Fix delta_kt_prime calculation for series with internal NaN values

docs/sphinx/source/whatsnew/v0.15.1.rst

@@ -14,6 +14,12 @@ Deprecations
 
 Bug fixes
 ~~~~~~~~~
+* Fix incorrect ``delta_kt_prime`` calculation in
+  :py:func:`pvlib.irradiance.dirint` for series containing internal NaN
+  values (e.g. multi-day data with nighttime gaps). Edge positions with
+  only one valid neighbor now correctly use that single delta value
+  (Perez eqn 3) instead of halving it.
+  (:issue:`1847`, :pull:`2698`)
 * Fix a division-by-zero condition in
   :py:func:`pvlib.transformer.simple_efficiency` when ``load_loss = 0``.
   (:issue:`2645`, :pull:`2646`)
@@ -57,6 +63,7 @@ Maintenance
 
 Contributors
 ~~~~~~~~~~~~
+* Ishaan Arora (:ghuser:`ishaan-arora-1`)
 * Aman Srivastava (:ghuser:`aman-coder03`)
 * Rajiv Daxini (:ghuser:`RDaxini`)
 * Echedey Luis (:ghuser:`echedey-ls`)

pvlib/irradiance.py

@@ -2026,16 +2026,15 @@ def _delta_kt_prime_dirint(kt_prime, use_delta_kt_prime, times):
     for use with :py:func:`_dirint_bins`.
     """
     if use_delta_kt_prime:
-        # Perez eqn 2
+        # Perez eqn 2 (both neighbors) and eqn 3 (one neighbor).
+        # mean(axis=1) skips NaN so that edge positions with only one
+        # valid neighbor return that single delta instead of halving it.
         kt_next = kt_prime.shift(-1)
         kt_previous = kt_prime.shift(1)
-        # replace nan with values that implement Perez Eq 3 for first and last
-        # positions. Use kt_previous and kt_next to handle series of length 1
-        kt_next.iloc[-1] = kt_previous.iloc[-1]
-        kt_previous.iloc[0] = kt_next.iloc[0]
-        delta_kt_prime = 0.5 * ((kt_prime - kt_next).abs().add(
-                                (kt_prime - kt_previous).abs(),
-                                fill_value=0))
+        delta_kt_prime = pd.DataFrame({
+            'next': (kt_prime - kt_next).abs(),
+            'prev': (kt_prime - kt_previous).abs(),
+        }).mean(axis=1)
     else:
         # do not change unless also modifying _dirint_bins
         delta_kt_prime = pd.Series(-1, index=times)

tests/test_irradiance.py

@@ -743,6 +743,38 @@ def test_dirint_no_delta_kt():
                         np.array([861.9,  670.4]), 1)
 
 
+def test_delta_kt_prime_dirint_multiday():
+    # GH 1847: _delta_kt_prime_dirint mishandled NaN boundaries between
+    # days, halving the delta at edge positions instead of using the
+    # single available neighbor value (Perez eqn 3).
+    times = pd.date_range('2014-01-01T05', periods=15, freq='1h',
+                          tz='Etc/GMT+0')
+    kt_prime = pd.Series(
+        [np.nan, np.nan, np.nan,
+         0.29458475, 0.21863506, 0.37650014,
+         0.41238529, 0.23375275, 0.23363453,
+         0.26348652, 0.25412631, 0.43794681,
+         np.nan, np.nan, np.nan],
+        index=times)
+    result = irradiance._delta_kt_prime_dirint(kt_prime, True, times)
+    # first valid value (index 3): only forward neighbor exists,
+    # delta_kt_prime == |kt[3] - kt[4]| (Perez eqn 3)
+    expected_first = abs(kt_prime.iloc[3] - kt_prime.iloc[4])
+    assert_almost_equal(result.iloc[3], expected_first)
+    # last valid value (index 11): only backward neighbor exists,
+    # delta_kt_prime == |kt[11] - kt[10]| (Perez eqn 3)
+    expected_last = abs(kt_prime.iloc[11] - kt_prime.iloc[10])
+    assert_almost_equal(result.iloc[11], expected_last)
+    # interior valid value (index 6): both neighbors exist,
+    # delta_kt_prime == mean of both deltas (Perez eqn 2)
+    expected_mid = 0.5 * (abs(kt_prime.iloc[6] - kt_prime.iloc[7])
+                          + abs(kt_prime.iloc[6] - kt_prime.iloc[5]))
+    assert_almost_equal(result.iloc[6], expected_mid)
+    # NaN positions should remain NaN
+    assert np.isnan(result.iloc[0])
+    assert np.isnan(result.iloc[-1])
+
+
 def test_dirint_coeffs():
     coeffs = irradiance._get_dirint_coeffs()
     assert coeffs[0, 0, 0, 0] == 0.385230