Cogeneration will be always an important concept for energy conversion in the future, since it proposes to optimize the use of the energy resources. In the transformation of conventional systems for operation in the cogeneration mode, there exists the necessity to estimate the electric and thermal load profiles hour by hour, so that the cogeneration system can be optimally designed and thus displace the electric energy that would be used by the conventional systems. This work develops a methodology for estimating the electric and thermal load profiles, hour by hour for each month of the year, from the few normally available data. For the electric profile, annual consumption data of electric energy measured at PUC-Rio, every 15 min in the period of I year, has been used to validate this methodology. For the thermal profile, a methodology was developed; it discusses how the input thermal energy can be estimated from values of ambient temperature, internal thermal loads and solar radiation incident on the buildings. As an example of this methodology, a thermal load calculation is detailed for a business building and the results compared to those obtained from an existing methodology. The results obtained with these models, allow more accurate predictions for estimating the electric energy with a generator, over a month period, when its capacity is smaller than the building peak demand. A model was also developed to calculate the contribution of the air conditioning electric energy consumption to the total electric energy load. (c) 2006 Elsevier B.V. All rights reserved.