
EnergyPlus models heating, cooling, lighting, ventilation, other energy flows and water use. It includes many innovative simulation capabilities: time-steps less than an hour, modular systems and plant integrated with heat balance-based zone simulation, multizone air flow, thermal comfort, water use, natural ventilation, and photovoltaic systems.
EnergyPlus runs on the Windows, Macintosh, and Linux platforms. Free add-ons and other third-party software products are available for use with EnergyPlus. The details are all here [PDF].