For more than two decades, the U.S. government supported the development of two hourly building energy simulation programs—BLAST and DOE-2. Designed in the days of mainframe computers, expanding their capabilities further became increasingly difficult, time-consuming, and expensive. For these reasons, a U.S. federal agency began developing a new building energy simulation tool in 1996. This new simulation program, known as EnergyPlus, builds on the capabilities and features of BLAST and DOE-2 and was released in April 2001. EnergyPlus includes a number of innovative simulation features (such as variable time steps, configurable modular systems that are integrated with a heat balance-based zone simulation) and input and output data structures tailored to facilitate third-party module and interface development. Other planned simulation capabilities include multizone airflow and electric power and solar thermal and photovoltaic simulation. EnergyPlus will also be linked to two other simulation programs—TRNSYS (solar thermal, photovoltaic, and complex HVAC systems) and SPARK (complex HVAC systems). As we neared completion of EnergyPlus version 1.0, the development team began working with interface developers so that easy-to-use interfaces would be available soon after EnergyPlus version 1.0 was released. Beginning in late 1999, a series of five beta versions were released for testing. Version 1.0 of EnergyPlus was released in April 2001.