EnergyPlus, DOE's flagship whole-building energy modeling engine, officially celebrated its 20th birthday earlier this year. The project itself started in 1996. At the time, scientists were already aware of the dangers of climate change and the need to curb energy use and greenhouse gas emissions, but building energy efficiency and building energy simulation were not the enterprises they are today. USGBC (the US Green Building Council) had only been formed three years earlier, IBPSA-USA (the US chapter of the International Building Performance Simulation Association) was not yet officially organized, and ASHRAE Standard 90 had yet to transition to continuous maintenance mode and become the now familiar Standard 90.1. Trane TRACE 600 was the dominant simulation tool and was primarily used for calculating peak loads and required HVAC equipment capacities.

DOE had been developing whole-building energy modeling tools since the early 1970’s, before its ascension to a cabinet-level agency. The lineage includes CAL-ERDA, named after the collaboration between the state of California and the Energy Research and Development Agency that developed it, DOE-1, and DOE-2. In 1996, DOE decided to create a state-of-the-art engine that would embody the latest and most advanced methods in building energy simulation. The new engine would combine aspects of DOE-2 and BLAST (Building Loads and System Thermodynamics), a simulation program developed at CERL (the US Army Corps of Engineers’ Construction Engineering Research Laboratory). The new engine would use integrated heat-balance methods to simultaneously solve for loads and HVAC system response, represent geometry in three dimensions to accurately account for shading, daylighting, radiant exchange and thermal comfort, and allow users to define flexible HVAC system configurations by explicitly connecting components. After five years of work, the development team consisting of researchers from DOE, LBNL (Lawrence Berkeley National Laboratory), CERL, UIUC (University of Illinois-Urbana Champaign), OSU (Oklahoma State University), and consultancy GARD Analytics, officially released EnergyPlus on April 12, 2001 at the National Building Museum in Washington, DC.

EnergyPlus launched on April 12, 2001 at the National Building Museum.
EnergyPlus officially launched on April 12, 2001 at the National Building Museum. Present were Dr. Curtis Pedersen (UIUC), Dr. Dru Crawley (DOE), Linda Lawrie (CERL), Dr. Richard Liesen (CERL), Dr. Fred Winkelman (LBNL), an Dr. Richard Strand (UIUC).
Dru Crawley

EnergyPlus immediately gained recognition as a significant engineering achievement, winning an R&D 100 Award in 2003. Advanced capabilities, dedicated federal support, continuous development and improvement and the availability of source code also attracted code organizations like ASHRAE and the California Energy Commission as well as the research and education community. DesignBuilder was one of the first and, according to data from the AIA 2030 Commitment reporting program, is still the EnergyPlus interface most commonly used by architects.

“When we first came across EnergyPlus v1 back in 2001, DesignBuilder was still a fresh new GUI, full of promise but rather lacking in technical pedigree,” recalls Andy Tindale, Managing Director at Design Builder Software. “It was an exciting prospect to be able to hook up to the next generation of simulation technology from DOE. The separation of GUI and engine was a key concept for both DesignBuilder and EnergyPlus, and so EnergyPlus slotted in perfectly and we had simulations up and running in no time.”

EnergyPlus has certainly evolved over the forty-one versions spanning 1.0 in April 2001 to 9.5 in April 2021. LBNL had always made EnergyPlus source code available, but for a fee, and the license prohibited redistribution of modified source code. In 2012, EnergyPlus was released under a permissive, open-source license that removed both the fee and the usage and redistribution restrictions. This change enabled the next significant change, when Autodesk contracted with Objexx Engineering to automatically translate the EnergyPlus source code—even then, nearly a million lines long!—from its native FORTRAN-90 to C++ and donated the translated code back to DOE. Although the translated code retained the FORTRAN procedural style and used only FORTRAN data structures, this transition allowed the team to begin a comprehensive modernization process. A permissive, open-source license and use of C++ also made it easier for vendors to adopt EnergyPlus in part or in whole and led to growth in the number of private-sector EnergyPlus-based applications and services, with products such as Sefaira Architecture and Systems, Autodesk Systems Analysis for Revit, Trane’s TRACE 3D Plus, and Honeybee from Ladybug Tools. These joined public EnergyPlus-based tools such as California’s CBECC-Com and DOE’s Asset Score.   

Recent years have seen new or improved EnergyPlus functionality that aim at expanding its use cases. Originally targeted at commercial buildings, EnergyPlus now models phenomena, building configurations, and equipment that is relevant to homes in detail, allowing DOE to retarget its Home Energy Score labeling tool from DOE-2.1E. User-defined control scripting via a facility called EMS (Energy Management System) has long been a unique feature of EnergyPlus. In 2020, this feature received a significant upgrade when it was reimplemented as an API (Application Programming Interface) with Python bindings allowing users to script using the popular Python language rather than EnergyPlus’ much more limited built-in scripting language. Only a year old, this feature has already been used to model grid-interactive buildings, new equipment, and novel envelope assemblies. Even the most recent version of EnergyPlus has an important if somewhat obscure capability—it is “thread safe,” allowing a single client application to run multiple EnergyPlus simulations in parallel with no interference and low communication overhead. This capability is especially important for co-simulation.

The EnergyPlus team itself has also evolved. Although their code lives on, a good number of the original developers have retired or moved on to other phases of their careers. NREL now leads the development team, which has a growing number of young contributors—many of whom learned EnergyPlus as students and used it in their research. One of the longtime team members is Dr. Mike Witte of GARD Analytics, who worked on the BLAST program as a UIUC Ph.D. student, and joined the EnergyPlus project shortly after its inception. "When I first started working on BLAST in 1983, DOE-2 was a competitor of sorts, and each team proudly extolled the virtues of their respective tools,” said Witte. “It was exciting to see the two groups join forces, and when the opportunity arose to work with this team again, I couldn’t pass it up. Many things have changed in the meantime, the language we program in, the tools we use to collaborate, the members of the team, and even the user and vendor communities which have expanded beyond what I could have imagined. Throughout, it has been a great privilege to be part of this team working to develop such important and useful software."

Andy Tindale adds, “EnergyPlus v1 was a major milestone in simulation technology, but it was limited and ran terribly slowly! In the 20 years since then it has been optimized and extended in a myriad of ways. It has been a privilege to work with the EnergyPlus team over the years and to watch the software evolve to become the state-of-the-art simulation engine that it is now.  DesignBuilder recently released v7 of its product, which provides access to many of EnergyPlus’ recent features including the Kiva ground heat transfer model.”

Dr. Dru Crawley of Bentley Systems was a program manager at BTO from 1994 until 2010. He launched the EnergyPlus project and shepherded it from its inception through the first ten years of its public life. He says “EnergyPlus grew out of a desire to get away from the monolithic code structures of DOE-2 and BLAST, which were almost impossible to maintain and extend, delaying any significant improvements. Building EnergyPlus from the ground up with a modular structure established a solid foundation that allowed many developers from around the world to collaborate and significantly improve the product over the past 20 years.” That work continues today!

EnergyPlus V1.0 was released on mini-CD.

Early versions of EnergyPlus were distributed on mini-CD. Today, new versions of EnergyPlus are downloaded from the web at

Dru Crawley