Non-intrusive load monitoring (NILM) is a novel detailed power load consumption monitoring technology. Research of electrical appliance load characteristics is the foundation of realizing NILM. Along with the increase of electronic and power electronic appliances equipment, harmonic characteristics have gradually become the focus of NILM. For a better application of harmonic characteristics to NILM, firstly, two indices for measuring the repeatability and discrimination of load characteristics are proposed. Based on the research on harmonic generation mechanism of electronic appliances, six kinds of harmonic characteristic indices are summerized, and their repeatability and discrimination are analyzed in detail according to the proposed two indices. Sample characteristics of different kinds and different brands of appliances from five family scenarios and two business scenarios are summerized, and the measured dataset is established. Analysis on the measured dataset shows that the 3rd and 5th harmonic amplitude value and total harmonic distortion have better repeatability and discrimination according to the proposed evaluation indices of harmonic characteristics. This conclusion corresponds to the accuracy results by different classifiers established on different harmonic characteristics, therefore it can not only guide the effective application of harmonic characteristics to NILM, but also prove the effectiveness of the two proposed load characteristics evaluation indices.