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The discrete Fourier transform (DFT) is still a widely used tool for analyzing and measuring both stationary and transient signals in power systems. However, the misapplications of DFT can lead to incorrect results caused by some problems, such as aliasing effect, spectral leakage, and picket-fence effect. This paper establishes a relationship between DFT-based spilled energy and harmonics/noninteger harmonics using the numerical induction method. Based on this study, a strategy of group-harmonics energy diffusion algorithm has been developed for both stationary and nonstationary harmonic/interharmonic evaluation in power systems. Especially, the frequency deviation range is used for frequency estimation, and the restored amplitude is used for retrieving dispersed amplitude. Accordingly, the harmonic/interharmonic frequency and its respective amplitude component can be accurately estimated using a simple mathematic computation. The numerical examples are presented to verify the performance of the proposed algorithm in terms of speed, accuracy, and efficiency.