A power monitor is used to estimate the amount of input power that is supplied to a power supply. The power monitor is protected by an EMI filter within the power supply. The power monitor measures a line frequency and a line voltage within the power supply and it also retrieves empirically derived coefficient values from memory where the coefficient values are used in formulas that have been determined to model the non-linear behavior of voltage and current in the power supply. The formulas are used to estimate the discrepancy between input power to the power supply and observed power values at a power monitor. The formulas used by the power monitor calculate power compensation factors based on the measured line frequency, the measured line voltage and the retrieved formula coefficient values. The power compensation factors are used to estimate the input power supplied to the power supply.