The number and power of loads that pollute, from an electric point of view, the electric network are constantly increasing. For this reason, this study aims to define a parameter to assess, in terms of distortion and unbalance, the quality of an electric network. Taking, as a starting point, the advantages and limitations of wavelets methods proposed in the literature, a new index useful for quantifying the load characteristic, is introduced. The efficiency of this index has been verified by computer simulations to assess the method applicability to three-wire measurements of instantaneous power theory, in many realistic situations.