What is the EPW file format?
An .epw file is a plain-text hourly weather dataset used by building-energy and daylighting simulation software. The format was introduced around 2001 alongside the first EnergyPlus release - the U.S. Department of Energy's flagship building simulation engine, developed and maintained by the National Renewable Energy Laboratory (NREL).
Each file covers one full year of hourly weather observations or modeled climate data, structured as eight named header records followed by 8,760 comma-separated data rows. The header records are:
LOCATION- site name, coordinates, elevation, time zoneDESIGN CONDITIONS- peak heating and cooling design valuesTYPICAL/EXTREME PERIODS- statistically representative weeksGROUND TEMPERATURES- monthly soil temperatures at several depthsHOLIDAYS/DAYLIGHT SAVINGS- calendar overridesCOMMENTS 1/COMMENTS 2- source and provenance notesDATA PERIODS- date range and interval declaration
Each hourly row carries dry-bulb temperature, dew-point, relative humidity, atmospheric pressure, global/direct/diffuse solar radiation, wind direction, wind speed, sky cover, and several additional parameters. Missing or uncertain values are flagged with sentinel codes (99.9, 999, or 9999 depending on field units) rather than blank cells. The file has no binary magic bytes - identification relies on the first line beginning with LOCATION,.
Beyond EnergyPlus itself, .epw files are consumed by OpenStudio, DesignBuilder, IES VE, Radiance-based workflows, and the Ladybug/Honeybee Grasshopper plug-ins. Free datasets for thousands of global locations are published at energyplus.net/weather and climate.onebuilding.org. The companion .stat file provides derived monthly and annual statistics for the same site, while .ddy files carry the associated design-day data for peak load calculations.
Security & safety
RISK: LOWEPW files are plain-text weather data with no executable content, so they are safe to open. The only practical caution is using the right file for the right location/scenario - an EPW for the wrong site or an out-of-date dataset gives misleading simulation results. Download files from official sources (energyplus.net/weather, climate.onebuilding.org) to ensure data quality.
Format details
in a nutshellPrograms that open EPW files
Windows5 apps
EnergyPlus
Open-source Free DOE/NREL engine; assign the EPW as the simulation weather file (also ships the Weather Statistics & Conversion tool to inspect EPWs). 
Elements (Big Ladder Software)
Free Free dedicated EPW editor/viewer - open the file to chart and edit hourly weather variables.
Microsoft Excel / any spreadsheet
Paid Open/import as CSV to read and chart the comma-separated hourly data (skip the 8 header lines). 
macOS2 apps
Linux2 apps
any text editor / spreadsheet
Open-source It is CSV-style text - open in LibreOffice Calc or a text editor.
Ladybug Tools EPW Map
Free Browse and download free EPW weather files for locations worldwide from a map. Technical details
deep spec| File structure | Eight named header records followed by 8,760 comma-separated hourly data rows (or 8,784 in a leap year) |
| Encoding | ASCII / UTF-8 plain text; no binary signature or byte-order mark required |
| First-line identifier | First line always starts with LOCATION, followed by site metadata - used for format detection in lieu of magic bytes |
| Data interval | One record per hour; a full non-leap year produces exactly 8,760 rows |
| Typical file size | 1.5 MB - 2.5 MB for a complete annual dataset |
| Missing-value sentinels | Field-specific placeholder codes replace absent readings: 99.9 for temperatures, 999 for wind parameters, 9999 for radiation values |
| Solar radiation fields | Global horizontal irradiance, direct normal irradiance, and diffuse horizontal irradiance, each in Wh/m² per hour |
| Meteorological fields per row | Includes dry-bulb temperature, dew-point, relative humidity, barometric pressure, wind speed, wind direction, total sky cover, opaque sky cover, ceiling height, and visibility |
| Header sections | LOCATION, DESIGN CONDITIONS, TYPICAL/EXTREME PERIODS, GROUND TEMPERATURES, HOLIDAYS/DAYLIGHT SAVINGS, COMMENTS 1, COMMENTS 2, DATA PERIODS |
| MIME type | text/plain; some tools also recognise text/csv |
| Companion formats | .stat (monthly and annual statistical summaries for the same site) and .ddy (design-day data for peak load sizing) |
| Developer | U.S. Department of Energy, National Renewable Energy Laboratory (NREL) |
| Ecosystem support | EnergyPlus, OpenStudio, DesignBuilder, IES VE, Radiance-based workflows, and Ladybug/Honeybee for Grasshopper and Dynamo |
| Data sources | Derived from TMY (Typical Meteorological Year), IWEC (International Weather for Energy Calculations), CWEC (Canadian Weather for Energy Calculations), and measured station archives |
| Geographic metadata | LOCATION header stores latitude, longitude, elevation in metres, UTC time-zone offset, and WMO/ASHRAE station identifier |
| File distribution | The .epw file itself is uncompressed; bulk downloads from energyplus.net are packaged as ZIP archives |
| Released | Around 2001 (with the first EnergyPlus releases) |
| Latest version | EPW format as documented in the EnergyPlus Auxiliary Programs guide (stable; tracks EnergyPlus, v25.x in 2025) |
| Open standard | Yes · royalty-free |
| Specification | bigladdersoftware.com |
EPW conversions
Community Q&A
asked by usersNo questions yet - be the first to ask about EPW files.
