Buildings account for 21% of global energy consumption. Reducing building energy consumption can result in an immediate impact on energy expenditures and the environment. Retrofitting existing buildings is a means toward reducing their energy footprint. Currently, building owners have several retrofit choices. Building energy simulations can be used to assist in this retrofit selection process, providing energy savings estimates from different retrofits. Currently available simulators, however, are not retrofit-oriented and are not built to enable comparisons among retrofits. In order to perform these comparisons, users are required to manually create building models for each retrofit, which is obviously a cumbersome process. In this paper, we introduce ROBESim, a retrofit-oriented building energy simulator. ROBESim is based on the popular EnergyPlus framework, and relies on EnergyPlus for most of the supported computations. By using the retrofit modules in ROBESim, the user can quickly and easily generate building models to perform retrofit comparison simulations. ROBESim was designed to be modular and extensible. We outline these modules and show how developers can build upon our work in ROBESim. In addition, we describe the retrofit module development process and discuss additional ease-of-use enhancements that make building energy simulation possible for users of all experience levels. (C) 2013 Elsevier B.V. All rights reserved.