arXiv:2611.05913v1 Announce Type: cross Abstract: Measured voltage and current waveforms may be represented in a reference frame whose frequency differs from the physical fundamental frequency. Reactive-power readouts calculated from such frequency-shifted coordinates may change if their readout operators are not transformed consistently. This paper presents a frequency-frame covariance formulation for finite-spectrum, zero-mean periodic waveforms with complete harmonic information over one physical period. A necessary and sufficient covariance condition is derived, requiring the readout operator to transform consistently with the waveform representation. Consequently, frequency-dependent harmonic weights must be evaluated at physical harmonic orders rather than relative frame labels. In particular, the derivative/Iliovici readout equals a relative-derivative contribution plus an explicit reference contribution. The formulation also separates readout-operator errors from errors caused by changes in the physical components retained by waveform projections. Three analytical cases illustrate conditional carrier recovery for a memoryless resistor, physical-frequency weighting at relative zero in an RLC branch, and the possibility that an exact reactive-power scalar can coexist with a substantial waveform error after projection. The scope is deterministic calculation consistency in frequency-shifted coordinates rather than instrument measurement uncertainty.