This paper describes a practical and simple method for determining the optimal size of renewable energy systems for building applications at the conceptual design stage. The method suggested in the paper is based on the use of the software tool RETScreen widely used for pre-feasibility or feasibility studies. The software is a convenient and relatively accurate tool for evaluating the feasibility of renewable systems. However, the tool does not provide with a method or function for optimal design. It seems there has been no publication or study to find optimal design parameters for renewable energy systems using the software. By using the features of this software, which can evaluate renewable technologies, this study provides a methodology to find the optimal design parameters for a renewable energy technology in the conceptual design of building energy systems. The paper describes a further development of the method for applications to integrated hybrid renewable energy resources in buildings. The paper also describes how to combine hybrid renewable heating systems, such as hybrid solar thermal and ground-source heat pump systems. Finally, this paper presents case studies on the optimal sizing of a single renewable system and an integrated renewable energy system for building applications. (C) 2012 Elsevier Ltd. All rights reserved.