According to international standards, power quality disturbances (PQD) are classified in agreement with amplitude or proportion, bandwidth and duration. In this paper an additional PQD classification is proposed, consisting in classifying PQD according to their occurrence in random or permanent disturbances. Random disturbances are those whose times of occurrence can not be determined in advance (surges, sags, swells, etc.) and permanent disturbances are those present in electric distribution systems continuously (harmonics, interharmonics, voltage fluctuations, flicker, etc). This paper shows a method for calculation of permanent disturbances indices based on measurements, distribution system modelling and Monte Carlo perfect simulation. Power definitions under non sinusoidal conditions are used to evaluate the effect of modelled disturbances