently, when the portable information handling system is powered on and operating on power provided by the AC/DC adapter, power usage can be determined and reported using the auto-calibrated input power measurement as an indication of the power usage of the portable information handling system.