EnergyPlus is a software package which "models heating, cooling, lighting, ventilating, and other energy flows as well as water in buildings."
"EnergyPlus is a stand-alone simulation program without a 'user friendly' graphical interface. EnergyPlus reads input and writes output as text files. A number of graphical interfaces are available or under development." One such graphical interface is the OpenStudio Google SketchUp plug-in.