EnergyPlus is the US Department of Energy's energy simulation platform. It's powerful, flexible, adaptable to new HVAC concepts and darn difficult to learn. Anticipating that EnergyPlus would become the premier energy simulation engine, we decided to learn it back in 2007 when the current version was v2. We're now glad we spent the time and effort and are helping to test beta versions for the development team. We've also assisted in the development of Simergy and worked with DesignBuilder, leading GUIs for EnergyPlus.