What EnergyPlus Is[LINK]
The primary website for EnergyPlus is http://www.energyplus.gov
EnergyPlus is an energy analysis and thermal load simulation program. Based on a user’s description of a building from the perspective of the building’s physical make-up, associated mechanical systems, etc., EnergyPlus will calculate the heating and cooling loads necessary to maintain thermal control set points, conditions throughout a secondary HVAC system and coil loads, and the energy consumption of primary plant equipment as well as many other simulation details that are necessary to verify that the simulation is performing as the actual building would. More details on what EnergyPlus is can be found in the GettingStarted Document.
No program is able to handle every simulation situation. However, it is the intent of EnergyPlus to handle as many building and HVAC design options either directly or indirectly through links to other programs in order to calculate thermal loads and/or energy consumption on for a design day or an extended period of time (up to, including, and beyond a year).