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In this paper, the authors deal with a smart total harmonic distortion (THD) meter that is able to determine the uncertainty of the measurements that it executes. Such an instrument, which is used for power quality monitoring through the measure of an index, acquires the signal samples from the low-voltage network and evaluates the THD factor. To estimate the uncertainty associated with this last result, a theoretical characterization of the whole instrument realized according to the International Organization for Standardization Guide recommendations (transducer stage included) is discussed first. This approach application has proved to be very useful to justify the necessity of looking for an alternative technique. Therefore, the authors proposed the ldquogrey boxrdquo calibration procedure. Considerations about the comparison of the two procedures have been made.