Fuel cell technologies are an emerging alternative to combustion-based cogeneration and traditional, centralized power generation. To accurately size fuel cell systems for houses and predict their performance, a system model that can be integrated into existing building simulation tools is required. A steady-state model of a generic PEM cogeneration fuel cell system was developed to fill this need. The model is useful for (i) estimating system fuel use, and electrical and thermal production, (ii) investigating the suitability of fuel cell systems in different climates, (iii) sizing fuel cell systems and ancillary equipment, and (iv) evaluating different control strategies. The model has been validated using empirical data and published estimates produced with other models. In this paper, the development of the model and the results of these validation studies are discussed. Also, the results from a Canadian case study investigating the effect of varying fuel cell size on the performance of a cogeneration system are presented. (C) 2004 Published by Elsevier B.V.