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Document the index convention of read_epw #2894

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Document the index convention of read_epw
cdeil Oct 9, 2026
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Add the pull request number to the what's new entry
cdeil Oct 9, 2026
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5 changes: 5 additions & 0 deletions docs/sphinx/source/whatsnew/v0.16.2.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -31,6 +31,10 @@ Documentation
* Fix the :py:func:`pvlib.spa.solar_position` docstring, which listed
outputs ``elevation`` and ``apparent_elevation`` in the wrong order.
(:issue:`2886`, :pull:`2887`)
* Document in :py:func:`pvlib.iotools.read_epw` that each record is labelled

@RDaxini RDaxini Oct 12, 2026

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pvlib uses US English

Suggested change
* Document in :py:func:`pvlib.iotools.read_epw` that each record is labelled
* Document in :py:func:`pvlib.iotools.read_epw` that each record is labeled

with the start of its hour, unlike :py:func:`pvlib.iotools.read_tmy3`, and
how to label it with the time given in the EPW file. (:issue:`1156`,
:pull:`2894`)


Testing
Expand All @@ -56,6 +60,7 @@ Contributors
* Chen Yi (:ghuser:`chenyi6758-source`)
* Rajiv Daxini (:ghuser:`rdaxini`)
* jason-writer (:ghuser:`jason-writer`)
* Christoph Deil (:ghuser:`cdeil`)



11 changes: 11 additions & 0 deletions pvlib/iotools/epw.py
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Expand Up @@ -208,6 +208,17 @@ def read_epw(filename, coerce_year=None):
| | liquid precipitation depth field, hour |
+-------------------------------+-----------------------------------------+

.. warning:: EPW irradiance and illuminance values are totals over the
hour *ending* at the time in the file, while temperatures, humidity
and pressure are values *at* that time [1]_. ``read_epw`` labels
each record with the *start* of its hour: EPW hour 1 (00:00-01:00)
becomes 00:00, so a full year runs from 00:00 on 1 January to 23:00
on 31 December. :py:func:`~pvlib.iotools.read_tmy3` labels each record
with the *end* of its hour instead, so the same data read from an EPW
file and from a TMY3 file are one hour apart. To label the records
with the time given in the EPW file, add one hour to the index:
``data.index = data.index + pd.Timedelta('1h')``.

Comment on lines +211 to +221

@RDaxini RDaxini Oct 12, 2026

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As far as I understand, illuminance values are averages, not totals.
Suggested some minor clarification regarding time labels and year-end record too.

Suggested change
.. warning:: EPW irradiance and illuminance values are totals over the
hour *ending* at the time in the file, while temperatures, humidity
and pressure are values *at* that time [1]_. ``read_epw`` labels
each record with the *start* of its hour: EPW hour 1 (00:00-01:00)
becomes 00:00, so a full year runs from 00:00 on 1 January to 23:00
on 31 December. :py:func:`~pvlib.iotools.read_tmy3` labels each record
with the *end* of its hour instead, so the same data read from an EPW
file and from a TMY3 file are one hour apart. To label the records
with the time given in the EPW file, add one hour to the index:
``data.index = data.index + pd.Timedelta('1h')``.
.. warning:: In hourly EPW files, radiation totals and illuminance
averages cover the hour *ending* at the file time; temperature,
humidity and pressure are values *at* that time [1]_.
``read_epw`` labels all fields with the *start* of the hour (EPW
hour 1 becomes 00:00), while :py:func:`~pvlib.iotools.read_tmy3`
uses the *end*.
To use end-of-hour labels, add one hour to the index:
``data.index = data.index + pd.Timedelta('1h')``. The record for
31 December's final hour then has the label 1 January 00:00 of
the following year. No records are added or removed.


References
----------
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