From 05f54087e7bca678f6735bfdc8f4a202d4bbece7 Mon Sep 17 00:00:00 2001
From: Will Hobbs <45701090+williamhobbs@users.noreply.github.com>
Date: Tue, 7 Oct 2025 08:03:14 -0500
Subject: [PATCH 01/41] add marion's adjustment

---
 pvlib/pvsystem.py | 54 +++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 52 insertions(+), 2 deletions(-)

diff --git a/pvlib/pvsystem.py b/pvlib/pvsystem.py
index 2b703f3a52..14c96ff6bf 100644
--- a/pvlib/pvsystem.py
+++ b/pvlib/pvsystem.py
@@ -2878,14 +2878,36 @@ def scale_voltage_current_power(data, voltage=1, current=1):
 
 @renamed_kwarg_warning(
     "0.13.0", "g_poa_effective", "effective_irradiance")
-def pvwatts_dc(effective_irradiance, temp_cell, pdc0, gamma_pdc, temp_ref=25.):
+def pvwatts_dc(effective_irradiance, temp_cell, pdc0, gamma_pdc, temp_ref=25.,
+               k=0.0, capped_adjustment=False):
     r"""
-    Implements NREL's PVWatts DC power model. The PVWatts DC model [1]_ is:
+    Implements NREL's PVWatts (Version 5) DC power model. The PVWatts Version
+    5 DC model [1]_ is:
 
     .. math::
 
         P_{dc} = \frac{G_{poa eff}}{1000} P_{dc0} ( 1 + \gamma_{pdc} (T_{cell} - T_{ref}))
 
+    This model has also been referred to as the power temperature coefficient
+    model.
+
+    This function accepts an optional irradiance adjustment factor, `k`, based
+    on based on [2]_. This applies a piece-wise adjustment to power based on
+    irradiance, where `k` is the reduction in actual power at 200 W/m^2
+    relative to ideal power calculated linearly from standard test conditions,
+    normalized to nameplate power at standard test conditions.
+
+    .. math::
+
+        k=\frac{0.2P_{dc0}-P_{200}}{P_{dc0}}
+
+    For example, a 500 W module that produces 95 W at 200 W/m^2 (a 5% relative
+    reduction in efficiency) would have a value of `k` = 0.01.
+
+    This adjustment increases relative efficiency for irradiance above 1000
+    W/m^2, which may not be desired. An optional input, `capped_adjustment`,
+    modifies the adjustment from [2]_ to only apply below 1000 W/m^2.
+   
     Note that ``pdc0`` is also used as a symbol in
     :py:func:`pvlib.inverter.pvwatts`. ``pdc0`` in this function refers to the DC
     power of the modules at reference conditions. ``pdc0`` in
@@ -2909,6 +2931,11 @@ def pvwatts_dc(effective_irradiance, temp_cell, pdc0, gamma_pdc, temp_ref=25.):
     temp_ref: numeric, default 25.0
         Cell reference temperature. PVWatts defines it to be 25 C and
         is included here for flexibility. [C]
+    k: numeric, default 0.005
+        Irradiance correction factor, defined in [2]_. [unitless]
+    modified_marion: Boolean, default False
+        Optional modification to [2]_, where no adjustment is applied above
+        1000 W/m^2.
 
     Returns
     -------
@@ -2920,11 +2947,34 @@ def pvwatts_dc(effective_irradiance, temp_cell, pdc0, gamma_pdc, temp_ref=25.):
     .. [1] A. P. Dobos, "PVWatts Version 5 Manual"
            http://pvwatts.nrel.gov/downloads/pvwattsv5.pdf
            (2014).
+    .. [2] B. Marion, "Comparison of Predictive Models for
+           Photovoltaic Module Performance," 
+           https://doi.org/10.1109/PVSC.2008.4922586,
+           https://docs.nrel.gov/docs/fy08osti/42511.pdf
+           (2008).
     """  # noqa: E501
 
     pdc = (effective_irradiance * 0.001 * pdc0 *
            (1 + gamma_pdc * (temp_cell - temp_ref)))
 
+    # apply Marion's correction if k is anything but zero
+    if k != 0:
+        err_1 = (k * (1 - (1 - effective_irradiance / 200)**4) /
+                 (effective_irradiance / 1000))
+        err_2 = (k * (1000 - effective_irradiance) / (1000 - 200))
+
+        pdc_marion = np.where(effective_irradiance <= 200,
+                              pdc * (1 - err_1),
+                              pdc * (1 - err_2))
+
+        # "cap" Marion's correction at 1000 W/m^2
+        if capped_adjustment is True:
+            pdc_marion = np.where(effective_irradiance >= 1000,
+                                  pdc,
+                                  pdc_marion)
+
+        pdc = pdc_marion
+
     return pdc
 
 

From 6d7ae12f6e0e3d352017dc2f3ca271519460f4d6 Mon Sep 17 00:00:00 2001
From: Will Hobbs <45701090+williamhobbs@users.noreply.github.com>
Date: Tue, 7 Oct 2025 15:52:45 -0500
Subject: [PATCH 02/41] linting

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

diff --git a/pvlib/pvsystem.py b/pvlib/pvsystem.py
index 14c96ff6bf..c3b241339c 100644
--- a/pvlib/pvsystem.py
+++ b/pvlib/pvsystem.py
@@ -2907,7 +2907,7 @@ def pvwatts_dc(effective_irradiance, temp_cell, pdc0, gamma_pdc, temp_ref=25.,
     This adjustment increases relative efficiency for irradiance above 1000
     W/m^2, which may not be desired. An optional input, `capped_adjustment`,
     modifies the adjustment from [2]_ to only apply below 1000 W/m^2.
-   
+
     Note that ``pdc0`` is also used as a symbol in
     :py:func:`pvlib.inverter.pvwatts`. ``pdc0`` in this function refers to the DC
     power of the modules at reference conditions. ``pdc0`` in
@@ -2948,7 +2948,7 @@ def pvwatts_dc(effective_irradiance, temp_cell, pdc0, gamma_pdc, temp_ref=25.,
            http://pvwatts.nrel.gov/downloads/pvwattsv5.pdf
            (2014).
     .. [2] B. Marion, "Comparison of Predictive Models for
-           Photovoltaic Module Performance," 
+           Photovoltaic Module Performance,"
            https://doi.org/10.1109/PVSC.2008.4922586,
            https://docs.nrel.gov/docs/fy08osti/42511.pdf
            (2008).

From 40328c836840880ecd4e8c4239044c907459cbdc Mon Sep 17 00:00:00 2001
From: Will Hobbs <45701090+williamhobbs@users.noreply.github.com>
Date: Tue, 7 Oct 2025 16:56:39 -0500
Subject: [PATCH 03/41] Update pvlib/pvsystem.py

Co-authored-by: Cliff Hansen <cwhanse@sandia.gov>
---
 pvlib/pvsystem.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/pvlib/pvsystem.py b/pvlib/pvsystem.py
index c3b241339c..14018f4fe3 100644
--- a/pvlib/pvsystem.py
+++ b/pvlib/pvsystem.py
@@ -2933,7 +2933,7 @@ def pvwatts_dc(effective_irradiance, temp_cell, pdc0, gamma_pdc, temp_ref=25.,
         is included here for flexibility. [C]
     k: numeric, default 0.005
         Irradiance correction factor, defined in [2]_. [unitless]
-    modified_marion: Boolean, default False
+    cap_adjustment: Boolean, default False
         Optional modification to [2]_, where no adjustment is applied above
         1000 W/m^2.
 

From a9dfb281277210f3f23733c455c314893c8a4eb3 Mon Sep 17 00:00:00 2001
From: Will Hobbs <45701090+williamhobbs@users.noreply.github.com>
Date: Tue, 7 Oct 2025 16:56:49 -0500
Subject: [PATCH 04/41] Update pvlib/pvsystem.py

Co-authored-by: Cliff Hansen <cwhanse@sandia.gov>
---
 pvlib/pvsystem.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/pvlib/pvsystem.py b/pvlib/pvsystem.py
index 14018f4fe3..5ce37bcdad 100644
--- a/pvlib/pvsystem.py
+++ b/pvlib/pvsystem.py
@@ -2931,7 +2931,7 @@ def pvwatts_dc(effective_irradiance, temp_cell, pdc0, gamma_pdc, temp_ref=25.,
     temp_ref: numeric, default 25.0
         Cell reference temperature. PVWatts defines it to be 25 C and
         is included here for flexibility. [C]
-    k: numeric, default 0.005
+    k: numeric, optional
         Irradiance correction factor, defined in [2]_. [unitless]
     cap_adjustment: Boolean, default False
         Optional modification to [2]_, where no adjustment is applied above

From 551e722632ae28519ccbc56df27affb14c6f707f Mon Sep 17 00:00:00 2001
From: Will Hobbs <45701090+williamhobbs@users.noreply.github.com>
Date: Tue, 7 Oct 2025 16:57:18 -0500
Subject: [PATCH 05/41] Update pvlib/pvsystem.py

Co-authored-by: Cliff Hansen <cwhanse@sandia.gov>
---
 pvlib/pvsystem.py | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/pvlib/pvsystem.py b/pvlib/pvsystem.py
index 5ce37bcdad..5379776e3a 100644
--- a/pvlib/pvsystem.py
+++ b/pvlib/pvsystem.py
@@ -2934,8 +2934,7 @@ def pvwatts_dc(effective_irradiance, temp_cell, pdc0, gamma_pdc, temp_ref=25.,
     k: numeric, optional
         Irradiance correction factor, defined in [2]_. [unitless]
     cap_adjustment: Boolean, default False
-        Optional modification to [2]_, where no adjustment is applied above
-        1000 W/m^2.
+        If True, apply the optional adjustment at and below 1000 W/m^2.
 
     Returns
     -------

From 73e80d3fccf1d7f36a7cdec0ce5dd818b7a41854 Mon Sep 17 00:00:00 2001
From: Will Hobbs <45701090+williamhobbs@users.noreply.github.com>
Date: Tue, 7 Oct 2025 16:57:25 -0500
Subject: [PATCH 06/41] Update pvlib/pvsystem.py

Co-authored-by: Cliff Hansen <cwhanse@sandia.gov>
---
 pvlib/pvsystem.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/pvlib/pvsystem.py b/pvlib/pvsystem.py
index 5379776e3a..1e54ea2c33 100644
--- a/pvlib/pvsystem.py
+++ b/pvlib/pvsystem.py
@@ -2957,7 +2957,7 @@ def pvwatts_dc(effective_irradiance, temp_cell, pdc0, gamma_pdc, temp_ref=25.,
            (1 + gamma_pdc * (temp_cell - temp_ref)))
 
     # apply Marion's correction if k is anything but zero
-    if k != 0:
+    if k is not None:
         err_1 = (k * (1 - (1 - effective_irradiance / 200)**4) /
                  (effective_irradiance / 1000))
         err_2 = (k * (1000 - effective_irradiance) / (1000 - 200))

From f25b42634b4d1dd3cc6efcc35ec9e14593ff46c2 Mon Sep 17 00:00:00 2001
From: Will Hobbs <45701090+williamhobbs@users.noreply.github.com>
Date: Tue, 7 Oct 2025 16:57:53 -0500
Subject: [PATCH 07/41] Update pvlib/pvsystem.py

Co-authored-by: Cliff Hansen <cwhanse@sandia.gov>
---
 pvlib/pvsystem.py | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/pvlib/pvsystem.py b/pvlib/pvsystem.py
index 1e54ea2c33..9f75b7cb22 100644
--- a/pvlib/pvsystem.py
+++ b/pvlib/pvsystem.py
@@ -2894,8 +2894,7 @@ def pvwatts_dc(effective_irradiance, temp_cell, pdc0, gamma_pdc, temp_ref=25.,
     This function accepts an optional irradiance adjustment factor, `k`, based
     on based on [2]_. This applies a piece-wise adjustment to power based on
     irradiance, where `k` is the reduction in actual power at 200 W/m^2
-    relative to ideal power calculated linearly from standard test conditions,
-    normalized to nameplate power at standard test conditions.
+    relative to power calculated at 200 W/m^2 as 0.2*`pdc0`.
 
     .. math::
 

From 380bb5895cee96bc34d4318093f95dd6010a9c13 Mon Sep 17 00:00:00 2001
From: Will Hobbs <45701090+williamhobbs@users.noreply.github.com>
Date: Tue, 7 Oct 2025 17:01:27 -0500
Subject: [PATCH 08/41] more suggested changes

---
 pvlib/pvsystem.py | 15 +++++++--------
 1 file changed, 7 insertions(+), 8 deletions(-)

diff --git a/pvlib/pvsystem.py b/pvlib/pvsystem.py
index 9f75b7cb22..064b75fdaf 100644
--- a/pvlib/pvsystem.py
+++ b/pvlib/pvsystem.py
@@ -2879,7 +2879,7 @@ def scale_voltage_current_power(data, voltage=1, current=1):
 @renamed_kwarg_warning(
     "0.13.0", "g_poa_effective", "effective_irradiance")
 def pvwatts_dc(effective_irradiance, temp_cell, pdc0, gamma_pdc, temp_ref=25.,
-               k=0.0, capped_adjustment=False):
+               k=None, cap_adjustment=False):
     r"""
     Implements NREL's PVWatts (Version 5) DC power model. The PVWatts Version
     5 DC model [1]_ is:
@@ -2892,17 +2892,16 @@ def pvwatts_dc(effective_irradiance, temp_cell, pdc0, gamma_pdc, temp_ref=25.,
     model.
 
     This function accepts an optional irradiance adjustment factor, `k`, based
-    on based on [2]_. This applies a piece-wise adjustment to power based on
-    irradiance, where `k` is the reduction in actual power at 200 W/m^2
-    relative to power calculated at 200 W/m^2 as 0.2*`pdc0`.
+    on [2]_. This applies a piece-wise adjustment to power based on irradiance,
+    where `k` is the reduction in actual power at 200 W/m^2 relative to power
+    calculated at 200 W/m^2 as 0.2*`pdc0`. For example, a 500 W module that
+    produces 95 W at 200 W/m^2 (a 5% relative reduction in efficiency) would
+    have a value of `k` = 0.01.
 
     .. math::
 
         k=\frac{0.2P_{dc0}-P_{200}}{P_{dc0}}
 
-    For example, a 500 W module that produces 95 W at 200 W/m^2 (a 5% relative
-    reduction in efficiency) would have a value of `k` = 0.01.
-
     This adjustment increases relative efficiency for irradiance above 1000
     W/m^2, which may not be desired. An optional input, `capped_adjustment`,
     modifies the adjustment from [2]_ to only apply below 1000 W/m^2.
@@ -2966,7 +2965,7 @@ def pvwatts_dc(effective_irradiance, temp_cell, pdc0, gamma_pdc, temp_ref=25.,
                               pdc * (1 - err_2))
 
         # "cap" Marion's correction at 1000 W/m^2
-        if capped_adjustment is True:
+        if cap_adjustment is True:
             pdc_marion = np.where(effective_irradiance >= 1000,
                                   pdc,
                                   pdc_marion)

From 5d31dfd0a2708d7d85a4c53b958c100c9142340f Mon Sep 17 00:00:00 2001
From: Will Hobbs <45701090+williamhobbs@users.noreply.github.com>
Date: Wed, 8 Oct 2025 11:41:41 -0500
Subject: [PATCH 09/41] Update pvlib/pvsystem.py

doi sphinx

Co-authored-by: RDaxini <143435106+RDaxini@users.noreply.github.com>
---
 pvlib/pvsystem.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/pvlib/pvsystem.py b/pvlib/pvsystem.py
index 064b75fdaf..8d5755dd14 100644
--- a/pvlib/pvsystem.py
+++ b/pvlib/pvsystem.py
@@ -2946,7 +2946,7 @@ def pvwatts_dc(effective_irradiance, temp_cell, pdc0, gamma_pdc, temp_ref=25.,
            (2014).
     .. [2] B. Marion, "Comparison of Predictive Models for
            Photovoltaic Module Performance,"
-           https://doi.org/10.1109/PVSC.2008.4922586,
+           :doi:`10.1109/PVSC.2008.4922586`,
            https://docs.nrel.gov/docs/fy08osti/42511.pdf
            (2008).
     """  # noqa: E501

From ad146b508b122d41ed7d252e5166d3a28e342999 Mon Sep 17 00:00:00 2001
From: Will Hobbs <45701090+williamhobbs@users.noreply.github.com>
Date: Wed, 8 Oct 2025 11:42:13 -0500
Subject: [PATCH 10/41] Update pvlib/pvsystem.py

Co-authored-by: RDaxini <143435106+RDaxini@users.noreply.github.com>
---
 pvlib/pvsystem.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/pvlib/pvsystem.py b/pvlib/pvsystem.py
index 8d5755dd14..be003e8d54 100644
--- a/pvlib/pvsystem.py
+++ b/pvlib/pvsystem.py
@@ -2893,7 +2893,7 @@ def pvwatts_dc(effective_irradiance, temp_cell, pdc0, gamma_pdc, temp_ref=25.,
 
     This function accepts an optional irradiance adjustment factor, `k`, based
     on [2]_. This applies a piece-wise adjustment to power based on irradiance,
-    where `k` is the reduction in actual power at 200 W/m^2 relative to power
+    where `k` is the reduction in actual power at 200 Wm⁻² relative to power
     calculated at 200 W/m^2 as 0.2*`pdc0`. For example, a 500 W module that
     produces 95 W at 200 W/m^2 (a 5% relative reduction in efficiency) would
     have a value of `k` = 0.01.

From 69ce5e743507cc00cb7d090f42a9ec91f4c72449 Mon Sep 17 00:00:00 2001
From: Will Hobbs <45701090+williamhobbs@users.noreply.github.com>
Date: Wed, 8 Oct 2025 11:42:26 -0500
Subject: [PATCH 11/41] Update pvlib/pvsystem.py

Co-authored-by: RDaxini <143435106+RDaxini@users.noreply.github.com>
---
 pvlib/pvsystem.py | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/pvlib/pvsystem.py b/pvlib/pvsystem.py
index be003e8d54..211ac2d5b2 100644
--- a/pvlib/pvsystem.py
+++ b/pvlib/pvsystem.py
@@ -2903,8 +2903,8 @@ def pvwatts_dc(effective_irradiance, temp_cell, pdc0, gamma_pdc, temp_ref=25.,
         k=\frac{0.2P_{dc0}-P_{200}}{P_{dc0}}
 
     This adjustment increases relative efficiency for irradiance above 1000
-    W/m^2, which may not be desired. An optional input, `capped_adjustment`,
-    modifies the adjustment from [2]_ to only apply below 1000 W/m^2.
+    Wm⁻², which may not be desired. An optional input, `capped_adjustment`,
+    modifies the adjustment from [2]_ to only apply below 1000 Wm⁻².
 
     Note that ``pdc0`` is also used as a symbol in
     :py:func:`pvlib.inverter.pvwatts`. ``pdc0`` in this function refers to the DC

From dc58507188ef4df9c1b4a6f4e28dd88bfea7538b Mon Sep 17 00:00:00 2001
From: Will Hobbs <45701090+williamhobbs@users.noreply.github.com>
Date: Wed, 8 Oct 2025 11:54:17 -0500
Subject: [PATCH 12/41] cleaning up docs

---
 pvlib/pvsystem.py | 63 +++++++++++++++++++++++++----------------------
 1 file changed, 33 insertions(+), 30 deletions(-)

diff --git a/pvlib/pvsystem.py b/pvlib/pvsystem.py
index 211ac2d5b2..bb9a8b903e 100644
--- a/pvlib/pvsystem.py
+++ b/pvlib/pvsystem.py
@@ -2881,36 +2881,7 @@ def scale_voltage_current_power(data, voltage=1, current=1):
 def pvwatts_dc(effective_irradiance, temp_cell, pdc0, gamma_pdc, temp_ref=25.,
                k=None, cap_adjustment=False):
     r"""
-    Implements NREL's PVWatts (Version 5) DC power model. The PVWatts Version
-    5 DC model [1]_ is:
-
-    .. math::
-
-        P_{dc} = \frac{G_{poa eff}}{1000} P_{dc0} ( 1 + \gamma_{pdc} (T_{cell} - T_{ref}))
-
-    This model has also been referred to as the power temperature coefficient
-    model.
-
-    This function accepts an optional irradiance adjustment factor, `k`, based
-    on [2]_. This applies a piece-wise adjustment to power based on irradiance,
-    where `k` is the reduction in actual power at 200 Wm⁻² relative to power
-    calculated at 200 W/m^2 as 0.2*`pdc0`. For example, a 500 W module that
-    produces 95 W at 200 W/m^2 (a 5% relative reduction in efficiency) would
-    have a value of `k` = 0.01.
-
-    .. math::
-
-        k=\frac{0.2P_{dc0}-P_{200}}{P_{dc0}}
-
-    This adjustment increases relative efficiency for irradiance above 1000
-    Wm⁻², which may not be desired. An optional input, `capped_adjustment`,
-    modifies the adjustment from [2]_ to only apply below 1000 Wm⁻².
-
-    Note that ``pdc0`` is also used as a symbol in
-    :py:func:`pvlib.inverter.pvwatts`. ``pdc0`` in this function refers to the DC
-    power of the modules at reference conditions. ``pdc0`` in
-    :py:func:`pvlib.inverter.pvwatts` refers to the DC power input limit of
-    the inverter.
+    Implement NREL's PVWatts (Version 5) DC power model. 
 
     Parameters
     ----------
@@ -2939,6 +2910,38 @@ def pvwatts_dc(effective_irradiance, temp_cell, pdc0, gamma_pdc, temp_ref=25.,
     pdc: numeric
         DC power. [W]
 
+    Notes
+    -----
+    The PVWatts Version 5 DC model [1]_ is:
+
+    .. math::
+
+        P_{dc} = \frac{G_{poa eff}}{1000} P_{dc0} ( 1 + \gamma_{pdc} (T_{cell} - T_{ref}))
+
+    This model has also been referred to as the power temperature coefficient
+    model.
+
+    This function accepts an optional irradiance adjustment factor, `k`, based
+    on [2]_. This applies a piece-wise adjustment to power based on irradiance,
+    where `k` is the reduction in actual power at 200 Wm⁻² relative to power
+    calculated at 200 Wm-2 as 0.2*`pdc0`. For example, a 500 W module that
+    produces 95 W at 200 Wm-2 (a 5% relative reduction in efficiency) would
+    have a value of `k` = 0.01.
+
+    .. math::
+
+        k=\frac{0.2P_{dc0}-P_{200}}{P_{dc0}}
+
+    This adjustment increases relative efficiency for irradiance above 1000
+    Wm⁻², which may not be desired. An optional input, `capped_adjustment`,
+    modifies the adjustment from [2]_ to only apply below 1000 Wm⁻².
+
+    Note that ``pdc0`` is also used as a symbol in
+    :py:func:`pvlib.inverter.pvwatts`. ``pdc0`` in this function refers to the DC
+    power of the modules at reference conditions. ``pdc0`` in
+    :py:func:`pvlib.inverter.pvwatts` refers to the DC power input limit of
+    the inverter.
+
     References
     ----------
     .. [1] A. P. Dobos, "PVWatts Version 5 Manual"

From 0d8b0b5f2b1e54b44f8b8cead089779a8a37bdbe Mon Sep 17 00:00:00 2001
From: Will Hobbs <45701090+williamhobbs@users.noreply.github.com>
Date: Wed, 8 Oct 2025 17:00:01 -0600
Subject: [PATCH 13/41] Update pvlib/pvsystem.py

Co-authored-by: Will Holmgren <william.holmgren@gmail.com>
---
 pvlib/pvsystem.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/pvlib/pvsystem.py b/pvlib/pvsystem.py
index bb9a8b903e..610519eea4 100644
--- a/pvlib/pvsystem.py
+++ b/pvlib/pvsystem.py
@@ -2968,7 +2968,7 @@ def pvwatts_dc(effective_irradiance, temp_cell, pdc0, gamma_pdc, temp_ref=25.,
                               pdc * (1 - err_2))
 
         # "cap" Marion's correction at 1000 W/m^2
-        if cap_adjustment is True:
+        if cap_adjustment:
             pdc_marion = np.where(effective_irradiance >= 1000,
                                   pdc,
                                   pdc_marion)

From 90b7c54e7b5c7ce6b3671971a15fde0ffd5e6c9a Mon Sep 17 00:00:00 2001
From: Will Hobbs <45701090+williamhobbs@users.noreply.github.com>
Date: Wed, 8 Oct 2025 19:58:24 -0500
Subject: [PATCH 14/41] prevent negative power

---
 pvlib/pvsystem.py | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/pvlib/pvsystem.py b/pvlib/pvsystem.py
index bb9a8b903e..57b48d644d 100644
--- a/pvlib/pvsystem.py
+++ b/pvlib/pvsystem.py
@@ -2901,7 +2901,8 @@ def pvwatts_dc(effective_irradiance, temp_cell, pdc0, gamma_pdc, temp_ref=25.,
         Cell reference temperature. PVWatts defines it to be 25 C and
         is included here for flexibility. [C]
     k: numeric, optional
-        Irradiance correction factor, defined in [2]_. [unitless]
+        Irradiance correction factor, defined in [2]_. Typically positive.
+        [unitless]
     cap_adjustment: Boolean, default False
         If True, apply the optional adjustment at and below 1000 W/m^2.
 
@@ -2973,6 +2974,10 @@ def pvwatts_dc(effective_irradiance, temp_cell, pdc0, gamma_pdc, temp_ref=25.,
                                   pdc,
                                   pdc_marion)
 
+        # large k values can result in negative power at low irradiance, so
+        # set negative power to zero
+        pdc_marion[pdc_marion < 0] = 0
+
         pdc = pdc_marion
 
     return pdc

From 4488e3ce497523f47c9796fe9a55b44ba81d5635 Mon Sep 17 00:00:00 2001
From: Will Hobbs <45701090+williamhobbs@users.noreply.github.com>
Date: Wed, 8 Oct 2025 20:04:15 -0500
Subject: [PATCH 15/41] fix typo in error eqn

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

diff --git a/pvlib/pvsystem.py b/pvlib/pvsystem.py
index a85861dfa0..2fb02cfd48 100644
--- a/pvlib/pvsystem.py
+++ b/pvlib/pvsystem.py
@@ -2960,9 +2960,8 @@ def pvwatts_dc(effective_irradiance, temp_cell, pdc0, gamma_pdc, temp_ref=25.,
 
     # apply Marion's correction if k is anything but zero
     if k is not None:
-        err_1 = (k * (1 - (1 - effective_irradiance / 200)**4) /
-                 (effective_irradiance / 1000))
-        err_2 = (k * (1000 - effective_irradiance) / (1000 - 200))
+        err_1 = k * (1 - (1 - effective_irradiance / 200)**4)
+        err_2 = k * (1000 - effective_irradiance) / (1000 - 200)
 
         pdc_marion = np.where(effective_irradiance <= 200,
                               pdc * (1 - err_1),

From 250af5a1df3287e3be97fee67581466b1827a0e7 Mon Sep 17 00:00:00 2001
From: Will Hobbs <45701090+williamhobbs@users.noreply.github.com>
Date: Wed, 8 Oct 2025 21:15:26 -0500
Subject: [PATCH 16/41] fix the right typo this time...

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

diff --git a/pvlib/pvsystem.py b/pvlib/pvsystem.py
index 2fb02cfd48..f8ba6a37e1 100644
--- a/pvlib/pvsystem.py
+++ b/pvlib/pvsystem.py
@@ -2960,8 +2960,10 @@ def pvwatts_dc(effective_irradiance, temp_cell, pdc0, gamma_pdc, temp_ref=25.,
 
     # apply Marion's correction if k is anything but zero
     if k is not None:
-        err_1 = k * (1 - (1 - effective_irradiance / 200)**4)
-        err_2 = k * (1000 - effective_irradiance) / (1000 - 200)
+        err_1 = (k * (1 - (1 - effective_irradiance / 200)**4) / 
+                 (effective_irradiance / 1000))
+        err_2 = (k * (1000 - effective_irradiance) / (1000 - 200) /
+                 (effective_irradiance / 1000))
 
         pdc_marion = np.where(effective_irradiance <= 200,
                               pdc * (1 - err_1),

From bd16ac2648dfe504bc3aec6233a8e82bef8edfe7 Mon Sep 17 00:00:00 2001
From: Will Hobbs <45701090+williamhobbs@users.noreply.github.com>
Date: Thu, 9 Oct 2025 16:07:23 -0500
Subject: [PATCH 17/41] reorder err eqns

---
 pvlib/pvsystem.py | 10 ++++------
 1 file changed, 4 insertions(+), 6 deletions(-)

diff --git a/pvlib/pvsystem.py b/pvlib/pvsystem.py
index f8ba6a37e1..04ecbf182c 100644
--- a/pvlib/pvsystem.py
+++ b/pvlib/pvsystem.py
@@ -2960,14 +2960,12 @@ def pvwatts_dc(effective_irradiance, temp_cell, pdc0, gamma_pdc, temp_ref=25.,
 
     # apply Marion's correction if k is anything but zero
     if k is not None:
-        err_1 = (k * (1 - (1 - effective_irradiance / 200)**4) / 
-                 (effective_irradiance / 1000))
-        err_2 = (k * (1000 - effective_irradiance) / (1000 - 200) /
-                 (effective_irradiance / 1000))
+        err_1 = k * (1 - (1 - effective_irradiance / 200)**4)
+        err_2 = k * (1000 - effective_irradiance) / (1000 - 200)
 
         pdc_marion = np.where(effective_irradiance <= 200,
-                              pdc * (1 - err_1),
-                              pdc * (1 - err_2))
+                              pdc - (pdc0 * err_1),
+                              pdc - (pdc0 * err_2))
 
         # "cap" Marion's correction at 1000 W/m^2
         if cap_adjustment:

From f3041c21c6817c4ee3c111e9216c27d6d744b642 Mon Sep 17 00:00:00 2001
From: Will Hobbs <45701090+williamhobbs@users.noreply.github.com>
Date: Thu, 9 Oct 2025 17:08:17 -0500
Subject: [PATCH 18/41] add more detail on `cap_adjustment`

---
 pvlib/pvsystem.py | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/pvlib/pvsystem.py b/pvlib/pvsystem.py
index 04ecbf182c..f07d698379 100644
--- a/pvlib/pvsystem.py
+++ b/pvlib/pvsystem.py
@@ -2933,9 +2933,12 @@ def pvwatts_dc(effective_irradiance, temp_cell, pdc0, gamma_pdc, temp_ref=25.,
 
         k=\frac{0.2P_{dc0}-P_{200}}{P_{dc0}}
 
-    This adjustment increases relative efficiency for irradiance above 1000
-    Wm⁻², which may not be desired. An optional input, `capped_adjustment`,
-    modifies the adjustment from [2]_ to only apply below 1000 Wm⁻².
+    For positive `k` values, and `k` is typically positive, this adjustment
+    increases relative efficiency when irradiance is above 1000 Wm⁻². This may
+    not be desired, as modules with nonlinear irradiance response often have
+    peak efficiency near 1000 Wm⁻², and it is either flat or declining at
+    higher irradiance. An optional parameter, `cap_adjustment`, can address
+    this by modifying the adjustment from [2]_ to only apply below 1000 Wm⁻².
 
     Note that ``pdc0`` is also used as a symbol in
     :py:func:`pvlib.inverter.pvwatts`. ``pdc0`` in this function refers to the DC

From 9f756bf24a233391e90f1e1cf09caf100cfd64b7 Mon Sep 17 00:00:00 2001
From: Will Hobbs <45701090+williamhobbs@users.noreply.github.com>
Date: Thu, 16 Oct 2025 09:22:46 -0500
Subject: [PATCH 19/41] return same object type that was input

---
 pvlib/pvsystem.py | 21 +++++++++++++++------
 1 file changed, 15 insertions(+), 6 deletions(-)

diff --git a/pvlib/pvsystem.py b/pvlib/pvsystem.py
index f07d698379..83ee09f820 100644
--- a/pvlib/pvsystem.py
+++ b/pvlib/pvsystem.py
@@ -2963,18 +2963,27 @@ def pvwatts_dc(effective_irradiance, temp_cell, pdc0, gamma_pdc, temp_ref=25.,
 
     # apply Marion's correction if k is anything but zero
     if k is not None:
+        pdc_marion = pdc
         err_1 = k * (1 - (1 - effective_irradiance / 200)**4)
         err_2 = k * (1000 - effective_irradiance) / (1000 - 200)
 
-        pdc_marion = np.where(effective_irradiance <= 200,
-                              pdc - (pdc0 * err_1),
-                              pdc - (pdc0 * err_2))
+        if hasattr(effective_irradiance, '__len__'):
+            pdc_marion[effective_irradiance <= 200] = pdc - (pdc0 * err_1)
+            pdc_marion[effective_irradiance > 200] = pdc - (pdc0 * err_2)
+        else:
+            if effective_irradiance <= 200:
+                pdc_marion = pdc - (pdc0 * err_1)
+            elif effective_irradiance > 200:
+                pdc_marion = pdc - (pdc0 * err_2)
+
 
         # "cap" Marion's correction at 1000 W/m^2
         if cap_adjustment:
-            pdc_marion = np.where(effective_irradiance >= 1000,
-                                  pdc,
-                                  pdc_marion)
+            if hasattr(effective_irradiance, '__len__'):
+                pdc_marion[effective_irradiance >= 1000] = pdc
+            else:
+                if effective_irradiance >= 1000:
+                    pdc_marion = pdc
 
         # large k values can result in negative power at low irradiance, so
         # set negative power to zero

From 17f7213b631ba411b43db5c3a32456c3dbc7ead7 Mon Sep 17 00:00:00 2001
From: Will Hobbs <45701090+williamhobbs@users.noreply.github.com>
Date: Thu, 16 Oct 2025 16:33:35 -0500
Subject: [PATCH 20/41] fix issue with scalars

---
 pvlib/pvsystem.py | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/pvlib/pvsystem.py b/pvlib/pvsystem.py
index 83ee09f820..8f7518ff1a 100644
--- a/pvlib/pvsystem.py
+++ b/pvlib/pvsystem.py
@@ -2976,7 +2976,6 @@ def pvwatts_dc(effective_irradiance, temp_cell, pdc0, gamma_pdc, temp_ref=25.,
             elif effective_irradiance > 200:
                 pdc_marion = pdc - (pdc0 * err_2)
 
-
         # "cap" Marion's correction at 1000 W/m^2
         if cap_adjustment:
             if hasattr(effective_irradiance, '__len__'):
@@ -2987,7 +2986,11 @@ def pvwatts_dc(effective_irradiance, temp_cell, pdc0, gamma_pdc, temp_ref=25.,
 
         # large k values can result in negative power at low irradiance, so
         # set negative power to zero
-        pdc_marion[pdc_marion < 0] = 0
+        if hasattr(effective_irradiance, '__len__'):
+            pdc_marion[pdc_marion < 0] = 0
+        else:
+            if pdc_marion < 0:
+                pdc_marion = 0
 
         pdc = pdc_marion
 

From a018525b49b3e818f158dd9ea29ba926d372ee62 Mon Sep 17 00:00:00 2001
From: Will Hobbs <45701090+williamhobbs@users.noreply.github.com>
Date: Thu, 16 Oct 2025 16:59:43 -0500
Subject: [PATCH 21/41] another fix

---
 pvlib/pvsystem.py | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/pvlib/pvsystem.py b/pvlib/pvsystem.py
index 8f7518ff1a..c024680f6d 100644
--- a/pvlib/pvsystem.py
+++ b/pvlib/pvsystem.py
@@ -2968,8 +2968,12 @@ def pvwatts_dc(effective_irradiance, temp_cell, pdc0, gamma_pdc, temp_ref=25.,
         err_2 = k * (1000 - effective_irradiance) / (1000 - 200)
 
         if hasattr(effective_irradiance, '__len__'):
-            pdc_marion[effective_irradiance <= 200] = pdc - (pdc0 * err_1)
-            pdc_marion[effective_irradiance > 200] = pdc - (pdc0 * err_2)
+            pdc_marion[effective_irradiance <= 200] = (
+                pdc[effective_irradiance <= 200] -
+                (pdc0 * err_1[effective_irradiance <= 200]))
+            pdc_marion[effective_irradiance > 200] = (
+                pdc[effective_irradiance > 200] -
+                (pdc0 * err_2[effective_irradiance > 200]))
         else:
             if effective_irradiance <= 200:
                 pdc_marion = pdc - (pdc0 * err_1)
@@ -2979,7 +2983,8 @@ def pvwatts_dc(effective_irradiance, temp_cell, pdc0, gamma_pdc, temp_ref=25.,
         # "cap" Marion's correction at 1000 W/m^2
         if cap_adjustment:
             if hasattr(effective_irradiance, '__len__'):
-                pdc_marion[effective_irradiance >= 1000] = pdc
+                pdc_marion[effective_irradiance >= 1000] = (
+                    pdc[effective_irradiance >= 1000])
             else:
                 if effective_irradiance >= 1000:
                     pdc_marion = pdc

From 816c82a9cfff5cb39d08ab76783cbd20f53d3256 Mon Sep 17 00:00:00 2001
From: Will Hobbs <45701090+williamhobbs@users.noreply.github.com>
Date: Thu, 16 Oct 2025 16:59:48 -0500
Subject: [PATCH 22/41] add tests

---
 tests/test_pvsystem.py | 40 ++++++++++++++++++++++++++++++++++++++++
 1 file changed, 40 insertions(+)

diff --git a/tests/test_pvsystem.py b/tests/test_pvsystem.py
index b7d8ba6173..2fb09b9797 100644
--- a/tests/test_pvsystem.py
+++ b/tests/test_pvsystem.py
@@ -2181,6 +2181,46 @@ def test_pvwatts_dc_series():
     assert_series_equal(expected, out)
 
 
+def test_pvwatts_dc_scalars_with_k():
+    expected = 8.9125
+    out = pvsystem.pvwatts_dc(100, 30, 100, -0.003, 25, 0.01)
+    assert_allclose(out, expected)
+
+
+def test_pvwatts_dc_arrays_with_k():
+    irrad_trans = np.array([np.nan, 100, 100])
+    temp_cell = np.array([30, np.nan, 30])
+    irrad_trans, temp_cell = np.meshgrid(irrad_trans, temp_cell)
+    expected = np.array([[nan,  8.9125,  8.9125],
+                         [nan,    nan,    nan],
+                         [nan,  8.9125,  8.9125]])
+    out = pvsystem.pvwatts_dc(irrad_trans, temp_cell, 100, -0.003, 25, 0.01)
+    assert_allclose(out, expected, equal_nan=True)
+
+
+def test_pvwatts_dc_series_with_k():
+    irrad_trans = pd.Series([np.nan, 100, 100])
+    temp_cell = pd.Series([30, np.nan, 30])
+    expected = pd.Series(np.array([   nan,    nan,  8.9125]))
+    out = pvsystem.pvwatts_dc(irrad_trans, temp_cell, 100, -0.003, 25, 0.01)
+    assert_series_equal(expected, out)
+
+
+def test_pvwatts_dc_with_k_and_cap_adjustment():
+    irrad_trans = [100, 1200]
+    temp_cell = 25
+    ks = [0.01, 0.02]
+    cap_adjustments = [False, True]
+    out = []
+    expected = [9.0625, 120.25, 8.125, 120.5, 9.0625, 120.0, 8.125, 120.0]
+    for cap_adjustment in cap_adjustments:
+        for k in ks:
+            for irrad in irrad_trans:
+                out.append(pvsystem.pvwatts_dc(irrad, temp_cell, 100, -0.003,
+                                               25, k, cap_adjustment))
+    assert_allclose(out, expected)
+
+
 def test_pvwatts_losses_default():
     expected = 14.075660688264469
     out = pvsystem.pvwatts_losses()

From a19b721239b1fafaee8f1e380fde1a40a1d5faa9 Mon Sep 17 00:00:00 2001
From: Will Hobbs <45701090+williamhobbs@users.noreply.github.com>
Date: Fri, 17 Oct 2025 08:53:21 -0500
Subject: [PATCH 23/41] increase test coverage

---
 tests/test_pvsystem.py | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/tests/test_pvsystem.py b/tests/test_pvsystem.py
index 2fb09b9797..d07a9be1e4 100644
--- a/tests/test_pvsystem.py
+++ b/tests/test_pvsystem.py
@@ -2199,9 +2199,9 @@ def test_pvwatts_dc_arrays_with_k():
 
 
 def test_pvwatts_dc_series_with_k():
-    irrad_trans = pd.Series([np.nan, 100, 100])
-    temp_cell = pd.Series([30, np.nan, 30])
-    expected = pd.Series(np.array([   nan,    nan,  8.9125]))
+    irrad_trans = pd.Series([np.nan, 100, 100, 1200])
+    temp_cell = pd.Series([30, np.nan, 30, 30])
+    expected = pd.Series(np.array([   nan,    nan,  8.9125, 118.45]))
     out = pvsystem.pvwatts_dc(irrad_trans, temp_cell, 100, -0.003, 25, 0.01)
     assert_series_equal(expected, out)
 
@@ -2209,10 +2209,10 @@ def test_pvwatts_dc_series_with_k():
 def test_pvwatts_dc_with_k_and_cap_adjustment():
     irrad_trans = [100, 1200]
     temp_cell = 25
-    ks = [0.01, 0.02]
+    ks = [0.01, 0.15]
     cap_adjustments = [False, True]
     out = []
-    expected = [9.0625, 120.25, 8.125, 120.5, 9.0625, 120.0, 8.125, 120.0]
+    expected = [9.0625, 120.25, 0, 123.75, 9.0625, 120.0, 0, 120.0]
     for cap_adjustment in cap_adjustments:
         for k in ks:
             for irrad in irrad_trans:

From 85016fab291af65b6e1ba9db35f10615c35eba96 Mon Sep 17 00:00:00 2001
From: Will Hobbs <45701090+williamhobbs@users.noreply.github.com>
Date: Fri, 17 Oct 2025 10:58:09 -0500
Subject: [PATCH 24/41] more test coverage

---
 tests/test_pvsystem.py | 18 +++++++++++++++---
 1 file changed, 15 insertions(+), 3 deletions(-)

diff --git a/tests/test_pvsystem.py b/tests/test_pvsystem.py
index d07a9be1e4..61873a9111 100644
--- a/tests/test_pvsystem.py
+++ b/tests/test_pvsystem.py
@@ -2188,12 +2188,12 @@ def test_pvwatts_dc_scalars_with_k():
 
 
 def test_pvwatts_dc_arrays_with_k():
-    irrad_trans = np.array([np.nan, 100, 100])
+    irrad_trans = np.array([np.nan, 100, 1200])
     temp_cell = np.array([30, np.nan, 30])
     irrad_trans, temp_cell = np.meshgrid(irrad_trans, temp_cell)
-    expected = np.array([[nan,  8.9125,  8.9125],
+    expected = np.array([[nan,  8.9125,  118.45],
                          [nan,    nan,    nan],
-                         [nan,  8.9125,  8.9125]])
+                         [nan,  8.9125,  118.45]])
     out = pvsystem.pvwatts_dc(irrad_trans, temp_cell, 100, -0.003, 25, 0.01)
     assert_allclose(out, expected, equal_nan=True)
 
@@ -2221,6 +2221,18 @@ def test_pvwatts_dc_with_k_and_cap_adjustment():
     assert_allclose(out, expected)
 
 
+def test_pvwatts_dc_arrays_with_k_and_cap_adjustment():
+    irrad_trans = np.array([np.nan, 100, 1200])
+    temp_cell = np.array([30, np.nan, 30])
+    irrad_trans, temp_cell = np.meshgrid(irrad_trans, temp_cell)
+    expected = np.array([[nan,  8.9125,  118.2],
+                         [nan,    nan,    nan],
+                         [nan,  8.9125,  118.2]])
+    out = pvsystem.pvwatts_dc(irrad_trans, temp_cell, 100, -0.003, 25, 0.01,
+                              True)
+    assert_allclose(out, expected, equal_nan=True)
+
+
 def test_pvwatts_losses_default():
     expected = 14.075660688264469
     out = pvsystem.pvwatts_losses()

From bed78f998c097c51580cc4ebe4c00f5faa757054 Mon Sep 17 00:00:00 2001
From: Will Hobbs <45701090+williamhobbs@users.noreply.github.com>
Date: Fri, 17 Oct 2025 10:58:24 -0500
Subject: [PATCH 25/41] fix shallow copy issue

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

diff --git a/pvlib/pvsystem.py b/pvlib/pvsystem.py
index c024680f6d..6c47e9a799 100644
--- a/pvlib/pvsystem.py
+++ b/pvlib/pvsystem.py
@@ -2881,7 +2881,7 @@ def scale_voltage_current_power(data, voltage=1, current=1):
 def pvwatts_dc(effective_irradiance, temp_cell, pdc0, gamma_pdc, temp_ref=25.,
                k=None, cap_adjustment=False):
     r"""
-    Implement NREL's PVWatts (Version 5) DC power model. 
+    Implement NREL's PVWatts (Version 5) DC power model.
 
     Parameters
     ----------
@@ -2963,7 +2963,8 @@ def pvwatts_dc(effective_irradiance, temp_cell, pdc0, gamma_pdc, temp_ref=25.,
 
     # apply Marion's correction if k is anything but zero
     if k is not None:
-        pdc_marion = pdc
+        pdc_marion = (effective_irradiance * 0.001 * pdc0 *
+                      (1 + gamma_pdc * (temp_cell - temp_ref)))
         err_1 = k * (1 - (1 - effective_irradiance / 200)**4)
         err_2 = k * (1000 - effective_irradiance) / (1000 - 200)
 

From c8b5cdee0deb87ddf0fc6e8b76727d86db228633 Mon Sep 17 00:00:00 2001
From: Will Hobbs <45701090+williamhobbs@users.noreply.github.com>
Date: Fri, 17 Oct 2025 11:01:20 -0500
Subject: [PATCH 26/41] Update pvlib/pvsystem.py

Co-authored-by: RDaxini <143435106+RDaxini@users.noreply.github.com>
---
 pvlib/pvsystem.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/pvlib/pvsystem.py b/pvlib/pvsystem.py
index 6c47e9a799..b84208eb96 100644
--- a/pvlib/pvsystem.py
+++ b/pvlib/pvsystem.py
@@ -2926,7 +2926,7 @@ def pvwatts_dc(effective_irradiance, temp_cell, pdc0, gamma_pdc, temp_ref=25.,
     on [2]_. This applies a piece-wise adjustment to power based on irradiance,
     where `k` is the reduction in actual power at 200 Wm⁻² relative to power
     calculated at 200 Wm-2 as 0.2*`pdc0`. For example, a 500 W module that
-    produces 95 W at 200 Wm-2 (a 5% relative reduction in efficiency) would
+    produces 95 W at 200 Wm⁻² (a 5% relative reduction in efficiency) would
     have a value of `k` = 0.01.
 
     .. math::

From 1f85ed25312464a1cab9a7932955d3ee330572f7 Mon Sep 17 00:00:00 2001
From: Will Hobbs <45701090+williamhobbs@users.noreply.github.com>
Date: Fri, 17 Oct 2025 11:01:44 -0500
Subject: [PATCH 27/41] Update pvlib/pvsystem.py

Co-authored-by: RDaxini <143435106+RDaxini@users.noreply.github.com>
---
 pvlib/pvsystem.py | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/pvlib/pvsystem.py b/pvlib/pvsystem.py
index b84208eb96..cb2c756e26 100644
--- a/pvlib/pvsystem.py
+++ b/pvlib/pvsystem.py
@@ -2898,8 +2898,8 @@ def pvwatts_dc(effective_irradiance, temp_cell, pdc0, gamma_pdc, temp_ref=25.,
         The temperature coefficient of power. Typically -0.002 to
         -0.005 per degree C. [1/C]
     temp_ref: numeric, default 25.0
-        Cell reference temperature. PVWatts defines it to be 25 C and
-        is included here for flexibility. [C]
+        Cell reference temperature. PVWatts defines it to be 25 °C and
+        is included here for flexibility. [°C]
     k: numeric, optional
         Irradiance correction factor, defined in [2]_. Typically positive.
         [unitless]

From e81b450d335b3c64df35212b62505ffb9c39d264 Mon Sep 17 00:00:00 2001
From: Will Hobbs <45701090+williamhobbs@users.noreply.github.com>
Date: Fri, 17 Oct 2025 11:01:56 -0500
Subject: [PATCH 28/41] Update pvlib/pvsystem.py

Co-authored-by: RDaxini <143435106+RDaxini@users.noreply.github.com>
---
 pvlib/pvsystem.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/pvlib/pvsystem.py b/pvlib/pvsystem.py
index cb2c756e26..0742bc4881 100644
--- a/pvlib/pvsystem.py
+++ b/pvlib/pvsystem.py
@@ -2904,7 +2904,7 @@ def pvwatts_dc(effective_irradiance, temp_cell, pdc0, gamma_pdc, temp_ref=25.,
         Irradiance correction factor, defined in [2]_. Typically positive.
         [unitless]
     cap_adjustment: Boolean, default False
-        If True, apply the optional adjustment at and below 1000 W/m^2.
+        If True, apply the optional adjustment at and below 1000 Wm⁻²
 
     Returns
     -------

From 3ea68efaa1bf49245ea65b17df005208fdd6e920 Mon Sep 17 00:00:00 2001
From: Will Hobbs <45701090+williamhobbs@users.noreply.github.com>
Date: Fri, 17 Oct 2025 11:36:51 -0500
Subject: [PATCH 29/41] update whatsnew

---
 docs/sphinx/source/whatsnew/v0.13.2.rst | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/docs/sphinx/source/whatsnew/v0.13.2.rst b/docs/sphinx/source/whatsnew/v0.13.2.rst
index 3177f7a022..1a680f24c5 100644
--- a/docs/sphinx/source/whatsnew/v0.13.2.rst
+++ b/docs/sphinx/source/whatsnew/v0.13.2.rst
@@ -27,8 +27,8 @@ Enhancements
   :py:func:`~pvlib.singlediode.bishop88_mpp`,
   :py:func:`~pvlib.singlediode.bishop88_v_from_i`, and
   :py:func:`~pvlib.singlediode.bishop88_i_from_v`. (:issue:`2497`, :pull:`2498`)
-
-
+* Add Marion 2008 non-linear irradiance adjustment factor to
+  :py:func:`pvlib.pvsystem.pvwatts_dc`. (:issue:`2566`, :pull:`2569`)
 
 Documentation
 ~~~~~~~~~~~~~
@@ -52,4 +52,4 @@ Maintenance
 
 Contributors
 ~~~~~~~~~~~~
-
+* Will Hobbs (:ghuser:`williamhobbs`)

From 158ff4195662f24106af2b7f1a9d04a5965b497b Mon Sep 17 00:00:00 2001
From: Will Hobbs <45701090+williamhobbs@users.noreply.github.com>
Date: Fri, 17 Oct 2025 12:00:36 -0500
Subject: [PATCH 30/41] reorganize to clean it up

---
 pvlib/pvsystem.py | 34 +++++++++++++++++++++-------------
 1 file changed, 21 insertions(+), 13 deletions(-)

diff --git a/pvlib/pvsystem.py b/pvlib/pvsystem.py
index 0742bc4881..f8bad096d2 100644
--- a/pvlib/pvsystem.py
+++ b/pvlib/pvsystem.py
@@ -2963,38 +2963,46 @@ def pvwatts_dc(effective_irradiance, temp_cell, pdc0, gamma_pdc, temp_ref=25.,
 
     # apply Marion's correction if k is anything but zero
     if k is not None:
-        pdc_marion = (effective_irradiance * 0.001 * pdc0 *
-                      (1 + gamma_pdc * (temp_cell - temp_ref)))
+        # calculate error adjustments
         err_1 = k * (1 - (1 - effective_irradiance / 200)**4)
         err_2 = k * (1000 - effective_irradiance) / (1000 - 200)
 
+        # if input is Series or array
         if hasattr(effective_irradiance, '__len__'):
+            # precalculate pdc before applying error adjustments
+            pdc_marion = (effective_irradiance * 0.001 * pdc0 *
+                          (1 + gamma_pdc * (temp_cell - temp_ref)))
+
+            # apply error adjustments
             pdc_marion[effective_irradiance <= 200] = (
                 pdc[effective_irradiance <= 200] -
                 (pdc0 * err_1[effective_irradiance <= 200]))
             pdc_marion[effective_irradiance > 200] = (
                 pdc[effective_irradiance > 200] -
                 (pdc0 * err_2[effective_irradiance > 200]))
+
+            # "cap" Marion's correction at 1000 W/m^2
+            if cap_adjustment:
+                pdc_marion[effective_irradiance >= 1000] = (
+                    pdc[effective_irradiance >= 1000])
+
+            # set negative power to zero
+            pdc_marion[pdc_marion < 0] = 0
+
+        # else (input is scalar)
         else:
+            # apply error adjustments
             if effective_irradiance <= 200:
                 pdc_marion = pdc - (pdc0 * err_1)
             elif effective_irradiance > 200:
                 pdc_marion = pdc - (pdc0 * err_2)
 
-        # "cap" Marion's correction at 1000 W/m^2
-        if cap_adjustment:
-            if hasattr(effective_irradiance, '__len__'):
-                pdc_marion[effective_irradiance >= 1000] = (
-                    pdc[effective_irradiance >= 1000])
-            else:
+            # "cap" Marion's correction at 1000 W/m^2 if needed
+            if cap_adjustment:
                 if effective_irradiance >= 1000:
                     pdc_marion = pdc
 
-        # large k values can result in negative power at low irradiance, so
-        # set negative power to zero
-        if hasattr(effective_irradiance, '__len__'):
-            pdc_marion[pdc_marion < 0] = 0
-        else:
+            # set negative power to zero
             if pdc_marion < 0:
                 pdc_marion = 0
 

From 1e3d41362a9820ad32e92bf9132accb8597fd4a4 Mon Sep 17 00:00:00 2001
From: Will Hobbs <45701090+williamhobbs@users.noreply.github.com>
Date: Fri, 17 Oct 2025 15:30:49 -0500
Subject: [PATCH 31/41] single comp path on numpy array

---
 pvlib/pvsystem.py | 57 ++++++++++++++---------------------------------
 1 file changed, 17 insertions(+), 40 deletions(-)

diff --git a/pvlib/pvsystem.py b/pvlib/pvsystem.py
index f8bad096d2..c8b0a7735a 100644
--- a/pvlib/pvsystem.py
+++ b/pvlib/pvsystem.py
@@ -2963,50 +2963,27 @@ def pvwatts_dc(effective_irradiance, temp_cell, pdc0, gamma_pdc, temp_ref=25.,
 
     # apply Marion's correction if k is anything but zero
     if k is not None:
+
+        # preserve input types
+        index = pdc.index if isinstance(pdc, pd.Series) else None
+        is_scalar = np.isscalar(pdc)
+
         # calculate error adjustments
         err_1 = k * (1 - (1 - effective_irradiance / 200)**4)
         err_2 = k * (1000 - effective_irradiance) / (1000 - 200)
+        err = np.where(effective_irradiance <= 200, err_1, err_2)
+        if cap_adjustment:
+            err = np.where(effective_irradiance >= 1000, 0, err)
 
-        # if input is Series or array
-        if hasattr(effective_irradiance, '__len__'):
-            # precalculate pdc before applying error adjustments
-            pdc_marion = (effective_irradiance * 0.001 * pdc0 *
-                          (1 + gamma_pdc * (temp_cell - temp_ref)))
-
-            # apply error adjustments
-            pdc_marion[effective_irradiance <= 200] = (
-                pdc[effective_irradiance <= 200] -
-                (pdc0 * err_1[effective_irradiance <= 200]))
-            pdc_marion[effective_irradiance > 200] = (
-                pdc[effective_irradiance > 200] -
-                (pdc0 * err_2[effective_irradiance > 200]))
-
-            # "cap" Marion's correction at 1000 W/m^2
-            if cap_adjustment:
-                pdc_marion[effective_irradiance >= 1000] = (
-                    pdc[effective_irradiance >= 1000])
-
-            # set negative power to zero
-            pdc_marion[pdc_marion < 0] = 0
-
-        # else (input is scalar)
-        else:
-            # apply error adjustments
-            if effective_irradiance <= 200:
-                pdc_marion = pdc - (pdc0 * err_1)
-            elif effective_irradiance > 200:
-                pdc_marion = pdc - (pdc0 * err_2)
-
-            # "cap" Marion's correction at 1000 W/m^2 if needed
-            if cap_adjustment:
-                if effective_irradiance >= 1000:
-                    pdc_marion = pdc
-
-            # set negative power to zero
-            if pdc_marion < 0:
-                pdc_marion = 0
-
-        pdc = pdc_marion
+        pdc = pdc - pdc0 * err
+
+        # set negative power to zero
+        pdc = np.where(pdc < 0, 0, pdc)
+
+        if index is not None:
+            pdc = pd.Series(pdc, index=index)
+        elif is_scalar:
+            pdc = float(pdc)
 
     return pdc
 

From bdf331d2cd311fae665be0e45445b24d30260697 Mon Sep 17 00:00:00 2001
From: Will Hobbs <45701090+williamhobbs@users.noreply.github.com>
Date: Fri, 17 Oct 2025 15:44:14 -0500
Subject: [PATCH 32/41] added some comments

---
 pvlib/pvsystem.py | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/pvlib/pvsystem.py b/pvlib/pvsystem.py
index c8b0a7735a..7ee982c732 100644
--- a/pvlib/pvsystem.py
+++ b/pvlib/pvsystem.py
@@ -2961,7 +2961,7 @@ def pvwatts_dc(effective_irradiance, temp_cell, pdc0, gamma_pdc, temp_ref=25.,
     pdc = (effective_irradiance * 0.001 * pdc0 *
            (1 + gamma_pdc * (temp_cell - temp_ref)))
 
-    # apply Marion's correction if k is anything but zero
+    # apply Marion's correction if k is provided
     if k is not None:
 
         # preserve input types
@@ -2972,14 +2972,18 @@ def pvwatts_dc(effective_irradiance, temp_cell, pdc0, gamma_pdc, temp_ref=25.,
         err_1 = k * (1 - (1 - effective_irradiance / 200)**4)
         err_2 = k * (1000 - effective_irradiance) / (1000 - 200)
         err = np.where(effective_irradiance <= 200, err_1, err_2)
+
+        # cap adjustment, if needed
         if cap_adjustment:
             err = np.where(effective_irradiance >= 1000, 0, err)
 
+        # make error adjustment
         pdc = pdc - pdc0 * err
 
         # set negative power to zero
         pdc = np.where(pdc < 0, 0, pdc)
 
+        # preserve input types
         if index is not None:
             pdc = pd.Series(pdc, index=index)
         elif is_scalar:

From 4c3e01d17dff62b671e9d3f856059a8a991b63c4 Mon Sep 17 00:00:00 2001
From: Will Hobbs <45701090+williamhobbs@users.noreply.github.com>
Date: Fri, 17 Oct 2025 16:32:30 -0500
Subject: [PATCH 33/41] unit formatting

---
 pvlib/pvsystem.py | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/pvlib/pvsystem.py b/pvlib/pvsystem.py
index 7ee982c732..836fe99f79 100644
--- a/pvlib/pvsystem.py
+++ b/pvlib/pvsystem.py
@@ -2889,14 +2889,14 @@ def pvwatts_dc(effective_irradiance, temp_cell, pdc0, gamma_pdc, temp_ref=25.,
         Irradiance transmitted to the PV cells. To be
         fully consistent with PVWatts, the user must have already
         applied angle of incidence losses, but not soiling, spectral,
-        etc. [W/m^2]
+        etc. [Wm⁻²]
     temp_cell: numeric
         Cell temperature [C].
     pdc0: numeric
-        Power of the modules at 1000 W/m^2 and cell reference temperature. [W]
+        Power of the modules at 1000 Wm⁻² and cell reference temperature. [W]
     gamma_pdc: numeric
         The temperature coefficient of power. Typically -0.002 to
-        -0.005 per degree C. [1/C]
+        -0.005 per degree C. [1/°C]
     temp_ref: numeric, default 25.0
         Cell reference temperature. PVWatts defines it to be 25 °C and
         is included here for flexibility. [°C]

From 996092a8aa1bec20fc0428b6be1ebeae1273bb33 Mon Sep 17 00:00:00 2001
From: Will Hobbs <45701090+williamhobbs@users.noreply.github.com>
Date: Mon, 20 Oct 2025 18:46:42 -0500
Subject: [PATCH 34/41] call keyword arguments as keyword arguments

---
 tests/test_pvsystem.py | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/tests/test_pvsystem.py b/tests/test_pvsystem.py
index 61873a9111..9a8af55942 100644
--- a/tests/test_pvsystem.py
+++ b/tests/test_pvsystem.py
@@ -2194,7 +2194,7 @@ def test_pvwatts_dc_arrays_with_k():
     expected = np.array([[nan,  8.9125,  118.45],
                          [nan,    nan,    nan],
                          [nan,  8.9125,  118.45]])
-    out = pvsystem.pvwatts_dc(irrad_trans, temp_cell, 100, -0.003, 25, 0.01)
+    out = pvsystem.pvwatts_dc(irrad_trans, temp_cell, 100, -0.003, k=0.01)
     assert_allclose(out, expected, equal_nan=True)
 
 
@@ -2202,7 +2202,7 @@ def test_pvwatts_dc_series_with_k():
     irrad_trans = pd.Series([np.nan, 100, 100, 1200])
     temp_cell = pd.Series([30, np.nan, 30, 30])
     expected = pd.Series(np.array([   nan,    nan,  8.9125, 118.45]))
-    out = pvsystem.pvwatts_dc(irrad_trans, temp_cell, 100, -0.003, 25, 0.01)
+    out = pvsystem.pvwatts_dc(irrad_trans, temp_cell, 100, -0.003, 25, k=0.01)
     assert_series_equal(expected, out)
 
 
@@ -2217,7 +2217,8 @@ def test_pvwatts_dc_with_k_and_cap_adjustment():
         for k in ks:
             for irrad in irrad_trans:
                 out.append(pvsystem.pvwatts_dc(irrad, temp_cell, 100, -0.003,
-                                               25, k, cap_adjustment))
+                                               k=k,
+                                               cap_adjustment=cap_adjustment))
     assert_allclose(out, expected)
 
 
@@ -2228,7 +2229,7 @@ def test_pvwatts_dc_arrays_with_k_and_cap_adjustment():
     expected = np.array([[nan,  8.9125,  118.2],
                          [nan,    nan,    nan],
                          [nan,  8.9125,  118.2]])
-    out = pvsystem.pvwatts_dc(irrad_trans, temp_cell, 100, -0.003, 25, 0.01,
+    out = pvsystem.pvwatts_dc(irrad_trans, temp_cell, 100, -0.003, k=0.01,
                               True)
     assert_allclose(out, expected, equal_nan=True)
 

From 5777613d3864bce6c233ea66b8c87cc854fbd3fd Mon Sep 17 00:00:00 2001
From: Will Hobbs <45701090+williamhobbs@users.noreply.github.com>
Date: Mon, 20 Oct 2025 18:47:52 -0500
Subject: [PATCH 35/41] quick fix

---
 tests/test_pvsystem.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tests/test_pvsystem.py b/tests/test_pvsystem.py
index 9a8af55942..a6968b2e25 100644
--- a/tests/test_pvsystem.py
+++ b/tests/test_pvsystem.py
@@ -2230,7 +2230,7 @@ def test_pvwatts_dc_arrays_with_k_and_cap_adjustment():
                          [nan,    nan,    nan],
                          [nan,  8.9125,  118.2]])
     out = pvsystem.pvwatts_dc(irrad_trans, temp_cell, 100, -0.003, k=0.01,
-                              True)
+                              cap_adjustment=True)
     assert_allclose(out, expected, equal_nan=True)
 
 

From 3c7f1e97fd60a6ec382517b142bf2707d273f173 Mon Sep 17 00:00:00 2001
From: Will Hobbs <45701090+williamhobbs@users.noreply.github.com>
Date: Mon, 20 Oct 2025 18:55:36 -0500
Subject: [PATCH 36/41] a few more missed keyword args

---
 tests/test_pvsystem.py | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/tests/test_pvsystem.py b/tests/test_pvsystem.py
index a6968b2e25..138715038e 100644
--- a/tests/test_pvsystem.py
+++ b/tests/test_pvsystem.py
@@ -2183,7 +2183,7 @@ def test_pvwatts_dc_series():
 
 def test_pvwatts_dc_scalars_with_k():
     expected = 8.9125
-    out = pvsystem.pvwatts_dc(100, 30, 100, -0.003, 25, 0.01)
+    out = pvsystem.pvwatts_dc(100, 30, 100, -0.003, k=0.01)
     assert_allclose(out, expected)
 
 
@@ -2202,7 +2202,7 @@ def test_pvwatts_dc_series_with_k():
     irrad_trans = pd.Series([np.nan, 100, 100, 1200])
     temp_cell = pd.Series([30, np.nan, 30, 30])
     expected = pd.Series(np.array([   nan,    nan,  8.9125, 118.45]))
-    out = pvsystem.pvwatts_dc(irrad_trans, temp_cell, 100, -0.003, 25, k=0.01)
+    out = pvsystem.pvwatts_dc(irrad_trans, temp_cell, 100, -0.003, k=0.01)
     assert_series_equal(expected, out)
 
 

From 86eb06ea92eb68490f0afd317338b249e401eb6b Mon Sep 17 00:00:00 2001
From: Will Hobbs <45701090+williamhobbs@users.noreply.github.com>
Date: Mon, 20 Oct 2025 18:58:48 -0500
Subject: [PATCH 37/41] simplify with_k_and_cap_adjustmen

---
 tests/test_pvsystem.py | 13 ++++---------
 1 file changed, 4 insertions(+), 9 deletions(-)

diff --git a/tests/test_pvsystem.py b/tests/test_pvsystem.py
index 138715038e..4fbd782e65 100644
--- a/tests/test_pvsystem.py
+++ b/tests/test_pvsystem.py
@@ -2209,16 +2209,11 @@ def test_pvwatts_dc_series_with_k():
 def test_pvwatts_dc_with_k_and_cap_adjustment():
     irrad_trans = [100, 1200]
     temp_cell = 25
-    ks = [0.01, 0.15]
-    cap_adjustments = [False, True]
     out = []
-    expected = [9.0625, 120.25, 0, 123.75, 9.0625, 120.0, 0, 120.0]
-    for cap_adjustment in cap_adjustments:
-        for k in ks:
-            for irrad in irrad_trans:
-                out.append(pvsystem.pvwatts_dc(irrad, temp_cell, 100, -0.003,
-                                               k=k,
-                                               cap_adjustment=cap_adjustment))
+    expected = [0, 120.0]
+    for irrad in irrad_trans:
+        out.append(pvsystem.pvwatts_dc(irrad, temp_cell, 100, -0.003, k=0.15,
+                                       cap_adjustment=True))
     assert_allclose(out, expected)
 
 

From 465044318627e98fb4f39cebfde01858cad12f49 Mon Sep 17 00:00:00 2001
From: Will Hobbs <45701090+williamhobbs@users.noreply.github.com>
Date: Tue, 21 Oct 2025 11:01:04 -0500
Subject: [PATCH 38/41] Update pvlib/pvsystem.py

Co-authored-by: Cliff Hansen <cwhanse@sandia.gov>
---
 pvlib/pvsystem.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/pvlib/pvsystem.py b/pvlib/pvsystem.py
index 836fe99f79..ee995c62aa 100644
--- a/pvlib/pvsystem.py
+++ b/pvlib/pvsystem.py
@@ -2904,7 +2904,7 @@ def pvwatts_dc(effective_irradiance, temp_cell, pdc0, gamma_pdc, temp_ref=25.,
         Irradiance correction factor, defined in [2]_. Typically positive.
         [unitless]
     cap_adjustment: Boolean, default False
-        If True, apply the optional adjustment at and below 1000 Wm⁻²
+        If True, only apply the optional adjustment at and below 1000 Wm⁻²
 
     Returns
     -------

From c646f21ba4f3073db3de775e0e6aa0f855e96d50 Mon Sep 17 00:00:00 2001
From: Will Hobbs <45701090+williamhobbs@users.noreply.github.com>
Date: Tue, 21 Oct 2025 11:02:26 -0500
Subject: [PATCH 39/41] Update pvlib/pvsystem.py

Co-authored-by: Cliff Hansen <cwhanse@sandia.gov>
---
 pvlib/pvsystem.py | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/pvlib/pvsystem.py b/pvlib/pvsystem.py
index ee995c62aa..7015d6dc18 100644
--- a/pvlib/pvsystem.py
+++ b/pvlib/pvsystem.py
@@ -2922,11 +2922,13 @@ def pvwatts_dc(effective_irradiance, temp_cell, pdc0, gamma_pdc, temp_ref=25.,
     This model has also been referred to as the power temperature coefficient
     model.
 
-    This function accepts an optional irradiance adjustment factor, `k`, based
-    on [2]_. This applies a piece-wise adjustment to power based on irradiance,
+    An optional adjustment can be applied to :math:`P_{dc}` as described in [2]_.
+    The adjustment accounts for the variation in module efficiency with
+    irradiance. The piece-wise adjustment to power is parameterized by `k`,
     where `k` is the reduction in actual power at 200 Wm⁻² relative to power
-    calculated at 200 Wm-2 as 0.2*`pdc0`. For example, a 500 W module that
-    produces 95 W at 200 Wm⁻² (a 5% relative reduction in efficiency) would
+    calculated at 200 Wm⁻² as 0.2*`pdc0`. For example, a module that
+    is rated at 500 W at STC but produces 95 W at 200 Wm⁻² 
+    (a 5% relative reduction in efficiency) would
     have a value of `k` = 0.01.
 
     .. math::

From f3e165e6139003d89144c915b19748d4a0c86fcf Mon Sep 17 00:00:00 2001
From: Will Hobbs <45701090+williamhobbs@users.noreply.github.com>
Date: Tue, 21 Oct 2025 11:02:40 -0500
Subject: [PATCH 40/41] Update pvlib/pvsystem.py

Co-authored-by: Cliff Hansen <cwhanse@sandia.gov>
---
 pvlib/pvsystem.py | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/pvlib/pvsystem.py b/pvlib/pvsystem.py
index 7015d6dc18..8a5085ffad 100644
--- a/pvlib/pvsystem.py
+++ b/pvlib/pvsystem.py
@@ -2936,7 +2936,8 @@ def pvwatts_dc(effective_irradiance, temp_cell, pdc0, gamma_pdc, temp_ref=25.,
         k=\frac{0.2P_{dc0}-P_{200}}{P_{dc0}}
 
     For positive `k` values, and `k` is typically positive, this adjustment
-    increases relative efficiency when irradiance is above 1000 Wm⁻². This may
+    would also increase relative efficiency when irradiance is above 1000 Wm⁻².
+    This may
     not be desired, as modules with nonlinear irradiance response often have
     peak efficiency near 1000 Wm⁻², and it is either flat or declining at
     higher irradiance. An optional parameter, `cap_adjustment`, can address

From 6624239a94318f7013153f7b545b55f669d74cc8 Mon Sep 17 00:00:00 2001
From: Will Hobbs <45701090+williamhobbs@users.noreply.github.com>
Date: Tue, 21 Oct 2025 11:51:10 -0500
Subject: [PATCH 41/41] doc formatting/linting

---
 pvlib/pvsystem.py | 40 +++++++++++++++++++---------------------
 1 file changed, 19 insertions(+), 21 deletions(-)

diff --git a/pvlib/pvsystem.py b/pvlib/pvsystem.py
index 8a5085ffad..1d993eed31 100644
--- a/pvlib/pvsystem.py
+++ b/pvlib/pvsystem.py
@@ -2886,20 +2886,19 @@ def pvwatts_dc(effective_irradiance, temp_cell, pdc0, gamma_pdc, temp_ref=25.,
     Parameters
     ----------
     effective_irradiance: numeric
-        Irradiance transmitted to the PV cells. To be
-        fully consistent with PVWatts, the user must have already
-        applied angle of incidence losses, but not soiling, spectral,
-        etc. [Wm⁻²]
+        Irradiance transmitted to the PV cells. To be fully consistent with
+        PVWatts, the user must have already applied angle of incidence losses,
+        but not soiling, spectral, etc. [Wm⁻²]
     temp_cell: numeric
         Cell temperature [C].
     pdc0: numeric
         Power of the modules at 1000 Wm⁻² and cell reference temperature. [W]
     gamma_pdc: numeric
-        The temperature coefficient of power. Typically -0.002 to
-        -0.005 per degree C. [1/°C]
+        The temperature coefficient of power. Typically -0.002 to -0.005 per
+        degree C. [1/°C]
     temp_ref: numeric, default 25.0
-        Cell reference temperature. PVWatts defines it to be 25 °C and
-        is included here for flexibility. [°C]
+        Cell reference temperature. PVWatts defines it to be 25 °C and is
+        included here for flexibility. [°C]
     k: numeric, optional
         Irradiance correction factor, defined in [2]_. Typically positive.
         [unitless]
@@ -2922,14 +2921,13 @@ def pvwatts_dc(effective_irradiance, temp_cell, pdc0, gamma_pdc, temp_ref=25.,
     This model has also been referred to as the power temperature coefficient
     model.
 
-    An optional adjustment can be applied to :math:`P_{dc}` as described in [2]_.
-    The adjustment accounts for the variation in module efficiency with
+    An optional adjustment can be applied to :math:`P_{dc}` as described in
+    [2]_. The adjustment accounts for the variation in module efficiency with
     irradiance. The piece-wise adjustment to power is parameterized by `k`,
     where `k` is the reduction in actual power at 200 Wm⁻² relative to power
-    calculated at 200 Wm⁻² as 0.2*`pdc0`. For example, a module that
-    is rated at 500 W at STC but produces 95 W at 200 Wm⁻² 
-    (a 5% relative reduction in efficiency) would
-    have a value of `k` = 0.01.
+    calculated at 200 Wm⁻² as 0.2*`pdc0`. For example, a module that is rated
+    at 500 W at STC but produces 95 W at 200 Wm⁻² (a 5% relative reduction in
+    efficiency) would have a value of `k` = 0.01.
 
     .. math::
 
@@ -2937,15 +2935,15 @@ def pvwatts_dc(effective_irradiance, temp_cell, pdc0, gamma_pdc, temp_ref=25.,
 
     For positive `k` values, and `k` is typically positive, this adjustment
     would also increase relative efficiency when irradiance is above 1000 Wm⁻².
-    This may
-    not be desired, as modules with nonlinear irradiance response often have
-    peak efficiency near 1000 Wm⁻², and it is either flat or declining at
-    higher irradiance. An optional parameter, `cap_adjustment`, can address
-    this by modifying the adjustment from [2]_ to only apply below 1000 Wm⁻².
+    This may not be desired, as modules with nonlinear irradiance response
+    often have peak efficiency near 1000 Wm⁻², and it is either flat or
+    declining at higher irradiance. An optional parameter, `cap_adjustment`,
+    can address this by modifying the adjustment from [2]_ to only apply below
+    1000 Wm⁻².
 
     Note that ``pdc0`` is also used as a symbol in
-    :py:func:`pvlib.inverter.pvwatts`. ``pdc0`` in this function refers to the DC
-    power of the modules at reference conditions. ``pdc0`` in
+    :py:func:`pvlib.inverter.pvwatts`. ``pdc0`` in this function refers to the
+    DC power of the modules at reference conditions. ``pdc0`` in
     :py:func:`pvlib.inverter.pvwatts` refers to the DC power input limit of
     the inverter.