From 5c7959050296a87b4c5d2d5743ae6b1aff4566fe Mon Sep 17 00:00:00 2001
From: Nate Croft <ncroft@bay4.com>
Date: Mon, 25 Jan 2021 13:10:52 -0700
Subject: [PATCH 01/16] Support functions for ohmic losses

---
 pvlib/pvsystem.py | 65 +++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 65 insertions(+)

diff --git a/pvlib/pvsystem.py b/pvlib/pvsystem.py
index afc1c195cf..15594ecf70 100644
--- a/pvlib/pvsystem.py
+++ b/pvlib/pvsystem.py
@@ -2763,6 +2763,71 @@ def pvwatts_losses(soiling=2, shading=3, snow=0, mismatch=2, wiring=2,
     return losses
 
 
+def dc_ohms_from_percent(V_mp_ref, I_mp_ref, dc_ohmic_percent,
+                         modules_per_string=1,
+                         strings_per_inverter=1):
+    """
+    Calculates the equivalent resistance of the wires from a percent
+    ohmic loss at STC.
+
+    Equivalent resistance is calculated with the function:
+
+    .. math::
+        Rw = (L_{stc} / 100) * (Varray / Iarray)
+
+    :math:`L_{stc}` is the input dc loss as a percent, e.g. 1.5% loss is
+    input as 1.5
+
+    Parameters
+    ----------
+    V_mp_ref: numeric
+    I_mp_ref: numeric
+    dc_ohmic_percent: numeric, default 0
+    modules_per_string: numeric, default 1
+    strings_per_inverter: numeric, default 1
+
+    Returns
+    ----------
+    Rw: numeric
+        Equivalent resistance in ohms
+
+    References
+    ----------
+    -- [1] PVsyst 7 Help. "Array ohmic wiring loss".
+            https://www.pvsyst.com/help/ohmic_loss.htm
+    """
+    vmp = modules_per_string * V_mp_ref
+
+    imp = strings_per_inverter * I_mp_ref
+
+    Rw = (dc_ohmic_percent / 100) * (vmp / imp)
+
+    return Rw
+
+
+def dc_ohmic_losses(ohms, current):
+    """
+    Returns ohmic losses in in units of power from the equivalent
+    resistance of of the wires and the operating current.
+
+    Parameters
+    ----------
+    ohms: numeric, float
+    current: numeric, float or array-like
+
+    Returns
+    ----------
+    numeric
+        Single or array-like value of the losses in units of power
+
+    References
+    ----------
+    -- [1] PVsyst 7 Help. "Array ohmic wiring loss".
+            https://www.pvsyst.com/help/ohmic_loss.htm
+    """
+    return ohms * current * current
+
+
 def combine_loss_factors(index, *losses, fill_method='ffill'):
     r"""
     Combines Series loss fractions while setting a common index.

From b6fa6f62ab0cadc8287bcb0eb9b57d5796668061 Mon Sep 17 00:00:00 2001
From: Nate Croft <ncroft@bay4.com>
Date: Mon, 15 Feb 2021 10:04:23 -0700
Subject: [PATCH 02/16] PVSystem dc ohmic losses and tests

---
 pvlib/pvsystem.py            | 52 ++++++++++++++++++++++++++++++++++--
 pvlib/tests/test_pvsystem.py | 52 ++++++++++++++++++++++++++++++++++++
 2 files changed, 102 insertions(+), 2 deletions(-)

diff --git a/pvlib/pvsystem.py b/pvlib/pvsystem.py
index 15594ecf70..952b58b287 100644
--- a/pvlib/pvsystem.py
+++ b/pvlib/pvsystem.py
@@ -191,6 +191,11 @@ def __init__(self,
                  racking_model=None, losses_parameters=None, name=None):
 
         if arrays is None:
+            if losses_parameters is None:
+                array_losses_parameters = {}
+            else:
+                array_losses_parameters = _build_kwargs(['dc_ohmic_percent'],
+                                                        losses_parameters)
             self.arrays = (Array(
                 surface_tilt,
                 surface_azimuth,
@@ -202,7 +207,8 @@ def __init__(self,
                 temperature_model_parameters,
                 modules_per_string,
                 strings_per_inverter,
-                racking_model
+                racking_model,
+                array_losses_parameters,
             ),)
         else:
             self.arrays = tuple(arrays)
@@ -970,6 +976,17 @@ def pvwatts_multi(self, p_dc):
         return inverter.pvwatts_multi(p_dc, self.inverter_parameters['pdc0'],
                                       **kwargs)
 
+    @_unwrap_single_value
+    def dc_ohms_from_percent(self):
+        """
+        Calculates the equivalent resistance of the wires for each array using
+        :py:func:`pvlib.pvsystem.dc_ohms_from_percent`
+
+        See :py:func:`pvlib.pvsystem.dc_ohms_from_percent` for details.
+        """
+
+        return tuple(array.dc_ohms_from_percent() for array in self.arrays)
+
     @property
     @_unwrap_single_value
     def module_parameters(self):
@@ -1102,6 +1119,9 @@ class Array:
         Valid strings are 'open_rack', 'close_mount', and 'insulated_back'.
         Used to identify a parameter set for the SAPM cell temperature model.
 
+    array_losses_parameters: None, dict or Series, default None.
+        Supported keys are dc_ohmic_percent.
+
     """
 
     def __init__(self,
@@ -1111,7 +1131,8 @@ def __init__(self,
                  module_parameters=None,
                  temperature_model_parameters=None,
                  modules_per_string=1, strings=1,
-                 racking_model=None, name=None):
+                 racking_model=None, array_losses_parameters=None,
+                 name=None):
         self.surface_tilt = surface_tilt
         self.surface_azimuth = surface_azimuth
 
@@ -1139,6 +1160,11 @@ def __init__(self,
         else:
             self.temperature_model_parameters = temperature_model_parameters
 
+        if array_losses_parameters is None:
+            self.array_losses_parameters = {}
+        else:
+            self.array_losses_parameters = array_losses_parameters
+
         self.name = name
 
     def __repr__(self):
@@ -1325,6 +1351,28 @@ def get_iam(self, aoi, iam_model='physical'):
         else:
             raise ValueError(model + ' is not a valid IAM model')
 
+    def dc_ohms_from_percent(self):
+        """
+        Calculates the equivalent resistance of the wires using
+        :py:func:`pvlib.pvsystem.dc_ohms_from_percent`,
+        `self.losses_parameters["dc_ohmic_percent"]`,
+        `self.module_parameters["V_mp_ref"]`,
+        `self.module_parameters["I_mp_ref"]`,
+        `self.modules_per_string`, and `self.strings_per_inverter`.
+        See :py:func:`pvlib.pvsystem.dc_ohms_from_percent` for details.
+        """
+
+        kwargs = _build_kwargs(['dc_ohmic_percent'],
+                               self.array_losses_parameters)
+
+        kwargs.update(_build_kwargs(['V_mp_ref', 'I_mp_ref'],
+                                    self.module_parameters))
+
+        kwargs.update({'modules_per_string': self.modules_per_string,
+                       'strings_per_inverter': self.strings})
+
+        return dc_ohms_from_percent(**kwargs)
+
 
 def calcparams_desoto(effective_irradiance, temp_cell,
                       alpha_sc, a_ref, I_L_ref, I_o_ref, R_sh_ref, R_s,
diff --git a/pvlib/tests/test_pvsystem.py b/pvlib/tests/test_pvsystem.py
index 311820828b..a58d42408c 100644
--- a/pvlib/tests/test_pvsystem.py
+++ b/pvlib/tests/test_pvsystem.py
@@ -1955,3 +1955,55 @@ def test_combine_loss_factors():
 def test_no_extra_kwargs():
     with pytest.raises(TypeError, match="arbitrary_kwarg"):
         pvsystem.PVSystem(arbitrary_kwarg='value')
+
+
+def test_dc_ohms_from_percent():
+    expected = .1425
+    out = pvsystem.dc_ohms_from_percent(38, 8, 3, 1, 1)
+    assert_allclose(out, expected)
+
+
+def test_PVSystem_dc_ohms_from_percent(mocker):
+    mocker.spy(pvsystem, 'dc_ohms_from_percent')
+
+    expected = .1425
+    system = pvsystem.PVSystem(losses_parameters={'dc_ohmic_percent': 3},
+                               module_parameters={'I_mp_ref': 8,
+                                                  'V_mp_ref': 38})
+    out = system.dc_ohms_from_percent()
+
+    pvsystem.dc_ohms_from_percent.assert_called_once_with(
+        dc_ohmic_percent=3,
+        V_mp_ref=38,
+        I_mp_ref=8,
+        modules_per_string=1,
+        strings_per_inverter=1
+    )
+
+    assert_allclose(out, expected)
+
+
+def test_dc_ohmic_losses():
+    expected = 9.12
+    out = pvsystem.dc_ohmic_losses(.1425, 8)
+    assert_allclose(out, expected)
+
+
+def test_Array_dc_ohms_from_percent(mocker):
+    mocker.spy(pvsystem, 'dc_ohms_from_percent')
+
+    expected = .1425
+    array = pvsystem.Array(array_losses_parameters={'dc_ohmic_percent': 3},
+                           module_parameters={'I_mp_ref': 8,
+                                              'V_mp_ref': 38})
+    out = array.dc_ohms_from_percent()
+
+    pvsystem.dc_ohms_from_percent.assert_called_once_with(
+        dc_ohmic_percent=3,
+        V_mp_ref=38,
+        I_mp_ref=8,
+        modules_per_string=1,
+        strings_per_inverter=1
+    )
+
+    assert_allclose(out, expected)

From 8b6769ca0af83654e2a0d086c788b16c2a8746b9 Mon Sep 17 00:00:00 2001
From: Nate Croft <ncroft@bay4.com>
Date: Mon, 15 Feb 2021 10:32:22 -0700
Subject: [PATCH 03/16] ModelChain dc ohimc losses as tests

---
 pvlib/modelchain.py            | 43 ++++++++++++++++++++++
 pvlib/tests/test_modelchain.py | 67 ++++++++++++++++++++++++++++++++++
 2 files changed, 110 insertions(+)

diff --git a/pvlib/modelchain.py b/pvlib/modelchain.py
index c9c578bf96..6a56b3a532 100644
--- a/pvlib/modelchain.py
+++ b/pvlib/modelchain.py
@@ -355,6 +355,11 @@ class ModelChain:
         The ModelChain instance will be passed as the first argument to a
         user-defined function.
 
+    dc_ohmic_model: None, str or function, default None
+        Valid strings are 'dc_ohms_from_percent', 'no_loss'. The ModelChain
+        instance will be passed as the first argument to a user-defined
+        function.
+
     losses_model: str or function, default 'no_loss'
         Valid strings are 'pvwatts', 'no_loss'. The ModelChain instance
         will be passed as the first argument to a user-defined function.
@@ -377,6 +382,7 @@ def __init__(self, system, location,
                  airmass_model='kastenyoung1989',
                  dc_model=None, ac_model=None, aoi_model=None,
                  spectral_model=None, temperature_model=None,
+                 dc_ohmic_model=None,
                  losses_model='no_loss', name=None):
 
         self.name = name
@@ -395,7 +401,9 @@ def __init__(self, system, location,
         self.spectral_model = spectral_model
         self.temperature_model = temperature_model
 
+        self.dc_ohmic_model = dc_ohmic_model
         self.losses_model = losses_model
+
         self.orientation_strategy = orientation_strategy
 
         self.weather = None
@@ -1058,6 +1066,40 @@ def faiman_temp(self):
     def fuentes_temp(self):
         return self._set_celltemp(self.system.fuentes_celltemp)
 
+    @property
+    def dc_ohmic_model(self):
+        return self._dc_ohmic_model
+
+    @dc_ohmic_model.setter
+    def dc_ohmic_model(self, model):
+        if model is None:
+            self._dc_ohmic_model = self.no_dc_ohmic_loss
+        elif isinstance(model, str):
+            model = model.lower()
+            if model == 'dc_ohms_from_percent':
+                self._dc_ohmic_model = self.dc_ohms_from_percent
+            elif model == 'no_loss':
+                self._dc_ohmic_model = self.no_dc_ohmic_loss
+            else:
+                raise ValueError(model + ' is not a valid losses model')
+        else:
+            self._dc_ohmic_model = partial(model, self)
+
+    def dc_ohms_from_percent(self):
+        """
+        Calculate time series of ohmic losses and apply those to the mpp power
+        output of the `dc_model` based on the pvsyst equivalent resistance
+        method. Uses a `dc_ohmic_percent` parameter in the `losses_parameters`
+        of the PVsystem.
+        """
+        Rw = self.system.dc_ohms_from_percent()
+        self.dc_ohmic_losses = pvsystem.dc_ohmic_losses(Rw, self.dc['i_mp'])
+        self.dc['p_mp'] = self.dc['p_mp'] - self.dc_ohmic_losses
+        return self
+
+    def no_dc_ohmic_loss(self):
+        return self
+
     @property
     def losses_model(self):
         return self._losses_model
@@ -1728,6 +1770,7 @@ def _run_from_effective_irrad(self, data=None):
         """
         self._prepare_temperature(data)
         self.dc_model()
+        self.dc_ohmic_model()
         self.losses_model()
         self.ac_model()
 
diff --git a/pvlib/tests/test_modelchain.py b/pvlib/tests/test_modelchain.py
index 9085b02b13..1a22ea7664 100644
--- a/pvlib/tests/test_modelchain.py
+++ b/pvlib/tests/test_modelchain.py
@@ -1378,6 +1378,73 @@ def constant_losses(mc):
     mc.results.dc *= mc.losses
 
 
+def dc_constant_losses(mc):
+    mc.results.dc['p_mp'] *= 0.9
+
+
+def test_dc_ohmic_model_ohms_from_percent(cec_dc_snl_ac_system,
+                                          location,
+                                          weather,
+                                          mocker):
+
+    m = mocker.spy(pvsystem, 'dc_ohms_from_percent')
+
+    system = cec_dc_snl_ac_system
+
+    for array in system.arrays:
+        array.array_losses_parameters = dict(dc_ohmic_percent=3)
+
+    mc = ModelChain(system, location,
+                    aoi_model='no_loss',
+                    spectral_model='no_loss',
+                    dc_ohmic_model='dc_ohms_from_percent')
+    mc.run_model(weather)
+
+    assert m.call_count == 1
+
+    assert isinstance(mc.dc_ohmic_losses, pd.Series)
+
+
+def test_dc_ohmic_model_no_dc_ohmic_loss(cec_dc_snl_ac_system,
+                                         location,
+                                         weather,
+                                         mocker):
+
+    m = mocker.spy(modelchain.ModelChain, 'no_dc_ohmic_loss')
+    mc = ModelChain(cec_dc_snl_ac_system, location,
+                    aoi_model='no_loss',
+                    spectral_model='no_loss',
+                    dc_ohmic_model='no_loss')
+    mc.run_model(weather)
+
+    assert m.call_count == 1
+    assert hasattr(mc, 'dc_ohmic_losses') is False
+
+
+def test_dc_ohmic_ext_def(cec_dc_snl_ac_system, location,
+                          weather, mocker):
+    m = mocker.spy(sys.modules[__name__], 'dc_constant_losses')
+    mc = ModelChain(cec_dc_snl_ac_system, location,
+                    aoi_model='no_loss',
+                    spectral_model='no_loss',
+                    dc_ohmic_model=dc_constant_losses)
+    mc.run_model(weather)
+
+    assert m.call_count == 1
+    assert isinstance(mc.results.ac, (pd.Series, pd.DataFrame))
+    assert not mc.results.ac.empty
+
+
+def test_dc_ohmic_not_a_model(cec_dc_snl_ac_system, location,
+                              weather, mocker):
+    exc_text = 'not_a_dc_model is not a valid losses model'
+    with pytest.raises(ValueError, match=exc_text):
+        ModelChain(cec_dc_snl_ac_system, location,
+                   aoi_model='no_loss',
+                   spectral_model='no_loss',
+                   dc_ohmic_model='not_a_dc_model')
+
+
 def test_losses_models_pvwatts(pvwatts_dc_pvwatts_ac_system, location, weather,
                                mocker):
     age = 1

From ab5ecffa47833763d62209bf59a7468c3d6b8025 Mon Sep 17 00:00:00 2001
From: Nate Croft <ncroft@bay4.com>
Date: Mon, 15 Feb 2021 13:31:40 -0700
Subject: [PATCH 04/16] updates to api.rst for dc_ohmic_losses

---
 docs/sphinx/source/api.rst | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/docs/sphinx/source/api.rst b/docs/sphinx/source/api.rst
index 8805d199a4..c36ab0a7bf 100644
--- a/docs/sphinx/source/api.rst
+++ b/docs/sphinx/source/api.rst
@@ -331,6 +331,7 @@ Pvsyst model
    temperature.pvsyst_cell
    pvsystem.calcparams_pvsyst
    pvsystem.singlediode
+   pvsystem.dc_ohms_from_percent
 
 PVWatts model
 ^^^^^^^^^^^^^
@@ -389,6 +390,8 @@ Loss models
    :toctree: generated/
 
    pvsystem.combine_loss_factors
+   pvsystem.dc_ohms_from_percent
+   modelchain.ModelChain.dc_ohmic_model
 
 Snow
 ----
@@ -615,6 +618,7 @@ ModelChain properties that are aliases for your specific modeling functions.
    modelchain.ModelChain.aoi_model
    modelchain.ModelChain.spectral_model
    modelchain.ModelChain.temperature_model
+   modelchain.ModelChain.dc_ohmic_model
    modelchain.ModelChain.losses_model
    modelchain.ModelChain.effective_irradiance_model
 
@@ -645,6 +649,8 @@ ModelChain model definitions.
    modelchain.ModelChain.pvsyst_temp
    modelchain.ModelChain.faiman_temp
    modelchain.ModelChain.fuentes_temp
+   modelchain.ModelChain.dc_ohmic_model
+   modelchain.ModelChain.no_dc_ohmic_loss
    modelchain.ModelChain.pvwatts_losses
    modelchain.ModelChain.no_extra_losses
 

From f63a24b6a1925b8e7170eaeab78de559155e5c2e Mon Sep 17 00:00:00 2001
From: Nate Croft <ncroft@bay4.com>
Date: Wed, 24 Feb 2021 10:26:57 -0700
Subject: [PATCH 05/16] Docstring formatting updates

---
 pvlib/pvsystem.py | 43 ++++++++++++++++++++++++++-----------------
 1 file changed, 26 insertions(+), 17 deletions(-)

diff --git a/pvlib/pvsystem.py b/pvlib/pvsystem.py
index d721321254..cd9a109cb0 100644
--- a/pvlib/pvsystem.py
+++ b/pvlib/pvsystem.py
@@ -1167,7 +1167,7 @@ class Array:
         Used to identify a parameter set for the SAPM cell temperature model.
 
     array_losses_parameters: None, dict or Series, default None.
-        Supported keys are dc_ohmic_percent.
+        Supported keys are 'dc_ohmic_percent'.
 
     """
 
@@ -2870,26 +2870,33 @@ def dc_ohms_from_percent(V_mp_ref, I_mp_ref, dc_ohmic_percent,
     .. math::
         Rw = (L_{stc} / 100) * (Varray / Iarray)
 
-    :math:`L_{stc}` is the input dc loss as a percent, e.g. 1.5% loss is
-    input as 1.5
+    :math:`Rw` is the equivalent resistance in ohms
+    :math:`Varray` is the Vmp of the modules times modules per string
+    :math:`Iarray` is the Imp of the modules times strings per array
+    :math:`L_{stc}` is the input dc loss percent
 
     Parameters
     ----------
     V_mp_ref: numeric
+        Voltage at maximum power in reference conditions [V]
     I_mp_ref: numeric
+        Current at maximum power in reference conditions [V]
     dc_ohmic_percent: numeric, default 0
-    modules_per_string: numeric, default 1
-    strings_per_inverter: numeric, default 1
+        input dc loss as a percent, e.g. 1.5% loss is input as 1.5
+    modules_per_string: int, default 1
+        Number of modules per string in the array.
+    strings: int, default 1
+        Number of parallel strings in the array.
 
     Returns
     ----------
     Rw: numeric
-        Equivalent resistance in ohms
+        Equivalent resistance [ohm]
 
     References
     ----------
-    -- [1] PVsyst 7 Help. "Array ohmic wiring loss".
-            https://www.pvsyst.com/help/ohmic_loss.htm
+    .. [1] PVsyst 7 Help. "Array ohmic wiring loss".
+        https://www.pvsyst.com/help/ohmic_loss.htm
     """
     vmp = modules_per_string * V_mp_ref
 
@@ -2900,27 +2907,29 @@ def dc_ohms_from_percent(V_mp_ref, I_mp_ref, dc_ohmic_percent,
     return Rw
 
 
-def dc_ohmic_losses(ohms, current):
+def dc_ohmic_losses(resistance, current):
     """
-    Returns ohmic losses in in units of power from the equivalent
-    resistance of of the wires and the operating current.
+    Returns ohmic losses in units of power from the equivalent
+    resistance of the wires and the operating current.
 
     Parameters
     ----------
-    ohms: numeric, float
+    resistance: numeric
+        Equivalent resistance of wires [ohm]
     current: numeric, float or array-like
+        Operating current [A]
 
     Returns
     ----------
-    numeric
-        Single or array-like value of the losses in units of power
+    loss: numeric
+        Power Loss [W]
 
     References
     ----------
-    -- [1] PVsyst 7 Help. "Array ohmic wiring loss".
-            https://www.pvsyst.com/help/ohmic_loss.htm
+    .. [1] PVsyst 7 Help. "Array ohmic wiring loss".
+        https://www.pvsyst.com/help/ohmic_loss.htm
     """
-    return ohms * current * current
+    return resistance * current * current
 
 
 def combine_loss_factors(index, *losses, fill_method='ffill'):

From 0db5afe0725326f8e8124b2c021609198e1608b4 Mon Sep 17 00:00:00 2001
From: Nate Croft <ncroft@bay4.com>
Date: Wed, 24 Feb 2021 11:42:34 -0700
Subject: [PATCH 06/16] make dc_ohms_from_percent work with other dc models

---
 pvlib/pvsystem.py            | 68 ++++++++++++++++++++++++++++--------
 pvlib/tests/test_pvsystem.py | 12 +++----
 2 files changed, 60 insertions(+), 20 deletions(-)

diff --git a/pvlib/pvsystem.py b/pvlib/pvsystem.py
index cd9a109cb0..3e82314dca 100644
--- a/pvlib/pvsystem.py
+++ b/pvlib/pvsystem.py
@@ -1401,22 +1401,62 @@ def get_iam(self, aoi, iam_model='physical'):
     def dc_ohms_from_percent(self):
         """
         Calculates the equivalent resistance of the wires using
-        :py:func:`pvlib.pvsystem.dc_ohms_from_percent`,
+        :py:func:`pvlib.pvsystem.dc_ohms_from_percent`
+
+        Makes use of array module parameters according to the
+        following DC models:
+        CEC:
+            `self.module_parameters["V_mp_ref"]`,
+            `self.module_parameters["I_mp_ref"]`,
+        SAPM:
+            `self.module_parameters["Vmpo"]`,
+            `self.module_parameters["Impo"]`,
+        PVsyst-like or other:
+            `self.module_parameters["Vmpp"]`,
+            `self.module_parameters["Impp"]`,
+
+        Other array parameters that are used are:
         `self.losses_parameters["dc_ohmic_percent"]`,
-        `self.module_parameters["V_mp_ref"]`,
-        `self.module_parameters["I_mp_ref"]`,
-        `self.modules_per_string`, and `self.strings_per_inverter`.
-        See :py:func:`pvlib.pvsystem.dc_ohms_from_percent` for details.
+        `self.modules_per_string`, and
+        `self.strings`.
+
+        See :py:func:`pvlib.pvsystem.dc_ohms_from_percent` for more details.
         """
 
         kwargs = _build_kwargs(['dc_ohmic_percent'],
                                self.array_losses_parameters)
 
-        kwargs.update(_build_kwargs(['V_mp_ref', 'I_mp_ref'],
-                                    self.module_parameters))
+        # add relevent Vmp and Imp parameters from CEC parameters
+        if all([elem in self.module_parameters
+                for elem in ['V_mp_ref', 'I_mp_ref']]):
+            kwargs.update({'vmp_ref': self.module_parameters['V_mp_ref'],
+                           'imp_ref': self.module_parameters['I_mp_ref']})
+
+
+        # add relevant Vmp and Imp parameters from SAPM parameters
+        elif all([elem in self.module_parameters
+                  for elem in ['Vmpo', 'Impo']]):
+            kwargs.update({'vmp_ref': self.module_parameters['Vmpo'],
+                           'imp_ref': self.module_parameters['Impo']})
+
+        # add relevant Vmp and Imp parameters if they are PVsyst-like
+        elif all([elem in self.module_parameters
+                  for elem in ['Vmpp', 'Impp']]):
+            kwargs.update({'vmp_ref': self.module_parameters['Vmpp'],
+                           'imp_ref': self.module_parameters['Impp']})
+
+        # raise error if relevant Vmp and Imp parameters are not found
+        else:
+            raise ValueError('Parameters for Vmp and Imp could not be found ',
+                             'in the array module parameters. Module ',
+                             'parameters must include one set of ',
+                             '{"V_mp_ref", "I_mp_Ref"}, '
+                             '{"Vmpo", "Impo"}, or '
+                             '{"Vmpp", "Impp"}, '
+                             )
 
         kwargs.update({'modules_per_string': self.modules_per_string,
-                       'strings_per_inverter': self.strings})
+                       'strings': self.strings})
 
         return dc_ohms_from_percent(**kwargs)
 
@@ -2858,9 +2898,9 @@ def pvwatts_losses(soiling=2, shading=3, snow=0, mismatch=2, wiring=2,
     return losses
 
 
-def dc_ohms_from_percent(V_mp_ref, I_mp_ref, dc_ohmic_percent,
+def dc_ohms_from_percent(vmp_ref, imp_ref, dc_ohmic_percent,
                          modules_per_string=1,
-                         strings_per_inverter=1):
+                         strings=1):
     """
     Calculates the equivalent resistance of the wires from a percent
     ohmic loss at STC.
@@ -2877,9 +2917,9 @@ def dc_ohms_from_percent(V_mp_ref, I_mp_ref, dc_ohmic_percent,
 
     Parameters
     ----------
-    V_mp_ref: numeric
+    vmp_ref: numeric
         Voltage at maximum power in reference conditions [V]
-    I_mp_ref: numeric
+    imp_ref: numeric
         Current at maximum power in reference conditions [V]
     dc_ohmic_percent: numeric, default 0
         input dc loss as a percent, e.g. 1.5% loss is input as 1.5
@@ -2898,9 +2938,9 @@ def dc_ohms_from_percent(V_mp_ref, I_mp_ref, dc_ohmic_percent,
     .. [1] PVsyst 7 Help. "Array ohmic wiring loss".
         https://www.pvsyst.com/help/ohmic_loss.htm
     """
-    vmp = modules_per_string * V_mp_ref
+    vmp = modules_per_string * vmp_ref
 
-    imp = strings_per_inverter * I_mp_ref
+    imp = strings * imp_ref
 
     Rw = (dc_ohmic_percent / 100) * (vmp / imp)
 
diff --git a/pvlib/tests/test_pvsystem.py b/pvlib/tests/test_pvsystem.py
index ea232dbfdd..23e0e4573c 100644
--- a/pvlib/tests/test_pvsystem.py
+++ b/pvlib/tests/test_pvsystem.py
@@ -2063,10 +2063,10 @@ def test_PVSystem_dc_ohms_from_percent(mocker):
 
     pvsystem.dc_ohms_from_percent.assert_called_once_with(
         dc_ohmic_percent=3,
-        V_mp_ref=38,
-        I_mp_ref=8,
+        vmp_ref=38,
+        imp_ref=8,
         modules_per_string=1,
-        strings_per_inverter=1
+        strings=1
     )
 
     assert_allclose(out, expected)
@@ -2089,10 +2089,10 @@ def test_Array_dc_ohms_from_percent(mocker):
 
     pvsystem.dc_ohms_from_percent.assert_called_once_with(
         dc_ohmic_percent=3,
-        V_mp_ref=38,
-        I_mp_ref=8,
+        vmp_ref=38,
+        imp_ref=8,
         modules_per_string=1,
-        strings_per_inverter=1
+        strings=1
     )
 
     assert_allclose(out, expected)

From a8a911efabf2f336820d4db6892c8a8e648fb2de Mon Sep 17 00:00:00 2001
From: Nate Croft <ncroft@bay4.com>
Date: Wed, 24 Feb 2021 12:31:48 -0700
Subject: [PATCH 07/16] format fixes

---
 pvlib/pvsystem.py | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/pvlib/pvsystem.py b/pvlib/pvsystem.py
index 3e82314dca..cb3d81915f 100644
--- a/pvlib/pvsystem.py
+++ b/pvlib/pvsystem.py
@@ -1432,7 +1432,6 @@ def dc_ohms_from_percent(self):
             kwargs.update({'vmp_ref': self.module_parameters['V_mp_ref'],
                            'imp_ref': self.module_parameters['I_mp_ref']})
 
-
         # add relevant Vmp and Imp parameters from SAPM parameters
         elif all([elem in self.module_parameters
                   for elem in ['Vmpo', 'Impo']]):
@@ -2936,7 +2935,7 @@ def dc_ohms_from_percent(vmp_ref, imp_ref, dc_ohmic_percent,
     References
     ----------
     .. [1] PVsyst 7 Help. "Array ohmic wiring loss".
-        https://www.pvsyst.com/help/ohmic_loss.htm
+       https://www.pvsyst.com/help/ohmic_loss.htm
     """
     vmp = modules_per_string * vmp_ref
 
@@ -2967,7 +2966,7 @@ def dc_ohmic_losses(resistance, current):
     References
     ----------
     .. [1] PVsyst 7 Help. "Array ohmic wiring loss".
-        https://www.pvsyst.com/help/ohmic_loss.htm
+       https://www.pvsyst.com/help/ohmic_loss.htm
     """
     return resistance * current * current
 

From 3c7b85d5f0852eda4aa8070277d9a8aec4179828 Mon Sep 17 00:00:00 2001
From: Nate Croft <ncroft@bay4.com>
Date: Wed, 24 Feb 2021 14:12:35 -0700
Subject: [PATCH 08/16] support for multiple arrays

---
 pvlib/modelchain.py | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/pvlib/modelchain.py b/pvlib/modelchain.py
index aca42fe791..5bb28ccfe6 100644
--- a/pvlib/modelchain.py
+++ b/pvlib/modelchain.py
@@ -1103,8 +1103,19 @@ def dc_ohms_from_percent(self):
         of the PVsystem.
         """
         Rw = self.system.dc_ohms_from_percent()
-        self.dc_ohmic_losses = pvsystem.dc_ohmic_losses(Rw, self.dc['i_mp'])
-        self.dc['p_mp'] = self.dc['p_mp'] - self.dc_ohmic_losses
+        if isinstance(self.results.dc, tuple):
+            self.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.dc_ohmic_losses):
+                df['p_mp'] = df['p_mp'] - loss
+        else:
+            self.dc_ohmic_losses = pvsystem.dc_ohmic_losses(
+                Rw, self.results.dc['i_mp']
+            )
+            self.results.dc['p_mp'] = (self.results.dc['p_mp']
+                                       - self.dc_ohmic_losses)
         return self
 
     def no_dc_ohmic_loss(self):

From 8749450a3941ea6a1007341e86d001a8f3ccddae Mon Sep 17 00:00:00 2001
From: Nate Croft <ncroft@bay4.com>
Date: Wed, 24 Feb 2021 15:23:01 -0700
Subject: [PATCH 09/16] added tests for new module parameters

---
 pvlib/pvsystem.py            |  8 ++++----
 pvlib/tests/test_pvsystem.py | 38 +++++++++++++++++++++++++++++++++++-
 2 files changed, 41 insertions(+), 5 deletions(-)

diff --git a/pvlib/pvsystem.py b/pvlib/pvsystem.py
index cb3d81915f..066e439fe0 100644
--- a/pvlib/pvsystem.py
+++ b/pvlib/pvsystem.py
@@ -1446,12 +1446,12 @@ def dc_ohms_from_percent(self):
 
         # raise error if relevant Vmp and Imp parameters are not found
         else:
-            raise ValueError('Parameters for Vmp and Imp could not be found ',
-                             'in the array module parameters. Module ',
-                             'parameters must include one set of ',
+            raise ValueError('Parameters for Vmp and Imp could not be found '
+                             'in the array module parameters. Module '
+                             'parameters must include one set of '
                              '{"V_mp_ref", "I_mp_Ref"}, '
                              '{"Vmpo", "Impo"}, or '
-                             '{"Vmpp", "Impp"}, '
+                             '{"Vmpp", "Impp"}.'
                              )
 
         kwargs.update({'modules_per_string': self.modules_per_string,
diff --git a/pvlib/tests/test_pvsystem.py b/pvlib/tests/test_pvsystem.py
index 23e0e4573c..21d0af5564 100644
--- a/pvlib/tests/test_pvsystem.py
+++ b/pvlib/tests/test_pvsystem.py
@@ -2082,17 +2082,53 @@ def test_Array_dc_ohms_from_percent(mocker):
     mocker.spy(pvsystem, 'dc_ohms_from_percent')
 
     expected = .1425
+
     array = pvsystem.Array(array_losses_parameters={'dc_ohmic_percent': 3},
                            module_parameters={'I_mp_ref': 8,
                                               'V_mp_ref': 38})
     out = array.dc_ohms_from_percent()
+    pvsystem.dc_ohms_from_percent.assert_called_with(
+        dc_ohmic_percent=3,
+        vmp_ref=38,
+        imp_ref=8,
+        modules_per_string=1,
+        strings=1
+    )
+    assert_allclose(out, expected)
 
-    pvsystem.dc_ohms_from_percent.assert_called_once_with(
+    array = pvsystem.Array(array_losses_parameters={'dc_ohmic_percent': 3},
+                           module_parameters={'Impo': 8,
+                                              'Vmpo': 38})
+    out = array.dc_ohms_from_percent()
+    pvsystem.dc_ohms_from_percent.assert_called_with(
         dc_ohmic_percent=3,
         vmp_ref=38,
         imp_ref=8,
         modules_per_string=1,
         strings=1
     )
+    assert_allclose(out, expected)
 
+    array = pvsystem.Array(array_losses_parameters={'dc_ohmic_percent': 3},
+                           module_parameters={'Impp': 8,
+                                              'Vmpp': 38})
+    out = array.dc_ohms_from_percent()
+
+    pvsystem.dc_ohms_from_percent.assert_called_with(
+        dc_ohmic_percent=3,
+        vmp_ref=38,
+        imp_ref=8,
+        modules_per_string=1,
+        strings=1
+    )
     assert_allclose(out, expected)
+
+    with pytest.raises(ValueError,
+                       match=('Parameters for Vmp and Imp could not be found '
+                              'in the array module parameters. Module '
+                              'parameters must include one set of '
+                              '{"V_mp_ref", "I_mp_Ref"}, '
+                              '{"Vmpo", "Impo"}, or '
+                              '{"Vmpp", "Impp"}.')):
+        array = pvsystem.Array(array_losses_parameters={'dc_ohmic_percent': 3})
+        out = array.dc_ohms_from_percent()

From 4500bc71a1349f29384cc8f3f54f59256555c4a7 Mon Sep 17 00:00:00 2001
From: Nate Croft <ncroft@bay4.com>
Date: Wed, 24 Feb 2021 15:42:16 -0700
Subject: [PATCH 10/16] modelchain tests for multiple array support

---
 pvlib/tests/test_modelchain.py | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/pvlib/tests/test_modelchain.py b/pvlib/tests/test_modelchain.py
index 1a62a74bb5..b95e282b3a 100644
--- a/pvlib/tests/test_modelchain.py
+++ b/pvlib/tests/test_modelchain.py
@@ -1496,6 +1496,7 @@ def dc_constant_losses(mc):
 
 
 def test_dc_ohmic_model_ohms_from_percent(cec_dc_snl_ac_system,
+                                          cec_dc_snl_ac_arrays,
                                           location,
                                           weather,
                                           mocker):
@@ -1517,6 +1518,22 @@ def test_dc_ohmic_model_ohms_from_percent(cec_dc_snl_ac_system,
 
     assert isinstance(mc.dc_ohmic_losses, pd.Series)
 
+    system = cec_dc_snl_ac_arrays
+
+    for array in system.arrays:
+        array.array_losses_parameters = dict(dc_ohmic_percent=3)
+
+    mc = ModelChain(system, location,
+                    aoi_model='no_loss',
+                    spectral_model='no_loss',
+                    dc_ohmic_model='dc_ohms_from_percent')
+    mc.run_model(weather)
+
+    assert m.call_count == 3
+    assert len(mc.dc_ohmic_losses) == len(mc.system.arrays)
+
+    assert isinstance(mc.dc_ohmic_losses, tuple)
+
 
 def test_dc_ohmic_model_no_dc_ohmic_loss(cec_dc_snl_ac_system,
                                          location,

From 547f6c192d85386bf0b982d2744a0a306d63f47d Mon Sep 17 00:00:00 2001
From: Nate Croft <ncroft@bay4.com>
Date: Fri, 5 Mar 2021 12:15:16 -0700
Subject: [PATCH 11/16] fix api, document render, and use explict variables

---
 docs/sphinx/source/api.rst |  2 +-
 pvlib/pvsystem.py          | 49 +++++++++++++++++++++-----------------
 2 files changed, 28 insertions(+), 23 deletions(-)

diff --git a/docs/sphinx/source/api.rst b/docs/sphinx/source/api.rst
index c36ab0a7bf..43f598c7c7 100644
--- a/docs/sphinx/source/api.rst
+++ b/docs/sphinx/source/api.rst
@@ -332,6 +332,7 @@ Pvsyst model
    pvsystem.calcparams_pvsyst
    pvsystem.singlediode
    pvsystem.dc_ohms_from_percent
+   pvsystem.dc_ohmic_losses
 
 PVWatts model
 ^^^^^^^^^^^^^
@@ -391,7 +392,6 @@ Loss models
 
    pvsystem.combine_loss_factors
    pvsystem.dc_ohms_from_percent
-   modelchain.ModelChain.dc_ohmic_model
 
 Snow
 ----
diff --git a/pvlib/pvsystem.py b/pvlib/pvsystem.py
index 066e439fe0..de37aa877e 100644
--- a/pvlib/pvsystem.py
+++ b/pvlib/pvsystem.py
@@ -1405,15 +1405,21 @@ def dc_ohms_from_percent(self):
 
         Makes use of array module parameters according to the
         following DC models:
+
         CEC:
-            `self.module_parameters["V_mp_ref"]`,
-            `self.module_parameters["I_mp_ref"]`,
+
+            * `self.module_parameters["V_mp_ref"]`,
+            * `self.module_parameters["I_mp_ref"]`
+
         SAPM:
-            `self.module_parameters["Vmpo"]`,
-            `self.module_parameters["Impo"]`,
+
+            * `self.module_parameters["Vmpo"]`,
+            * `self.module_parameters["Impo"]`
+
         PVsyst-like or other:
-            `self.module_parameters["Vmpp"]`,
-            `self.module_parameters["Impp"]`,
+
+            * `self.module_parameters["Vmpp"]`,
+            * `self.module_parameters["Impp"]`
 
         Other array parameters that are used are:
         `self.losses_parameters["dc_ohmic_percent"]`,
@@ -1423,26 +1429,23 @@ def dc_ohms_from_percent(self):
         See :py:func:`pvlib.pvsystem.dc_ohms_from_percent` for more details.
         """
 
-        kwargs = _build_kwargs(['dc_ohmic_percent'],
-                               self.array_losses_parameters)
-
-        # add relevent Vmp and Imp parameters from CEC parameters
+        # get relevent Vmp and Imp parameters from CEC parameters
         if all([elem in self.module_parameters
                 for elem in ['V_mp_ref', 'I_mp_ref']]):
-            kwargs.update({'vmp_ref': self.module_parameters['V_mp_ref'],
-                           'imp_ref': self.module_parameters['I_mp_ref']})
+            vmp_ref = self.module_parameters['V_mp_ref']
+            imp_ref = self.module_parameters['I_mp_ref']
 
-        # add relevant Vmp and Imp parameters from SAPM parameters
+        # get relevant Vmp and Imp parameters from SAPM parameters
         elif all([elem in self.module_parameters
                   for elem in ['Vmpo', 'Impo']]):
-            kwargs.update({'vmp_ref': self.module_parameters['Vmpo'],
-                           'imp_ref': self.module_parameters['Impo']})
+            vmp_ref = self.module_parameters['Vmpo']
+            imp_ref = self.module_parameters['Impo']
 
-        # add relevant Vmp and Imp parameters if they are PVsyst-like
+        # get relevant Vmp and Imp parameters if they are PVsyst-like
         elif all([elem in self.module_parameters
                   for elem in ['Vmpp', 'Impp']]):
-            kwargs.update({'vmp_ref': self.module_parameters['Vmpp'],
-                           'imp_ref': self.module_parameters['Impp']})
+            vmp_ref = self.module_parameters['Vmpp']
+            imp_ref = self.module_parameters['Impp']
 
         # raise error if relevant Vmp and Imp parameters are not found
         else:
@@ -1454,10 +1457,12 @@ def dc_ohms_from_percent(self):
                              '{"Vmpp", "Impp"}.'
                              )
 
-        kwargs.update({'modules_per_string': self.modules_per_string,
-                       'strings': self.strings})
-
-        return dc_ohms_from_percent(**kwargs)
+        return dc_ohms_from_percent(
+            vmp_ref,
+            imp_ref,
+            self.array_losses_parameters['dc_ohmic_percent'],
+            self.modules_per_string,
+            self.strings)
 
 
 def calcparams_desoto(effective_irradiance, temp_cell,

From 68857abf63b97e7580cbe0797cfe8f4c05e27c12 Mon Sep 17 00:00:00 2001
From: Nate Croft <ncroft@bay4.com>
Date: Fri, 5 Mar 2021 13:06:43 -0700
Subject: [PATCH 12/16] resolve conflict and small format fix

---
 pvlib/modelchain.py | 2 --
 pvlib/pvsystem.py   | 6 +++---
 2 files changed, 3 insertions(+), 5 deletions(-)

diff --git a/pvlib/modelchain.py b/pvlib/modelchain.py
index 5bb28ccfe6..294f280d61 100644
--- a/pvlib/modelchain.py
+++ b/pvlib/modelchain.py
@@ -429,8 +429,6 @@ def __init__(self, system, location,
         self.dc_ohmic_model = dc_ohmic_model
         self.losses_model = losses_model
 
-        self.orientation_strategy = orientation_strategy
-
         self.weather = None
         self.times = None
 
diff --git a/pvlib/pvsystem.py b/pvlib/pvsystem.py
index de37aa877e..612dc69e11 100644
--- a/pvlib/pvsystem.py
+++ b/pvlib/pvsystem.py
@@ -1408,17 +1408,17 @@ def dc_ohms_from_percent(self):
 
         CEC:
 
-            * `self.module_parameters["V_mp_ref"]`,
+            * `self.module_parameters["V_mp_ref"]`
             * `self.module_parameters["I_mp_ref"]`
 
         SAPM:
 
-            * `self.module_parameters["Vmpo"]`,
+            * `self.module_parameters["Vmpo"]`
             * `self.module_parameters["Impo"]`
 
         PVsyst-like or other:
 
-            * `self.module_parameters["Vmpp"]`,
+            * `self.module_parameters["Vmpp"]`
             * `self.module_parameters["Impp"]`
 
         Other array parameters that are used are:

From 26be1fe96b6e11ff9aeb9973a3e6b1db943f237d Mon Sep 17 00:00:00 2001
From: Nate Croft <ncroft@bay4.com>
Date: Fri, 5 Mar 2021 14:14:49 -0700
Subject: [PATCH 13/16] dc_ohmic_losses as results attribute

---
 pvlib/modelchain.py            | 20 +++++++++++---------
 pvlib/tests/test_modelchain.py |  8 ++++----
 2 files changed, 15 insertions(+), 13 deletions(-)

diff --git a/pvlib/modelchain.py b/pvlib/modelchain.py
index 294f280d61..21c88f440e 100644
--- a/pvlib/modelchain.py
+++ b/pvlib/modelchain.py
@@ -273,7 +273,8 @@ class ModelChainResult:
     _singleton_tuples: bool = field(default=False)
     _per_array_fields = {'total_irrad', 'aoi', 'aoi_modifier',
                          'spectral_modifier', 'cell_temperature',
-                         'effective_irradiance', 'dc', 'diode_params'}
+                         'effective_irradiance', 'dc', 'diode_params',
+                         'dc_ohmic_losses'}
 
     # system-level information
     solar_position: Optional[pd.DataFrame] = field(default=None)
@@ -293,6 +294,7 @@ class ModelChainResult:
     dc: Optional[PerArray[Union[pd.Series, pd.DataFrame]]] = \
         field(default=None)
     diode_params: Optional[PerArray[pd.DataFrame]] = field(default=None)
+    dc_ohmic_losses: Optional[PerArray[pd.Series]] = field(default=None)
 
     def _result_type(self, value):
         """Coerce `value` to the correct type according to
@@ -407,7 +409,7 @@ def __init__(self, system, location,
                  airmass_model='kastenyoung1989',
                  dc_model=None, ac_model=None, aoi_model=None,
                  spectral_model=None, temperature_model=None,
-                 dc_ohmic_model=None,
+                 dc_ohmic_model='no_loss',
                  losses_model='no_loss', name=None):
 
         self.name = name
@@ -1080,9 +1082,9 @@ def dc_ohmic_model(self):
 
     @dc_ohmic_model.setter
     def dc_ohmic_model(self, model):
-        if model is None:
-            self._dc_ohmic_model = self.no_dc_ohmic_loss
-        elif isinstance(model, str):
+        # if model is None:
+        #     self._dc_ohmic_model = self.no_dc_ohmic_loss
+        if isinstance(model, str):
             model = model.lower()
             if model == 'dc_ohms_from_percent':
                 self._dc_ohmic_model = self.dc_ohms_from_percent
@@ -1102,18 +1104,18 @@ def dc_ohms_from_percent(self):
         """
         Rw = self.system.dc_ohms_from_percent()
         if isinstance(self.results.dc, tuple):
-            self.dc_ohmic_losses = tuple(
+            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.dc_ohmic_losses):
+            for df, loss in zip(self.results.dc, self.results.dc_ohmic_losses):
                 df['p_mp'] = df['p_mp'] - loss
         else:
-            self.dc_ohmic_losses = pvsystem.dc_ohmic_losses(
+            self.results.dc_ohmic_losses = pvsystem.dc_ohmic_losses(
                 Rw, self.results.dc['i_mp']
             )
             self.results.dc['p_mp'] = (self.results.dc['p_mp']
-                                       - self.dc_ohmic_losses)
+                                       - self.results.dc_ohmic_losses)
         return self
 
     def no_dc_ohmic_loss(self):
diff --git a/pvlib/tests/test_modelchain.py b/pvlib/tests/test_modelchain.py
index b95e282b3a..81e3f53628 100644
--- a/pvlib/tests/test_modelchain.py
+++ b/pvlib/tests/test_modelchain.py
@@ -1516,7 +1516,7 @@ def test_dc_ohmic_model_ohms_from_percent(cec_dc_snl_ac_system,
 
     assert m.call_count == 1
 
-    assert isinstance(mc.dc_ohmic_losses, pd.Series)
+    assert isinstance(mc.results.dc_ohmic_losses, pd.Series)
 
     system = cec_dc_snl_ac_arrays
 
@@ -1530,9 +1530,9 @@ def test_dc_ohmic_model_ohms_from_percent(cec_dc_snl_ac_system,
     mc.run_model(weather)
 
     assert m.call_count == 3
-    assert len(mc.dc_ohmic_losses) == len(mc.system.arrays)
+    assert len(mc.results.dc_ohmic_losses) == len(mc.system.arrays)
 
-    assert isinstance(mc.dc_ohmic_losses, tuple)
+    assert isinstance(mc.results.dc_ohmic_losses, tuple)
 
 
 def test_dc_ohmic_model_no_dc_ohmic_loss(cec_dc_snl_ac_system,
@@ -1548,7 +1548,7 @@ def test_dc_ohmic_model_no_dc_ohmic_loss(cec_dc_snl_ac_system,
     mc.run_model(weather)
 
     assert m.call_count == 1
-    assert hasattr(mc, 'dc_ohmic_losses') is False
+    assert mc.results.dc_ohmic_losses is None
 
 
 def test_dc_ohmic_ext_def(cec_dc_snl_ac_system, location,

From f96ac03e6f99f122d028828250cbc1a6fbc65713 Mon Sep 17 00:00:00 2001
From: Nate Croft <ncroft@bay4.com>
Date: Fri, 5 Mar 2021 14:42:10 -0700
Subject: [PATCH 14/16] fix tests and None model

---
 pvlib/modelchain.py            |  6 +++---
 pvlib/tests/test_modelchain.py | 12 ++++++++++++
 2 files changed, 15 insertions(+), 3 deletions(-)

diff --git a/pvlib/modelchain.py b/pvlib/modelchain.py
index 21c88f440e..9d177bf1ac 100644
--- a/pvlib/modelchain.py
+++ b/pvlib/modelchain.py
@@ -1082,9 +1082,9 @@ def dc_ohmic_model(self):
 
     @dc_ohmic_model.setter
     def dc_ohmic_model(self, model):
-        # if model is None:
-        #     self._dc_ohmic_model = self.no_dc_ohmic_loss
-        if isinstance(model, str):
+        if model is None:
+            self._dc_ohmic_model = self.no_dc_ohmic_loss
+        elif isinstance(model, str):
             model = model.lower()
             if model == 'dc_ohms_from_percent':
                 self._dc_ohmic_model = self.dc_ohms_from_percent
diff --git a/pvlib/tests/test_modelchain.py b/pvlib/tests/test_modelchain.py
index 81e3f53628..c49c632ebb 100644
--- a/pvlib/tests/test_modelchain.py
+++ b/pvlib/tests/test_modelchain.py
@@ -1547,6 +1547,7 @@ def test_dc_ohmic_model_no_dc_ohmic_loss(cec_dc_snl_ac_system,
                     dc_ohmic_model='no_loss')
     mc.run_model(weather)
 
+    assert mc.dc_ohmic_model == mc.no_dc_ohmic_loss
     assert m.call_count == 1
     assert mc.results.dc_ohmic_losses is None
 
@@ -1564,6 +1565,17 @@ def test_dc_ohmic_ext_def(cec_dc_snl_ac_system, location,
     assert isinstance(mc.results.ac, (pd.Series, pd.DataFrame))
     assert not mc.results.ac.empty
 
+    mc = ModelChain(cec_dc_snl_ac_system, location,
+                    aoi_model='no_loss',
+                    spectral_model='no_loss',
+                    dc_ohmic_model=None)
+    mc.run_model(weather)
+
+    assert m.call_count == 1
+    assert mc.dc_ohmic_model == mc.no_dc_ohmic_loss
+    assert isinstance(mc.results.ac, (pd.Series, pd.DataFrame))
+    assert not mc.results.ac.empty
+
 
 def test_dc_ohmic_not_a_model(cec_dc_snl_ac_system, location,
                               weather, mocker):

From ba92da61df138bf5f4842dcf51648e429cb928c9 Mon Sep 17 00:00:00 2001
From: Nate Croft <ncroft@bay4.com>
Date: Mon, 15 Mar 2021 14:34:15 -0700
Subject: [PATCH 15/16] remove none as model option

---
 pvlib/modelchain.py | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/pvlib/modelchain.py b/pvlib/modelchain.py
index 9d177bf1ac..95411b3359 100644
--- a/pvlib/modelchain.py
+++ b/pvlib/modelchain.py
@@ -382,7 +382,7 @@ class ModelChain:
         The ModelChain instance will be passed as the first argument to a
         user-defined function.
 
-    dc_ohmic_model: None, str or function, default None
+    dc_ohmic_model: str or function, default 'no_loss'
         Valid strings are 'dc_ohms_from_percent', 'no_loss'. The ModelChain
         instance will be passed as the first argument to a user-defined
         function.
@@ -1082,9 +1082,7 @@ def dc_ohmic_model(self):
 
     @dc_ohmic_model.setter
     def dc_ohmic_model(self, model):
-        if model is None:
-            self._dc_ohmic_model = self.no_dc_ohmic_loss
-        elif isinstance(model, str):
+        if isinstance(model, str):
             model = model.lower()
             if model == 'dc_ohms_from_percent':
                 self._dc_ohmic_model = self.dc_ohms_from_percent

From 19728cd2be7ca1e488fdb92d52812e3971da9ac1 Mon Sep 17 00:00:00 2001
From: Nate Croft <ncroft@bay4.com>
Date: Mon, 15 Mar 2021 14:41:10 -0700
Subject: [PATCH 16/16] remove none from tests

---
 pvlib/tests/test_modelchain.py | 11 -----------
 1 file changed, 11 deletions(-)

diff --git a/pvlib/tests/test_modelchain.py b/pvlib/tests/test_modelchain.py
index c49c632ebb..28a3adef45 100644
--- a/pvlib/tests/test_modelchain.py
+++ b/pvlib/tests/test_modelchain.py
@@ -1565,17 +1565,6 @@ def test_dc_ohmic_ext_def(cec_dc_snl_ac_system, location,
     assert isinstance(mc.results.ac, (pd.Series, pd.DataFrame))
     assert not mc.results.ac.empty
 
-    mc = ModelChain(cec_dc_snl_ac_system, location,
-                    aoi_model='no_loss',
-                    spectral_model='no_loss',
-                    dc_ohmic_model=None)
-    mc.run_model(weather)
-
-    assert m.call_count == 1
-    assert mc.dc_ohmic_model == mc.no_dc_ohmic_loss
-    assert isinstance(mc.results.ac, (pd.Series, pd.DataFrame))
-    assert not mc.results.ac.empty
-
 
 def test_dc_ohmic_not_a_model(cec_dc_snl_ac_system, location,
                               weather, mocker):