EnergyPlus — The Complete Guide: Free Building Energy Simulation
EnergyPlus is the US Department of Energy's building energy simulation engine, open source. It calculates, hour by hour over a full year, a building's energy balance: heat entering and leaving through the envelope, solar radiation, loads from people and lighting, and heating, cooling and ventilation systems. Many commercial energy simulation programs use it as their engine, and it is accepted in energy compliance and rating paths worldwide.
EnergyPlus
US Department of Energy
Annual building energy simulation — envelope, HVAC, thermal comfort and daylight. The US DOE engine.
Get it from the vendorWhat EnergyPlus calculates
- Heating and cooling loads — for every thermal zone, every hour, using a weather file for the site.
- Envelope — walls, roofs and windows by material layers, including thermal mass, shading and radiation.
- HVAC systems — detailed HVAC systems, heat pumps, chillers and ventilation.
- Lighting and daylight — daylight-responsive lighting control.
- Comfort and energy use — zone temperatures, annual consumption and emissions.
How you work with it
EnergyPlus itself is an engine: the input is a text file (IDF or epJSON) and a weather file (EPW). In practice most users build the model through an interface. The main free interface is OpenStudio from NREL — geometry, materials, schedules and systems, plus results reports. EnergyPlus also ships with a basic IDF Editor and EP-Launch for direct runs.
OpenStudio
National Renewable Energy Laboratory
Graphical workflow on top of EnergyPlus — geometry, systems and scenario comparison.
Get it from the vendorWhat's missing
The learning curve is significant, complex geometry takes time to build, and defining HVAC systems needs systems knowledge. Results depend heavily on assumptions — occupancy schedules, infiltration, equipment efficiency — so document them. Checking compliance with a specific local standard usually needs a dedicated tool or methodology, not just an engine run.
Who it suits
Energy and green building consultants, HVAC engineers, architects who want to test early design options, and researchers and students.
Companion tools
OpenStudio as the interface, Radiance for accurate daylight analysis, THERM and WINDOW for thermal bridges and windows, and the CBE Thermal Comfort Tool for comfort checks.
Radiance
Lawrence Berkeley National Laboratory
Physically accurate daylight and lighting simulation — the basis for green building light calculations.
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THERM
Lawrence Berkeley National Laboratory
2D heat transfer analysis of envelope sections — thermal bridges, U-value and condensation checks.
Get it from the vendorWINDOW
Lawrence Berkeley National Laboratory
Thermal and optical properties of glazing systems — U-value, SHGC and visible transmittance.
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CBE Thermal Comfort Tool
Center for the Built Environment, UC Berkeley
Thermal comfort calculation to ASHRAE 55 and EN 16798, in the browser.
Get it from the vendorBottom line
EnergyPlus is the professional engine behind a large share of the world's energy simulations — and with OpenStudio you can use it without paying for a licence.