What is an EPW file? The EnergyPlus weather format, explained

What is an EPW file?

EPW stands for EnergyPlus Weather. Drury Crawley, Jon Hand and Linda Lawrie described a new, generalised weather format for two simulation programs, EnergyPlus and ESP-r, in 1999; it later became known as EPW. It is based on the data in the older TMY2 format, rearranged into simple comma-separated text in SI units so that people and spreadsheets can read it as easily as software.

Because the format is shared, one file can feed an EnergyPlus energy model, a Grasshopper script in Ladybug Tools or a web tool such as the CBE Clima Tool. By 2015 two of its authors, Drury Crawley and Linda Lawrie, counted more than 20 building simulation programs that read EPW.

How is an EPW file structured?

The file opens with eight header lines, each starting with a keyword. Then come the data records, one line per time step. Most weather data is hourly, so a full year has 8,760 data lines (365 days × 24 hours). The format also allows several records per hour.

• LOCATION: city, region, country, data source, WMO station number, latitude, longitude, time zone (hours from GMT) and elevation in metres.
• DESIGN CONDITIONS: heating and cooling design conditions matched to the station's WMO number, usually taken from the ASHRAE Handbook of Fundamentals.
• TYPICAL/EXTREME PERIODS: typical and extreme weather periods found in the data by the EnergyPlus Weather Converter.
• GROUND TEMPERATURES: monthly undisturbed ground temperatures at several depths, calculated from the air temperature record.
• HOLIDAYS/DAYLIGHT SAVING: whether 29 February is observed, daylight saving dates and any holidays.
• COMMENTS 1 and COMMENTS 2: free text that may describe the data source and how the file was made.
• DATA PERIODS: the number of records per hour and the start and end dates of the data.

Which weather variables does each record contain?

Each data line has 35 fields. The first five give the date and time (year, month, day, hour and minute), the sixth holds data-source and uncertainty flags, and the rest are weather values:

• Temperature and humidity: dry-bulb and dew-point temperature (°C), relative humidity (%) and station pressure (Pa).
• Radiation: extraterrestrial, global horizontal, direct normal and diffuse horizontal solar radiation, plus infrared radiation from the sky (all in Wh/m²).
• Daylight: global, direct normal and diffuse horizontal illuminance (lux) and zenith luminance (cd/m²).
• Wind: direction in degrees, with north at 0 and east at 90, and speed in m/s.
• Sky and visibility: total and opaque sky cover in tenths, visibility, ceiling height and present-weather codes.
• Water and ground: precipitable water, aerosol optical depth, snow depth, days since last snowfall, albedo, and liquid precipitation depth (mm) with its accumulation period.

Where do EPW files come from?

An EPW is only a container; the source of the data inside matters more. The file name and the source field in the LOCATION line usually tell you where it came from.

• Typical years from weather stations. Climate.OneBuilding.Org, run by Linda Lawrie and Drury Crawley, two of the format's creators, publishes free TMYx files for 17,315 locations, built from NOAA's Integrated Surface Database with TMY and ISO 15927-4 methods. In the 2026 release their solar radiation comes from the ERA5 reanalysis. The site also hosts national sets such as TMY3 for the United States and CWEC for Canada.
• Typical years from satellite and reanalysis data. PVGIS, from the European Commission's Joint Research Centre, generates a typical year in EPW format for a chosen location, using satellite or reanalysis solar radiation and ERA reanalysis data for the other variables.
• Experimental national products. NOAA's National Centers for Environmental Information publishes an experimental typical-year product for 1,972 stations, based on 1998–2022 data, in CSV and EPW formats.
• Actual and future years. The same format can hold one real calendar year, for example to test a design against a recorded heatwave, or a typical year adjusted to a future climate.

How to open, check and use an EPW file

Any text editor opens an EPW, and the eight header lines are readable as they are. A spreadsheet can import the file as comma-separated text; skip the first eight lines to get one row per hour and one column per field. The EnergyPlus Weather Converter can also turn an EPW into a CSV version and produce a statistics report.

Two conventions catch people out. Hours run from 1 to 24, with hour 1 covering 00:01 to 01:00. And radiation values are totals over the period before the timestamp, while temperatures are values at the timestamp. Missing data is flagged with codes such as 99.9 for temperature and 9999 for radiation, and not every field is used by every program: EnergyPlus ignores the global horizontal radiation field and works from direct normal and diffuse horizontal radiation.

Every chart and simulation inherits the file's assumptions, so choosing it is a design decision. Before using one, check:

• Location and elevation: is the station close to the site and at a similar altitude, and does it share the site's exposure, for example coastal or inland?
• Source and period: a typical year built from 2011–2025 data can describe a noticeably different climate from one built from 1976–2005 data.
• The Year column: in a typical year it changes from month to month, because each month comes from a different source year.
• Purpose: a typical year leaves out extremes by design, so it is not the right file on its own for overheating checks or peak loads.

How C4B uses EPW files

C4B does not ask you to find a weather file. For every search it loads the nearest Climate.OneBuilding.Org weather file for the location you pick, usually a TMYx file, on every plan including Free. In projects on paid plans it also compiles a typical year from reanalysis data at the site's exact coordinates, so you can compare the station with the site.

Downloading EPW files from C4B, whether a custom site file or a future-climate file, is available on the Technical and Enterprise plans. The files use the standard format, so they work in EnergyPlus-based tools.