What is the K-Factor rating?

"K-factor is defined as a ratio between the additional losses due to harmonics and the eddy current losses at 60Hz. It is used to specify transformers for non-linear loads. This factor is used to specify the size of the transformer to meet the magnitude of the harmonic load in the circuit. A standard general-purpose transformer does not have the shielding, conductor sizes, core cross-section, or the capacity in the neutral to provide the same service. Transformers with a rated K factor of 4, 7, 13, 20 and 30 are available. For balanced loading, a transformer with a K factor of 4 should be specified when no more than 50% of the total load is non-linear. A transformer with K factor 13 should be specified when 100% of the load is non-linear. "