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EnergyPlus is a whole-building energy simulation program. A researcher who reads this page should be able to say what goes into a run, what comes out, and what to record so another team can reproduce the result.
What is EnergyPlus used for?
EnergyPlus is used to simulate how a building and its systems consume energy and water. The project describes it as a whole-building energy simulation program for engineers, architects and researchers, and its quick-start documentation calls it an energy analysis and thermal load simulation program: from a description of a building and its systems it computes heating and cooling loads, system conditions and energy consumption.
What is an energy model?
An energy model is the input description a simulation program runs on. In EnergyPlus the building and its associated systems are written as a set of objects in an IDF (Input Data File), which the documentation defines as a plain text file of objects representing the building and its systems. Weather comes from a separate EPW file. Running the program on those inputs produces simulated loads and energy use, which is the model’s output.
Who maintains EnergyPlus and how is it licensed?
The source code is public on GitHub (the NREL/EnergyPlus repository, now hosted under the NatLabRockies organization). The repository states that the U.S. Department of Energy maintains the public EnergyPlus website, that the software is open source under a permissive BSD-style license (LICENSE.txt in the repository), and that formal releases appear twice a year, usually in March and September. It advises using formal releases for production work rather than pre-releases. Documentation lives at energyplus.readthedocs.io (Quick Start, Getting Started, Input Output Reference, Engineering Reference, epJSON schema, C and Python APIs). Community support is offered through Unmet Hours and the EnergyPlus Helpdesk. The quick start says compiled versions are provided for Windows, Mac and Linux.
Is EnergyPlus in Texas?
EnergyPlus is not tied to a state. It is a U.S. Department of Energy-supported simulation program that you run on your own computer, and the location of a building is set by the weather file you supply, not by where the software comes from. The quick start notes that it ships with TMY weather files for US locations, with more available from energyplus.net.
Inputs, weather files and outputs
The table maps each file type to its role, using the wording of the Quick Start documentation.
| File | Direction | What it is |
|---|---|---|
| IDF | Input | Plain text file of objects representing the building and its systems |
| EPW | Input | Plain text file of weather information |
| ESO | Output | Raw report variable output in text form |
| SQL | Output | Output variables in SQLite format, produced only if requested in the input file |
| RDD | Output | List of output variables available from the run |
| MDD | Output | List of output meters available from the run |
| EIO | Output | Additional results, including diagnostic messages |
| ERR | Output | List of errors and warnings produced by the run |
| MTR | Output | Raw report meter output, formatted as the ESO |
| MTD | Output | List of meter component variables |
The RDD and MDD are useful before you analyse anything: they list which variables and meters a given run can report. The ERR file lists the run’s warnings and errors, so read it before trusting any result.
How to run it
The Quick Start describes EnergyPlus as a command-line program that graphical interfaces can also drive. Three options matter most:
-i(or--idd) specifies the location of the IDD file.-w(or--weather) specifies the input EPW weather file.-r(or--readvars) runs the ReadVarsESO program after the simulation to generate a CSV file from the ESO.
--help lists every option available in your installed version.
Reporting EnergyPlus results reproducibly
Because releases arrive twice a year and the project recommends formal releases for production use, a result is only meaningful with the version that produced it. For a methods section or data package, record the following so that others can repeat the run (see reproducibility and reproducible research practices):
- the exact EnergyPlus release number used;
- the IDF (or epJSON) input file, archived with the data;
- the EPW weather file, including its source and file name, since the weather determines the loads;
- the command-line options used, and the ERR file showing how the run finished.
Re-running the same inputs on the same release repeats a computation; whether the model represents the building is a separate question, discussed in reproducibility versus replicability.
Strengths and limits
- Open and documented. The source is public under a permissive license, and the Engineering Reference and Input Output Reference are published for each release, so the calculations behind a result can be inspected.
- Scriptable. It runs from the command line, and the documentation lists C and Python APIs, which supports batch runs.
- Input-driven. The program computes from the building description you give it, so result quality depends on the care taken building that description and choosing weather.
- Raw outputs. The primary outputs are text and SQLite files; turning them into figures is the researcher’s job (the
-roption produces CSV).
Related tools
The quick start notes that graphical interfaces can drive EnergyPlus. One such workflow tool is covered on CASRAI in the OpenStudio guide; it is a way of building and running models around the EnergyPlus engine rather than a substitute for understanding its inputs and outputs.
How much does energy modeling cost?
EnergyPlus itself is open-source software under a permissive license, so the program carries no license fee. The cost of an energy modeling project depends on who builds the model and how much work the building requires; the EnergyPlus documentation does not state service prices, so none are given here.
FAQ
Is EnergyPlus free?
Yes. The repository states it is open source under a permissive BSD-style license.
Where do I get weather files?
The quick start says EnergyPlus ships with TMY weather files for US locations and that more are available at energyplus.net.
How often is it updated?
The repository states that formal releases happen twice a year, usually in March and September.
Can I use it without a graphical interface?
Yes. It is a command-line program; graphical interfaces are optional wrappers.
Sources
- EnergyPlus Quick Start, energyplus.readthedocs.io (read 2026-11-07)
- EnergyPlus repository README and license, GitHub (read 2026-11-07)
