In one embodiment, a system and method perform a spectral analysis in a power measurement system to determine the total harmonic distortion (“THD”) in a power signal by correlating one or more THD calculation parameters to a sensed period of a voltage input signal. In at least one embodiment, the one or more THD calculation parameters are a number of samples of a voltage, current, or voltage and current components of the power signal that correlate to the sensed period. Because, for example, the period of the power signal can vary or the clock frequency can drift over time, the power measurement system correlates the number of samples with the period and, thus, varies the count of samples over which THD is calculated when the period varies. By correlating the samples with the period, the samples more closely represent a period of the sampled component of the power signal.