Electric energy prices in Brazil are higher during 3 h of peak consumption on working days: however, consumers could use a generator system for partially reducing the electricity costs from utility company during peak hours. The process of sizing a generator system begins developing the so called load capacity curve, using acquired building electricity consumption data. The local utility company supplies this data only for the last month of operation; consequently, data are unavailable for the rest of the year. In order to overcome the difficulty, this paper develops two methodologies to estimate the curve for every month of the year. The first one uses the frequency of occurrence of the measured values of the load power in each bin and the last one the monthly values of load power and energy consumption, as supplied by the local utility monthly bills. In order to validate the methodologies, monthly utility bills and consumption data of electric energy measured at Pontifical Catholic University of Rio de Janeiro, every 15 min over a four year period, have been used. Finally a generator system was sized, minimizing the electrical energy cost supplied by it, when the generator meets partially the building load. (C) 2012 Elsevier B.V. All rights reserved.