Until now, load modelling has encountered the following problems: on the one hand, the modelling method based on aggregating analytically the individual loads up to the bulk power level suffers from the uncertain composition obtained by statistics; on the other hand, the modelling method, based on observing on-line input-output load behavior, may avoid very well many uncertain factors. But it raises a doubt: could the coefficients determined by small natural variations describe the load behavior for large voltage and frequency variations? After experiments in laboratory and comparison between three different treatments of data, a new load model being a function of voltage, frequency and time has been defined.