Fluorescent lamps are among the most diffuse electrical components that introduce significant waveform distortions in low voltage distribution systems. These distortions involve a wide range of frequencies (i.e., from 0 kHz to 150 kHz), and spectral components characterized by a non-uniform behavior in time and frequency domains. Early IEC standards covered formally only the range from 0 kHz to 9 kHz, defining grouping and group total harmonic distortion both for the low-frequencies (i.e., from 0 kHz to 2 kHz) and for frequencies ranging from 2 kHz to 9 kHz. Recently, the IEC standards have suggested to extend the grouping, previously defined for the range from 2 kHz to 9 kHz, also to the range from 9 kHz to 150 kHz. In this paper, the waveform distortions of a variable number of fluorescent lamps were quantified by applying the main Power Quality indices currently available for such devices. Indices are evaluated using both the IEC method and an hybrid method proposed recently in the relevant literature, i.e., the sliding-window Wavelet-Modified ESPRIT method. Numerical applications on measured data show the waveform distortion levels versus the number of lamps, quantifying also the different performances of the aforesaid spectral analysis methods