A method and apparatus for allowing the user of a power generator coupled to a time-varying load, to define an alternative reference impedance to enable on or more metrics to be provided relative to the alternative reference impedance. The metrics, for example, may provide indicia of performance of the power generator system. One illustrative embodiment provides a power delivery system that applies power to a plasma chamber to create a plasma therein; determines a reference impedance of the plasma at an operating condition; and controls the power delivery system based on the determined reference impedance.