Fix TypeError in dc_ohms_from_percent for single-array list input

docs/sphinx/source/whatsnew/v0.16.0.rst

@@ -19,6 +19,10 @@ Deprecations
 
 Bug fixes
 ~~~~~~~~~
+* Fixed a ``TypeError`` in :py:class:`~pvlib.modelchain.ModelChain` when
+  ``dc_ohmic_model="dc_ohms_from_percent"`` is used with a single-Array system
+  and the weather/irradiance input is passed as a length-1 list or tuple.
+  (:issue:`2829`)
 
 
 Enhancements
@@ -48,3 +52,4 @@ Maintenance
 Contributors
 ~~~~~~~~~~~~
 * Carolina Crespo (:ghuser:`cbcrespo`)
+* Sai Asish Y (:ghuser:`SAY-5`)

pvlib/modelchain.py

@@ -1032,15 +1032,16 @@ def dc_ohms_from_percent(self):
         method. Uses a `dc_ohmic_percent` parameter in the `losses_parameters`
         of the PVsystem.
         """
-        Rw = self.system.dc_ohms_from_percent()
         if isinstance(self.results.dc, tuple):
+            Rw = self.system.dc_ohms_from_percent(unwrap=False)
             self.results.dc_ohmic_losses = tuple(
                 pvsystem.dc_ohmic_losses(Rw, df['i_mp'])
                 for Rw, df in zip(Rw, self.results.dc)
             )
             for df, loss in zip(self.results.dc, self.results.dc_ohmic_losses):
                 df['p_mp'] = df['p_mp'] - loss
         else:
+            Rw = self.system.dc_ohms_from_percent()
             self.results.dc_ohmic_losses = pvsystem.dc_ohmic_losses(
                 Rw, self.results.dc['i_mp']
             )

tests/test_modelchain.py

@@ -1642,6 +1642,28 @@ def test_dc_ohmic_model_ohms_from_percent(cec_dc_snl_ac_system,
     assert isinstance(mc.results.dc_ohmic_losses, tuple)
 
 
+@pytest.mark.parametrize("input_type", [tuple, list])
+def test_dc_ohmic_model_ohms_from_percent_single_array_list_input(
+        cec_dc_snl_ac_system, location, weather, total_irrad, input_type):
+    # GH 2829: a single-Array system given list/tuple input stores
+    # results.dc as a tuple, so dc_ohms_from_percent must not unwrap Rw
+    for array in cec_dc_snl_ac_system.arrays:
+        array.array_losses_parameters = dict(dc_ohmic_percent=3)
+
+    data = weather.copy()
+    data[['poa_global', 'poa_diffuse', 'poa_direct']] = total_irrad
+    data['effective_irradiance'] = data['poa_global']
+
+    mc = ModelChain(cec_dc_snl_ac_system, location,
+                    aoi_model='no_loss',
+                    spectral_model='no_loss',
+                    dc_ohmic_model='dc_ohms_from_percent')
+    mc.run_model_from_effective_irradiance(input_type((data,)))
+
+    assert isinstance(mc.results.dc_ohmic_losses, tuple)
+    assert len(mc.results.dc_ohmic_losses) == 1
+
+
 def test_dc_ohmic_model_no_dc_ohmic_loss(cec_dc_snl_ac_system,
                                          location,
                                          weather,