From 6a1196968c076059468058381538be3cd0c963f9 Mon Sep 17 00:00:00 2001
From: Kevin Anderson <kevin.anderso@gmail.com>
Date: Tue, 8 Apr 2025 14:09:58 -0400
Subject: [PATCH 01/17] add function

---
 pvlib/ivtools/sdm/__init__.py |   1 +
 pvlib/ivtools/sdm/pvsyst.py   | 310 ++++++++++++++++++++++++++++++++++
 2 files changed, 311 insertions(+)

diff --git a/pvlib/ivtools/sdm/__init__.py b/pvlib/ivtools/sdm/__init__.py
index b45519fc16..d0b658c198 100644
--- a/pvlib/ivtools/sdm/__init__.py
+++ b/pvlib/ivtools/sdm/__init__.py
@@ -15,5 +15,6 @@
 
 from pvlib.ivtools.sdm.pvsyst import (  # noqa: F401
     fit_pvsyst_sandia,
+    fit_pvsyst_iec61853_sandia,
     pvsyst_temperature_coeff,
 )
diff --git a/pvlib/ivtools/sdm/pvsyst.py b/pvlib/ivtools/sdm/pvsyst.py
index 6233c38c72..4724c0b55f 100644
--- a/pvlib/ivtools/sdm/pvsyst.py
+++ b/pvlib/ivtools/sdm/pvsyst.py
@@ -14,6 +14,9 @@
     _extract_sdm_params, _initial_iv_params, _update_iv_params
 )
 
+from pvlib.pvsystem import (
+    _pvsyst_Rsh, _pvsyst_IL, _pvsyst_Io, _pvsyst_nNsVth, _pvsyst_gamma
+)
 
 CONSTANTS = {'E0': 1000.0, 'T0': 25.0, 'k': constants.k, 'q': constants.e}
 
@@ -305,3 +308,310 @@ def maxp(temp_cell, irrad_ref, alpha_sc, gamma_ref, mu_gamma, I_L_ref,
     gamma_pdc = _first_order_centered_difference(maxp, x0=temp_ref, args=args)
 
     return gamma_pdc / pmp
+
+
+def fit_pvsyst_iec61853_sandia(effective_irradiance, temp_cell,
+                               i_sc, v_oc, i_mp, v_mp,
+                               cells_in_series, EgRef=1.121,
+                               alpha_sc=None, beta_mp=None,
+                               r_sh_coeff=0.12, R_s=None,
+                               min_Rsh_irradiance=None):
+    """
+    Estimate parameters for the PVsyst module performance model using
+    IEC 61853-1 matrix measurements.
+
+    Parameters
+    ----------
+    effective_irradiance : array
+        Effective irradiance for each test condition [W/m²]
+    temp_cell : array
+        Cell temperature for each test condition [C]
+    i_sc : array
+        Short circuit current for each test condition [A]
+    v_oc : array
+        Open circuit voltage for each test condition [V]
+    i_mp : array
+        Current at maximum power point for each test condition [A]
+    v_mp : array
+        Voltage at maximum power point for each test condition [V]
+    cells_in_series : int
+        The number of cells connected in series.
+    EgRef : float, optional
+        The energy bandgap at reference temperature in units of eV.
+        1.121 eV for crystalline silicon. EgRef must be >0.
+    alpha_sc : float, optional
+        Temperature coefficient of short circuit current.  If not specified,
+        it will be estimated using the ``i_sc`` values at irradiance of
+        1000 W/m2. [A/K]
+    beta_mp : float, optional
+        Temperature coefficient of maximum power voltage.  If not specified,
+        it will be estimated using the ``v_mp`` values at irradiance of
+        1000 W/m2. [1/K]
+    r_sh_coeff : float, default 0.12
+        Shunt resistance fitting coefficient.  The default value is taken
+        from [1]_.
+    R_s : float, optional
+        Series resistance value.  If not provided, a value will be estimated
+        from the input measurements. [ohm]
+    min_Rsh_irradiance : float, optional
+        Irradiance threshold below which values are excluded when estimating
+        shunt resistance parameter values.  May be useful for modules
+        with problematic low-light measurements. [W/m²]
+
+    Returns
+    -------
+    dict
+        alpha_sc : float
+            short circuit current temperature coefficient [A/K]
+        gamma_ref : float
+            diode (ideality) factor at STC [unitless]
+        mu_gamma : float
+            temperature coefficient for diode (ideality) factor [1/K]
+        I_L_ref : float
+            light current at STC [A]
+        I_o_ref : float
+            dark current at STC [A]
+        R_sh_ref : float
+            shunt resistance at STC [ohm]
+        R_sh_0 : float
+            shunt resistance at zero irradiance [ohm]
+        R_sh_exp : float
+            exponential factor defining decrease in shunt resistance with
+            increasing effective irradiance
+        R_s : float
+            series resistance at STC [ohm]
+        cells_in_series : int
+            number of cells in series
+        EgRef : float
+            effective band gap at STC [eV]
+
+    See also
+    --------
+    pvlib.pvsystem.calcparams_pvsyst
+    pvlib.ivtools.sdm.fit_pvsyst_sandia
+
+    Notes
+    -----
+    This method is non-iterative.  In some cases, it may be desirable to
+    refine the estimated parameter values using a numerical optimizer like
+    those provided in ``scipy.optimize``.
+
+    References
+    ----------
+    .. [1] K. S. Anderson, C. W. Hansen, and M. Theristis, "A Noniterative
+       Method of Estimating Parameter Values for the PVsyst Version 6
+       Single-Diode Model From IEC 61853-1 Matrix Measurements," IEEE Journal
+       of Photovoltaics, vol. 15, 3, 2025. :doi:`10.1109/JPHOTOV.2025.3554338`
+    """
+
+    try:
+        import statsmodels.api as sm
+    except ImportError:
+        raise ImportError('fit_pvsyst_iec61853_sandia requires statsmodels')
+
+    if alpha_sc is None:
+        mu_i_sc = _fit_tempco(i_sc[effective_irradiance==1000],
+                              temp_cell[effective_irradiance==1000])
+        i_sc_ref = float(i_sc[(effective_irradiance==1000) & (temp_cell==25)])
+        alpha_sc = mu_i_sc * i_sc_ref
+
+    if beta_mp is None:
+        beta_mp = _fit_tempco(v_mp[effective_irradiance==1000],
+                              temp_cell[effective_irradiance==1000])
+
+    R_sh_ref, R_sh_0, R_sh_exp = _fit_shunt_resistances(
+        i_sc, i_mp, v_mp, effective_irradiance, temp_cell, beta_mp,
+        coeff=r_sh_coeff, min_irradiance=min_Rsh_irradiance
+    )
+
+    if R_s is None:
+        R_s = _fit_series_resistance(sm, v_oc, i_mp, v_mp)
+
+    filt = temp_cell == 25
+    gamma_ref, mu_gamma = _fit_diode_ideality_factor(
+        sm, i_sc[filt], v_oc[filt], i_mp[filt], v_mp[filt],
+        effective_irradiance[filt], temp_cell[filt],
+        R_sh_ref, R_sh_0, R_sh_exp, R_s, cells_in_series
+    )
+
+    I_o_ref = _fit_saturation_current(
+        i_sc, v_oc, effective_irradiance, temp_cell, gamma_ref, mu_gamma,
+        R_sh_ref, R_sh_0, R_sh_exp, R_s, cells_in_series, EgRef
+    )
+
+    I_L_ref = _fit_photocurrent(
+        sm, i_sc, effective_irradiance, temp_cell, alpha_sc,
+        gamma_ref, mu_gamma,
+        I_o_ref, R_sh_ref, R_sh_0, R_sh_exp, R_s, cells_in_series, EgRef
+    )
+
+    gamma_ref, mu_gamma = \
+        _fit_diode_ideality_factor_post(sm, i_mp, v_mp, effective_irradiance,
+                                        temp_cell, alpha_sc, I_L_ref, I_o_ref, 
+                                        R_sh_ref, R_sh_0, R_sh_exp, R_s,
+                                        cells_in_series, EgRef)
+
+    fitted_params = dict(
+        alpha_sc=alpha_sc,
+        gamma_ref=gamma_ref,
+        mu_gamma=mu_gamma,
+        I_L_ref=I_L_ref,
+        I_o_ref=I_o_ref,
+        R_sh_ref=R_sh_ref,
+        R_sh_0=R_sh_0,
+        R_sh_exp=R_sh_exp,
+        R_s=R_s,
+        cells_in_series=cells_in_series,
+        EgRef=EgRef,
+    )
+    return fitted_params
+
+
+def _fit_tempco(values, temp_cell, temp_cell_ref=25):
+    fit = np.polynomial.polynomial.Polynomial.fit(temp_cell, values, deg=1)
+    intercept, slope = fit.convert().coef
+    value_ref = intercept + slope*temp_cell_ref
+    return slope / value_ref
+
+
+def _fit_shunt_resistances(i_sc, i_mp, v_mp, effective_irradiance, temp_cell,
+                           beta_v_mp, coeff=0.2, min_irradiance=None):
+    if min_irradiance is None:
+        min_irradiance = 0
+
+    mask = effective_irradiance >= min_irradiance
+    i_sc = i_sc[mask]
+    i_mp = i_mp[mask]
+    v_mp = v_mp[mask]
+    effective_irradiance = effective_irradiance[mask]
+    temp_cell = temp_cell[mask]
+    
+    # Equation 10
+    Rsh_est = (
+        (v_mp / (1 + beta_v_mp * (temp_cell - 25)))
+        / (coeff * (i_sc - i_mp))
+    )
+    Rshexp = 5.5
+    
+    # Eq 11
+    y = Rsh_est
+    x = np.exp(-Rshexp * effective_irradiance / 1000)
+    
+    fit = np.polynomial.polynomial.Polynomial.fit(x, y, deg=1)
+    intercept, slope = fit.convert().coef
+    Rshbase = intercept
+    Rsh0 = slope + Rshbase
+
+    # Eq 12
+    expRshexp = np.exp(-Rshexp)
+    Rshref = Rshbase * (1 - expRshexp) + Rsh0 * expRshexp
+
+    return Rshref, Rsh0, Rshexp
+
+
+def _fit_series_resistance(sm, v_oc, i_mp, v_mp):
+    # Stein et al 2014, https://doi.org/10.1109/PVSC.2014.6925326
+    
+    # Eq 13
+    x = np.array([i_mp, np.log(i_mp), v_mp]).T
+    x = sm.add_constant(x)
+    y = v_oc
+    
+    results = sm.OLS(endog=y, exog=x).fit()
+    R_s = results.params['x1']
+    return R_s
+
+
+def _fit_diode_ideality_factor(sm, i_sc, v_oc, i_mp, v_mp,
+                               effective_irradiance, temp_cell,
+                               R_sh_ref, R_sh_0, R_sh_exp, R_s,
+                               cells_in_series):
+
+    NsVth = _pvsyst_nNsVth(temp_cell, gamma=1, cells_in_series=cells_in_series)
+    Rsh = _pvsyst_Rsh(effective_irradiance, R_sh_ref, R_sh_0, R_sh_exp)
+    term1 = (i_sc * (1 + R_s/Rsh) - v_oc / Rsh)  # Eq 15
+    term2 = (i_sc - i_mp) * (1 + R_s/Rsh) - v_mp / Rsh  # Eq 16
+
+    # Eq 14
+    x1 = NsVth * np.log(term2 / term1)
+
+    x = np.array([x1]).T
+    y = v_mp + i_mp*R_s - v_oc
+
+    results = sm.OLS(endog=y, exog=x).fit()
+    gamma_ref = results.params[0]
+    return gamma_ref, 0
+
+
+def _fit_saturation_current(i_sc, v_oc, effective_irradiance, temp_cell,
+                            gamma_ref, mu_gamma,
+                            R_sh_ref, R_sh_0, R_sh_exp, R_s,
+                            cells_in_series, EgRef):
+    R_sh = _pvsyst_Rsh(effective_irradiance, R_sh_ref, R_sh_0, R_sh_exp)
+    gamma = _pvsyst_gamma(temp_cell, gamma_ref, mu_gamma)
+    nNsVth = _pvsyst_nNsVth(temp_cell, gamma, cells_in_series)
+
+    # Eq 17
+    I_o_est = (i_sc * (1 + R_s/R_sh) - v_oc/R_sh) / (np.expm1(v_oc / nNsVth))
+    x = _pvsyst_Io(temp_cell, gamma, I_o_ref=1, EgRef=EgRef)
+
+    # Eq 18
+    log_I_o_ref = np.mean(np.log(I_o_est) - np.log(x))
+    I_o_ref = np.exp(log_I_o_ref)
+
+    return I_o_ref
+
+
+def _fit_photocurrent(sm, i_sc, effective_irradiance, temp_cell,
+                      alpha_sc, gamma_ref, mu_gamma, I_o_ref, 
+                      R_sh_ref, R_sh_0, R_sh_exp, R_s,
+                      cells_in_series, EgRef):
+    R_sh = _pvsyst_Rsh(effective_irradiance, R_sh_ref, R_sh_0, R_sh_exp)
+    gamma = _pvsyst_gamma(temp_cell, gamma_ref, mu_gamma)
+    I_o = _pvsyst_Io(temp_cell, gamma, I_o_ref, EgRef)
+    nNsVth = _pvsyst_nNsVth(temp_cell, gamma, cells_in_series)
+
+    # Eq 19
+    I_L_est = i_sc + I_o * (np.expm1(i_sc * R_s / nNsVth)) + i_sc * R_s / R_sh
+
+    # Eq 20
+    x = np.array([effective_irradiance / 1000]).T
+    y = I_L_est - effective_irradiance / 1000 * alpha_sc * (temp_cell - 25)
+    results = sm.OLS(endog=y, exog=x).fit()
+    I_L_ref = results.params['x1']
+    return I_L_ref
+
+
+def _fit_diode_ideality_factor_post(sm, i_mp, v_mp, effective_irradiance,
+                                    temp_cell, alpha_sc, I_L_ref, I_o_ref, 
+                                    R_sh_ref, R_sh_0, R_sh_exp, R_s,
+                                    cells_in_series, EgRef):
+
+    Rsh = _pvsyst_Rsh(effective_irradiance, R_sh_ref, R_sh_0, R_sh_exp)
+    I_L = _pvsyst_IL(effective_irradiance, temp_cell, I_L_ref, alpha_sc)
+    NsVth = _pvsyst_nNsVth(temp_cell, gamma=1, cells_in_series=cells_in_series)
+
+    Tref_K = 25 + 273.15
+    Tcell_K = temp_cell + 273.15
+
+    # Eq 21
+    k = constants.k  # Boltzmann constant in J/K
+    q = constants.e  # elementary charge in coulomb
+    numerator = (
+        (q * EgRef / k) * (1/Tref_K - 1/Tcell_K)
+        + (v_mp + i_mp*R_s) / NsVth
+    )
+    denominator = (
+        np.log((I_L - i_mp - (v_mp+i_mp*R_s) / Rsh) / I_o_ref)
+        - 3 * np.log(Tcell_K / Tref_K)
+    )
+    gamma_est = numerator / denominator
+
+    # Eq 22
+    x = np.array([temp_cell - 25]).T
+    x = sm.add_constant(x)
+    y = gamma_est
+
+    results = sm.OLS(endog=y, exog=x).fit()
+    gamma_ref, mu_gamma = results.params
+    return gamma_ref, mu_gamma

From 9cb218c18d56747d7974ee30d88455867fa9ceec Mon Sep 17 00:00:00 2001
From: Kevin Anderson <kevin.anderso@gmail.com>
Date: Tue, 8 Apr 2025 14:10:13 -0400
Subject: [PATCH 02/17] add tests

---
 tests/ivtools/sdm/test_pvsyst.py | 106 +++++++++++++++++++++++++++++++
 1 file changed, 106 insertions(+)

diff --git a/tests/ivtools/sdm/test_pvsyst.py b/tests/ivtools/sdm/test_pvsyst.py
index 6927a8c1d3..5d246904c6 100644
--- a/tests/ivtools/sdm/test_pvsyst.py
+++ b/tests/ivtools/sdm/test_pvsyst.py
@@ -7,6 +7,8 @@
 
 from tests.conftest import requires_statsmodels, TESTS_DATA_DIR
 
+import pytest
+
 
 def _read_iv_curves_for_test(datafile, npts):
     """ read constants and npts IV curves from datafile """
@@ -198,3 +200,107 @@ def test_pvsyst_temperature_coeff():
         params['I_L_ref'], params['I_o_ref'], params['R_sh_ref'],
         params['R_sh_0'], params['R_s'], params['cells_in_series'])
     assert_allclose(gamma_pdc, expected, rtol=0.0005)
+
+
+@pytest.fixture
+def pvsyst_iec61853_table3():
+    # test cases in Table 3 of https://doi.org/10.1109/JPHOTOV.2025.3554338
+    parameters = ['alpha_sc', 'I_L_ref', 'I_o_ref', 'gamma_ref', 'mu_gamma',
+                  'R_sh_ref', 'R_sh_0', 'R_sh_exp', 'R_s', 'cells_in_series',
+                  'EgRef']
+    high_current = {
+        'true': [1e-3, 10, 1e-11, 1.0, -1e-4, 300, 1500, 5.5, 0.2, 60, 1.121],
+        'estimated': [9.993e-4, 9.997, 1.408e-10, 1.109, -1.666e-5, 543.7,
+                      6130, 5.5, 0.176, 60, 1.121]
+    }
+    high_voltage = {
+        'true': [1e-3, 1.8, 1e-9, 1.5, 1e-4, 3000, 10000, 5.5, 5.0, 108, 1.4],
+        'estimated': [9.983e-4, 1.799, 1.225e-8, 1.706, 2.662e-4, 4288, 47560,
+                      5.5, 4.292, 108, 1.4],
+    }
+    for d in [high_current, high_voltage]:
+        for key in d:
+            d[key] = dict(zip(parameters, np.array(d[key])))
+
+    return {
+        'high current': high_current, 'high voltage': high_voltage
+    }
+
+
+@pytest.fixture
+def iec61853_conditions():
+    ee = np.array([100, 100, 200, 200, 400, 400, 400, 600, 600, 600, 600,
+                   800, 800, 800, 800, 1000, 1000, 1000, 1000, 1100, 1100,
+                   1100], dtype=np.float64)
+    tc = np.array([15, 25, 15, 25, 15, 25, 50, 15, 25, 50, 75,
+                   15, 25, 50, 75, 15, 25, 50, 75, 25, 50, 75],
+                  dtype=np.float64)
+    return ee, tc
+
+
+@requires_statsmodels
+def test_fit_pvsyst_iec61853_sandia(pvsyst_iec61853_table3,
+                                    iec61853_conditions):
+    ee, tc = iec61853_conditions
+    for _, case in pvsyst_iec61853_table3.items():
+        true_params = case['true']
+        expected_params = case['estimated']
+
+        sde_params = pvsystem.calcparams_pvsyst(ee, tc, **true_params)
+        iv = pvsystem.singlediode(*sde_params)
+
+        fitted_params = sdm.fit_pvsyst_iec61853_sandia(
+            ee, tc, iv['i_sc'], iv['v_oc'], iv['i_mp'], iv['v_mp'],
+            true_params['cells_in_series'], EgRef=true_params['EgRef'],
+        )
+        for key in expected_params.keys():
+            assert np.isclose(fitted_params[key], expected_params[key],
+                              atol=0, rtol=1e-3)
+
+
+@requires_statsmodels
+def test_fit_pvsyst_iec61853_sandia_optional(pvsyst_iec61853_table3,
+                                             iec61853_conditions):
+    # verify that modifying each optional parameter results in different output
+    ee, tc = iec61853_conditions
+    case = pvsyst_iec61853_table3['high current']
+    true_params = case['true']
+    expected = case['estimated']
+    sde_params = pvsystem.calcparams_pvsyst(ee, tc, **true_params)
+    iv = pvsystem.singlediode(*sde_params)
+
+    main_inputs = dict(
+        effective_irradiance=ee, temp_cell=tc, i_sc=iv['i_sc'],
+        v_oc=iv['v_oc'], i_mp=iv['i_mp'], v_mp=iv['v_mp'],
+        cells_in_series=true_params['cells_in_series']
+    )
+
+    fitted_params = sdm.fit_pvsyst_iec61853_sandia(**main_inputs,
+                                                   EgRef=1.0)
+    assert not np.isclose(fitted_params['I_o_ref'], expected['I_o_ref'],
+                          atol=0, rtol=1e-3)
+
+    fitted_params = sdm.fit_pvsyst_iec61853_sandia(**main_inputs,
+                                                   alpha_sc=0.5e-3)
+    assert not np.isclose(fitted_params['alpha_sc'], expected['alpha_sc'],
+                          atol=0, rtol=1e-3)
+
+    fitted_params = sdm.fit_pvsyst_iec61853_sandia(**main_inputs,
+                                                   beta_mp=-1e-4)
+    assert not np.isclose(fitted_params['R_sh_ref'], expected['R_sh_ref'],
+                          atol=0, rtol=1e-3)
+
+    fitted_params = sdm.fit_pvsyst_iec61853_sandia(**main_inputs,
+                                                   r_sh_coeff=0.3)
+    assert not np.isclose(fitted_params['R_sh_ref'], expected['R_sh_ref'],
+                          atol=0, rtol=1e-3)
+    
+    fitted_params = sdm.fit_pvsyst_iec61853_sandia(**main_inputs,
+                                                   R_s=0.5)
+    assert not np.isclose(fitted_params['R_s'], expected['R_s'],
+                          atol=0, rtol=1e-3)
+
+    fitted_params = sdm.fit_pvsyst_iec61853_sandia(**main_inputs,
+                                                   min_Rsh_irradiance=500)
+    assert not np.isclose(fitted_params['R_sh_ref'], expected['R_sh_ref'],
+                          atol=0, rtol=1e-3)

From f7682ae7ef14d3c09784c44deedba3d68c3cfed8 Mon Sep 17 00:00:00 2001
From: Kevin Anderson <kevin.anderso@gmail.com>
Date: Tue, 8 Apr 2025 14:10:25 -0400
Subject: [PATCH 03/17] add to API reference

---
 docs/sphinx/source/reference/pv_modeling/parameters.rst | 1 +
 1 file changed, 1 insertion(+)

diff --git a/docs/sphinx/source/reference/pv_modeling/parameters.rst b/docs/sphinx/source/reference/pv_modeling/parameters.rst
index 9b1817bd01..69e8ba942f 100644
--- a/docs/sphinx/source/reference/pv_modeling/parameters.rst
+++ b/docs/sphinx/source/reference/pv_modeling/parameters.rst
@@ -12,6 +12,7 @@ Functions for fitting single diode models
     ivtools.sdm.fit_cec_sam
     ivtools.sdm.fit_desoto
     ivtools.sdm.fit_pvsyst_sandia
+    ivtools.sdm.fit_pvsyst_iec61853_sandia
     ivtools.sdm.fit_desoto_sandia
 
 Functions for fitting the single diode equation

From f48485c994cbfe6f35fa5bf24664254bcbfb6a5f Mon Sep 17 00:00:00 2001
From: Kevin Anderson <kevin.anderso@gmail.com>
Date: Tue, 8 Apr 2025 14:10:32 -0400
Subject: [PATCH 04/17] whatsnew

---
 docs/sphinx/source/whatsnew/v0.12.1.rst | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/docs/sphinx/source/whatsnew/v0.12.1.rst b/docs/sphinx/source/whatsnew/v0.12.1.rst
index f1c9b7e69d..a58d792471 100644
--- a/docs/sphinx/source/whatsnew/v0.12.1.rst
+++ b/docs/sphinx/source/whatsnew/v0.12.1.rst
@@ -14,8 +14,9 @@ Bug fixes
 
 Enhancements
 ~~~~~~~~~~~~
-* ``pvlib.ivtools.sdm`` is now a subpackage. (:issue:`2252`, :pull:`2256`)
-
+* :py:mod:`pvlib.ivtools.sdm` is now a subpackage. (:issue:`2252`, :pull:`2256`)
+* Add a function for estimating PVsyst SDM parameters from IEC 61853-1 matrix
+  data (:py:func:`~pvlib.ivtools.sdm.fit_pvsyst_iec61853_sandia`). (:issue:`2185`, :pull:`2429`)
 
 Documentation
 ~~~~~~~~~~~~~

From 07540a1e9293380b7e48cd5a39a26cee6075165f Mon Sep 17 00:00:00 2001
From: Kevin Anderson <kevin.anderso@gmail.com>
Date: Tue, 8 Apr 2025 14:32:46 -0400
Subject: [PATCH 05/17] lint

---
 pvlib/ivtools/sdm/pvsyst.py      | 32 ++++++++++++++++----------------
 tests/ivtools/sdm/test_pvsyst.py |  2 +-
 2 files changed, 17 insertions(+), 17 deletions(-)

diff --git a/pvlib/ivtools/sdm/pvsyst.py b/pvlib/ivtools/sdm/pvsyst.py
index 4724c0b55f..78d066d6a8 100644
--- a/pvlib/ivtools/sdm/pvsyst.py
+++ b/pvlib/ivtools/sdm/pvsyst.py
@@ -409,15 +409,16 @@ def fit_pvsyst_iec61853_sandia(effective_irradiance, temp_cell,
     except ImportError:
         raise ImportError('fit_pvsyst_iec61853_sandia requires statsmodels')
 
+    is_g_stc = effective_irradiance == 1000
+    is_t_stc = temp_cell == 25
+
     if alpha_sc is None:
-        mu_i_sc = _fit_tempco(i_sc[effective_irradiance==1000],
-                              temp_cell[effective_irradiance==1000])
-        i_sc_ref = float(i_sc[(effective_irradiance==1000) & (temp_cell==25)])
+        mu_i_sc = _fit_tempco(i_sc[is_g_stc], temp_cell[is_g_stc])
+        i_sc_ref = float(i_sc[is_g_stc & is_t_stc])
         alpha_sc = mu_i_sc * i_sc_ref
 
     if beta_mp is None:
-        beta_mp = _fit_tempco(v_mp[effective_irradiance==1000],
-                              temp_cell[effective_irradiance==1000])
+        beta_mp = _fit_tempco(v_mp[is_g_stc], temp_cell[is_g_stc])
 
     R_sh_ref, R_sh_0, R_sh_exp = _fit_shunt_resistances(
         i_sc, i_mp, v_mp, effective_irradiance, temp_cell, beta_mp,
@@ -427,10 +428,9 @@ def fit_pvsyst_iec61853_sandia(effective_irradiance, temp_cell,
     if R_s is None:
         R_s = _fit_series_resistance(sm, v_oc, i_mp, v_mp)
 
-    filt = temp_cell == 25
     gamma_ref, mu_gamma = _fit_diode_ideality_factor(
-        sm, i_sc[filt], v_oc[filt], i_mp[filt], v_mp[filt],
-        effective_irradiance[filt], temp_cell[filt],
+        sm, i_sc[is_t_stc], v_oc[is_t_stc], i_mp[is_t_stc], v_mp[is_t_stc],
+        effective_irradiance[is_t_stc], temp_cell[is_t_stc],
         R_sh_ref, R_sh_0, R_sh_exp, R_s, cells_in_series
     )
 
@@ -447,7 +447,7 @@ def fit_pvsyst_iec61853_sandia(effective_irradiance, temp_cell,
 
     gamma_ref, mu_gamma = \
         _fit_diode_ideality_factor_post(sm, i_mp, v_mp, effective_irradiance,
-                                        temp_cell, alpha_sc, I_L_ref, I_o_ref, 
+                                        temp_cell, alpha_sc, I_L_ref, I_o_ref,
                                         R_sh_ref, R_sh_0, R_sh_exp, R_s,
                                         cells_in_series, EgRef)
 
@@ -485,18 +485,18 @@ def _fit_shunt_resistances(i_sc, i_mp, v_mp, effective_irradiance, temp_cell,
     v_mp = v_mp[mask]
     effective_irradiance = effective_irradiance[mask]
     temp_cell = temp_cell[mask]
-    
+
     # Equation 10
     Rsh_est = (
         (v_mp / (1 + beta_v_mp * (temp_cell - 25)))
         / (coeff * (i_sc - i_mp))
     )
     Rshexp = 5.5
-    
+
     # Eq 11
     y = Rsh_est
     x = np.exp(-Rshexp * effective_irradiance / 1000)
-    
+
     fit = np.polynomial.polynomial.Polynomial.fit(x, y, deg=1)
     intercept, slope = fit.convert().coef
     Rshbase = intercept
@@ -511,12 +511,12 @@ def _fit_shunt_resistances(i_sc, i_mp, v_mp, effective_irradiance, temp_cell,
 
 def _fit_series_resistance(sm, v_oc, i_mp, v_mp):
     # Stein et al 2014, https://doi.org/10.1109/PVSC.2014.6925326
-    
+
     # Eq 13
     x = np.array([i_mp, np.log(i_mp), v_mp]).T
     x = sm.add_constant(x)
     y = v_oc
-    
+
     results = sm.OLS(endog=y, exog=x).fit()
     R_s = results.params['x1']
     return R_s
@@ -563,7 +563,7 @@ def _fit_saturation_current(i_sc, v_oc, effective_irradiance, temp_cell,
 
 
 def _fit_photocurrent(sm, i_sc, effective_irradiance, temp_cell,
-                      alpha_sc, gamma_ref, mu_gamma, I_o_ref, 
+                      alpha_sc, gamma_ref, mu_gamma, I_o_ref,
                       R_sh_ref, R_sh_0, R_sh_exp, R_s,
                       cells_in_series, EgRef):
     R_sh = _pvsyst_Rsh(effective_irradiance, R_sh_ref, R_sh_0, R_sh_exp)
@@ -583,7 +583,7 @@ def _fit_photocurrent(sm, i_sc, effective_irradiance, temp_cell,
 
 
 def _fit_diode_ideality_factor_post(sm, i_mp, v_mp, effective_irradiance,
-                                    temp_cell, alpha_sc, I_L_ref, I_o_ref, 
+                                    temp_cell, alpha_sc, I_L_ref, I_o_ref,
                                     R_sh_ref, R_sh_0, R_sh_exp, R_s,
                                     cells_in_series, EgRef):
 
diff --git a/tests/ivtools/sdm/test_pvsyst.py b/tests/ivtools/sdm/test_pvsyst.py
index 5d246904c6..178e68ab77 100644
--- a/tests/ivtools/sdm/test_pvsyst.py
+++ b/tests/ivtools/sdm/test_pvsyst.py
@@ -294,7 +294,7 @@ def test_fit_pvsyst_iec61853_sandia_optional(pvsyst_iec61853_table3,
                                                    r_sh_coeff=0.3)
     assert not np.isclose(fitted_params['R_sh_ref'], expected['R_sh_ref'],
                           atol=0, rtol=1e-3)
-    
+
     fitted_params = sdm.fit_pvsyst_iec61853_sandia(**main_inputs,
                                                    R_s=0.5)
     assert not np.isclose(fitted_params['R_s'], expected['R_s'],

From f91884c304bb0681556c9061dc24e46bf6bcae2e Mon Sep 17 00:00:00 2001
From: Kevin Anderson <kevin.anderso@gmail.com>
Date: Wed, 9 Apr 2025 09:48:56 -0400
Subject: [PATCH 06/17] add suffix to names of private functions

---
 pvlib/ivtools/sdm/pvsyst.py | 62 ++++++++++++++++++-------------------
 1 file changed, 31 insertions(+), 31 deletions(-)

diff --git a/pvlib/ivtools/sdm/pvsyst.py b/pvlib/ivtools/sdm/pvsyst.py
index 78d066d6a8..b5dd6175de 100644
--- a/pvlib/ivtools/sdm/pvsyst.py
+++ b/pvlib/ivtools/sdm/pvsyst.py
@@ -413,43 +413,45 @@ def fit_pvsyst_iec61853_sandia(effective_irradiance, temp_cell,
     is_t_stc = temp_cell == 25
 
     if alpha_sc is None:
-        mu_i_sc = _fit_tempco(i_sc[is_g_stc], temp_cell[is_g_stc])
         i_sc_ref = float(i_sc[is_g_stc & is_t_stc])
+        mu_i_sc = _fit_tempco_pvsyst_iec61853_sandia(i_sc[is_g_stc],
+                                                     temp_cell[is_g_stc])
         alpha_sc = mu_i_sc * i_sc_ref
 
     if beta_mp is None:
-        beta_mp = _fit_tempco(v_mp[is_g_stc], temp_cell[is_g_stc])
+        beta_mp = _fit_tempco_pvsyst_iec61853_sandia(v_mp[is_g_stc],
+                                                     temp_cell[is_g_stc])
 
-    R_sh_ref, R_sh_0, R_sh_exp = _fit_shunt_resistances(
+    R_sh_ref, R_sh_0, R_sh_exp = _fit_shunt_resistances_pvsyst_iec61853_sandia(
         i_sc, i_mp, v_mp, effective_irradiance, temp_cell, beta_mp,
         coeff=r_sh_coeff, min_irradiance=min_Rsh_irradiance
     )
 
     if R_s is None:
-        R_s = _fit_series_resistance(sm, v_oc, i_mp, v_mp)
+        R_s = _fit_series_resistance_pvsyst_iec61853_sandia(sm, v_oc,
+                                                            i_mp, v_mp)
 
-    gamma_ref, mu_gamma = _fit_diode_ideality_factor(
+    gamma_ref, mu_gamma = _fit_diode_ideality_factor_pvsyst_iec61853_sandia(
         sm, i_sc[is_t_stc], v_oc[is_t_stc], i_mp[is_t_stc], v_mp[is_t_stc],
         effective_irradiance[is_t_stc], temp_cell[is_t_stc],
         R_sh_ref, R_sh_0, R_sh_exp, R_s, cells_in_series
     )
 
-    I_o_ref = _fit_saturation_current(
+    I_o_ref = _fit_saturation_current_pvsyst_iec61853_sandia(
         i_sc, v_oc, effective_irradiance, temp_cell, gamma_ref, mu_gamma,
         R_sh_ref, R_sh_0, R_sh_exp, R_s, cells_in_series, EgRef
     )
 
-    I_L_ref = _fit_photocurrent(
+    I_L_ref = _fit_photocurrent_pvsyst_iec61853_sandia(
         sm, i_sc, effective_irradiance, temp_cell, alpha_sc,
         gamma_ref, mu_gamma,
         I_o_ref, R_sh_ref, R_sh_0, R_sh_exp, R_s, cells_in_series, EgRef
     )
 
     gamma_ref, mu_gamma = \
-        _fit_diode_ideality_factor_post(sm, i_mp, v_mp, effective_irradiance,
-                                        temp_cell, alpha_sc, I_L_ref, I_o_ref,
-                                        R_sh_ref, R_sh_0, R_sh_exp, R_s,
-                                        cells_in_series, EgRef)
+        _fit_diode_ideality_factor_post_pvsyst_iec61853_sandia(
+            sm, i_mp, v_mp, effective_irradiance, temp_cell, alpha_sc, I_L_ref,
+            I_o_ref, R_sh_ref, R_sh_0, R_sh_exp, R_s, cells_in_series, EgRef)
 
     fitted_params = dict(
         alpha_sc=alpha_sc,
@@ -467,15 +469,16 @@ def fit_pvsyst_iec61853_sandia(effective_irradiance, temp_cell,
     return fitted_params
 
 
-def _fit_tempco(values, temp_cell, temp_cell_ref=25):
+def _fit_tempco_pvsyst_iec61853_sandia(values, temp_cell, temp_cell_ref=25):
     fit = np.polynomial.polynomial.Polynomial.fit(temp_cell, values, deg=1)
     intercept, slope = fit.convert().coef
     value_ref = intercept + slope*temp_cell_ref
     return slope / value_ref
 
 
-def _fit_shunt_resistances(i_sc, i_mp, v_mp, effective_irradiance, temp_cell,
-                           beta_v_mp, coeff=0.2, min_irradiance=None):
+def _fit_shunt_resistances_pvsyst_iec61853_sandia(
+        i_sc, i_mp, v_mp, effective_irradiance, temp_cell,
+        beta_v_mp, coeff=0.2, min_irradiance=None):
     if min_irradiance is None:
         min_irradiance = 0
 
@@ -509,7 +512,7 @@ def _fit_shunt_resistances(i_sc, i_mp, v_mp, effective_irradiance, temp_cell,
     return Rshref, Rsh0, Rshexp
 
 
-def _fit_series_resistance(sm, v_oc, i_mp, v_mp):
+def _fit_series_resistance_pvsyst_iec61853_sandia(sm, v_oc, i_mp, v_mp):
     # Stein et al 2014, https://doi.org/10.1109/PVSC.2014.6925326
 
     # Eq 13
@@ -522,10 +525,9 @@ def _fit_series_resistance(sm, v_oc, i_mp, v_mp):
     return R_s
 
 
-def _fit_diode_ideality_factor(sm, i_sc, v_oc, i_mp, v_mp,
-                               effective_irradiance, temp_cell,
-                               R_sh_ref, R_sh_0, R_sh_exp, R_s,
-                               cells_in_series):
+def _fit_diode_ideality_factor_pvsyst_iec61853_sandia(
+        sm, i_sc, v_oc, i_mp, v_mp, effective_irradiance, temp_cell,
+        R_sh_ref, R_sh_0, R_sh_exp, R_s, cells_in_series):
 
     NsVth = _pvsyst_nNsVth(temp_cell, gamma=1, cells_in_series=cells_in_series)
     Rsh = _pvsyst_Rsh(effective_irradiance, R_sh_ref, R_sh_0, R_sh_exp)
@@ -543,10 +545,9 @@ def _fit_diode_ideality_factor(sm, i_sc, v_oc, i_mp, v_mp,
     return gamma_ref, 0
 
 
-def _fit_saturation_current(i_sc, v_oc, effective_irradiance, temp_cell,
-                            gamma_ref, mu_gamma,
-                            R_sh_ref, R_sh_0, R_sh_exp, R_s,
-                            cells_in_series, EgRef):
+def _fit_saturation_current_pvsyst_iec61853_sandia(
+        i_sc, v_oc, effective_irradiance, temp_cell, gamma_ref, mu_gamma,
+        R_sh_ref, R_sh_0, R_sh_exp, R_s, cells_in_series, EgRef):
     R_sh = _pvsyst_Rsh(effective_irradiance, R_sh_ref, R_sh_0, R_sh_exp)
     gamma = _pvsyst_gamma(temp_cell, gamma_ref, mu_gamma)
     nNsVth = _pvsyst_nNsVth(temp_cell, gamma, cells_in_series)
@@ -562,10 +563,10 @@ def _fit_saturation_current(i_sc, v_oc, effective_irradiance, temp_cell,
     return I_o_ref
 
 
-def _fit_photocurrent(sm, i_sc, effective_irradiance, temp_cell,
-                      alpha_sc, gamma_ref, mu_gamma, I_o_ref,
-                      R_sh_ref, R_sh_0, R_sh_exp, R_s,
-                      cells_in_series, EgRef):
+def _fit_photocurrent_pvsyst_iec61853_sandia(
+        sm, i_sc, effective_irradiance, temp_cell, alpha_sc, gamma_ref,
+        mu_gamma, I_o_ref, R_sh_ref, R_sh_0, R_sh_exp, R_s, cells_in_series,
+        EgRef):
     R_sh = _pvsyst_Rsh(effective_irradiance, R_sh_ref, R_sh_0, R_sh_exp)
     gamma = _pvsyst_gamma(temp_cell, gamma_ref, mu_gamma)
     I_o = _pvsyst_Io(temp_cell, gamma, I_o_ref, EgRef)
@@ -582,10 +583,9 @@ def _fit_photocurrent(sm, i_sc, effective_irradiance, temp_cell,
     return I_L_ref
 
 
-def _fit_diode_ideality_factor_post(sm, i_mp, v_mp, effective_irradiance,
-                                    temp_cell, alpha_sc, I_L_ref, I_o_ref,
-                                    R_sh_ref, R_sh_0, R_sh_exp, R_s,
-                                    cells_in_series, EgRef):
+def _fit_diode_ideality_factor_post_pvsyst_iec61853_sandia(
+        sm, i_mp, v_mp, effective_irradiance, temp_cell, alpha_sc, I_L_ref,
+        I_o_ref, R_sh_ref, R_sh_0, R_sh_exp, R_s, cells_in_series, EgRef):
 
     Rsh = _pvsyst_Rsh(effective_irradiance, R_sh_ref, R_sh_0, R_sh_exp)
     I_L = _pvsyst_IL(effective_irradiance, temp_cell, I_L_ref, alpha_sc)

From df724bc23b1347e8232313536dc9d736aa794ada Mon Sep 17 00:00:00 2001
From: Kevin Anderson <kevin.anderso@gmail.com>
Date: Wed, 9 Apr 2025 09:49:49 -0400
Subject: [PATCH 07/17] add irrad/temp tolerance parameters

---
 pvlib/ivtools/sdm/pvsyst.py | 18 +++++++++++++++---
 1 file changed, 15 insertions(+), 3 deletions(-)

diff --git a/pvlib/ivtools/sdm/pvsyst.py b/pvlib/ivtools/sdm/pvsyst.py
index b5dd6175de..7a3cdef412 100644
--- a/pvlib/ivtools/sdm/pvsyst.py
+++ b/pvlib/ivtools/sdm/pvsyst.py
@@ -315,7 +315,9 @@ def fit_pvsyst_iec61853_sandia(effective_irradiance, temp_cell,
                                cells_in_series, EgRef=1.121,
                                alpha_sc=None, beta_mp=None,
                                r_sh_coeff=0.12, R_s=None,
-                               min_Rsh_irradiance=None):
+                               min_Rsh_irradiance=None,
+                               irradiance_tolerance=20,
+                               temperature_tolerance=1):
     """
     Estimate parameters for the PVsyst module performance model using
     IEC 61853-1 matrix measurements.
@@ -357,6 +359,14 @@ def fit_pvsyst_iec61853_sandia(effective_irradiance, temp_cell,
         Irradiance threshold below which values are excluded when estimating
         shunt resistance parameter values.  May be useful for modules
         with problematic low-light measurements. [W/m²]
+    irradiance_tolerance : float, default 20
+        Tolerance for irradiance variation around the STC value.
+        The default value corresponds to a +/- 2% interval around the STC
+        value of 1000 W/m². [W/m²]
+    temperature_tolerance : float, default 1
+        Tolerance for temperature variation around the STC value.
+        The default value corresponds to a +/- 1 degree interval around the STC
+        value of 25 degrees. [C]
 
     Returns
     -------
@@ -409,8 +419,10 @@ def fit_pvsyst_iec61853_sandia(effective_irradiance, temp_cell,
     except ImportError:
         raise ImportError('fit_pvsyst_iec61853_sandia requires statsmodels')
 
-    is_g_stc = effective_irradiance == 1000
-    is_t_stc = temp_cell == 25
+    is_g_stc = np.isclose(effective_irradiance, 1000, rtol=0,
+                          atol=irradiance_tolerance)
+    is_t_stc = np.isclose(temp_cell, 25, rtol=0,
+                          atol=temperature_tolerance)
 
     if alpha_sc is None:
         i_sc_ref = float(i_sc[is_g_stc & is_t_stc])

From 283d80f5b97d140dd349db4ff2fa7715bf3f5d51 Mon Sep 17 00:00:00 2001
From: Kevin Anderson <kevin.anderso@gmail.com>
Date: Wed, 9 Apr 2025 09:49:56 -0400
Subject: [PATCH 08/17] add tolerance tests

---
 tests/ivtools/sdm/test_pvsyst.py | 33 ++++++++++++++++++++++++++++++++
 1 file changed, 33 insertions(+)

diff --git a/tests/ivtools/sdm/test_pvsyst.py b/tests/ivtools/sdm/test_pvsyst.py
index 178e68ab77..43421e0e38 100644
--- a/tests/ivtools/sdm/test_pvsyst.py
+++ b/tests/ivtools/sdm/test_pvsyst.py
@@ -304,3 +304,36 @@ def test_fit_pvsyst_iec61853_sandia_optional(pvsyst_iec61853_table3,
                                                    min_Rsh_irradiance=500)
     assert not np.isclose(fitted_params['R_sh_ref'], expected['R_sh_ref'],
                           atol=0, rtol=1e-3)
+
+
+@requires_statsmodels
+def test_fit_pvsyst_iec61853_sandia_tolerance(pvsyst_iec61853_table3,
+                                              iec61853_conditions):
+    # verify that the *_tolerance parameters allow non-"perfect" irradiance
+    # and temperature values
+    ee, tc = iec61853_conditions
+    ee[ee == 1000] = 999
+    tc[tc == 25] = 25.1
+    case = pvsyst_iec61853_table3['high current']
+    true_params = case['true']
+    expected = case['estimated']
+    sde_params = pvsystem.calcparams_pvsyst(ee, tc, **true_params)
+    iv = pvsystem.singlediode(*sde_params)
+
+    inputs = dict(
+        effective_irradiance=ee, temp_cell=tc, i_sc=iv['i_sc'],
+        v_oc=iv['v_oc'], i_mp=iv['i_mp'], v_mp=iv['v_mp'],
+        cells_in_series=true_params['cells_in_series']
+    )
+    fitted_params = sdm.fit_pvsyst_iec61853_sandia(**inputs)
+    # still get approximately the expected values
+    for key in expected.keys():
+        assert np.isclose(fitted_params[key], expected[key], atol=0, rtol=1e-2)
+
+    # but if the changes exceed the specified tolerance, then error:
+    with pytest.raises(ValueError, match='Coefficient array is empty'):
+        sdm.fit_pvsyst_iec61853_sandia(**inputs, irradiance_tolerance=0.1)
+
+    with pytest.raises(ValueError, match='can only convert an array of size 1'):
+        sdm.fit_pvsyst_iec61853_sandia(**inputs, temperature_tolerance=0.1)
+    

From 400f2c1ded7e3c28a284b1206dd0798b0c562aeb Mon Sep 17 00:00:00 2001
From: Kevin Anderson <kevin.anderso@gmail.com>
Date: Wed, 9 Apr 2025 09:50:11 -0400
Subject: [PATCH 09/17] fix a couple warnings

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

diff --git a/pvlib/ivtools/sdm/pvsyst.py b/pvlib/ivtools/sdm/pvsyst.py
index 7a3cdef412..d5e78e764b 100644
--- a/pvlib/ivtools/sdm/pvsyst.py
+++ b/pvlib/ivtools/sdm/pvsyst.py
@@ -425,9 +425,9 @@ def fit_pvsyst_iec61853_sandia(effective_irradiance, temp_cell,
                           atol=temperature_tolerance)
 
     if alpha_sc is None:
-        i_sc_ref = float(i_sc[is_g_stc & is_t_stc])
         mu_i_sc = _fit_tempco_pvsyst_iec61853_sandia(i_sc[is_g_stc],
                                                      temp_cell[is_g_stc])
+        i_sc_ref = float(i_sc[is_g_stc & is_t_stc].item())
         alpha_sc = mu_i_sc * i_sc_ref
 
     if beta_mp is None:
@@ -553,7 +553,7 @@ def _fit_diode_ideality_factor_pvsyst_iec61853_sandia(
     y = v_mp + i_mp*R_s - v_oc
 
     results = sm.OLS(endog=y, exog=x).fit()
-    gamma_ref = results.params[0]
+    gamma_ref = results.params['x1']
     return gamma_ref, 0
 
 

From a143fa5e944b492cf264ac0d885f735ad8ae7e7b Mon Sep 17 00:00:00 2001
From: Kevin Anderson <kevin.anderso@gmail.com>
Date: Wed, 9 Apr 2025 09:58:01 -0400
Subject: [PATCH 10/17] lint

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

diff --git a/tests/ivtools/sdm/test_pvsyst.py b/tests/ivtools/sdm/test_pvsyst.py
index 43421e0e38..19fc9ddbec 100644
--- a/tests/ivtools/sdm/test_pvsyst.py
+++ b/tests/ivtools/sdm/test_pvsyst.py
@@ -334,6 +334,6 @@ def test_fit_pvsyst_iec61853_sandia_tolerance(pvsyst_iec61853_table3,
     with pytest.raises(ValueError, match='Coefficient array is empty'):
         sdm.fit_pvsyst_iec61853_sandia(**inputs, irradiance_tolerance=0.1)
 
-    with pytest.raises(ValueError, match='can only convert an array of size 1'):
+    with pytest.raises(ValueError,
+                       match='can only convert an array of size 1'):
         sdm.fit_pvsyst_iec61853_sandia(**inputs, temperature_tolerance=0.1)
-    

From 8e353122d24b5434275f77fb36f1d2f27d8ffea9 Mon Sep 17 00:00:00 2001
From: Kevin Anderson <kevin.anderso@gmail.com>
Date: Thu, 10 Apr 2025 08:25:48 -0400
Subject: [PATCH 11/17] change order of optional parameters

---
 pvlib/ivtools/sdm/pvsyst.py | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/pvlib/ivtools/sdm/pvsyst.py b/pvlib/ivtools/sdm/pvsyst.py
index d5e78e764b..ef3173c0bf 100644
--- a/pvlib/ivtools/sdm/pvsyst.py
+++ b/pvlib/ivtools/sdm/pvsyst.py
@@ -314,7 +314,7 @@ def fit_pvsyst_iec61853_sandia(effective_irradiance, temp_cell,
                                i_sc, v_oc, i_mp, v_mp,
                                cells_in_series, EgRef=1.121,
                                alpha_sc=None, beta_mp=None,
-                               r_sh_coeff=0.12, R_s=None,
+                               R_s=None, r_sh_coeff=0.12,
                                min_Rsh_irradiance=None,
                                irradiance_tolerance=20,
                                temperature_tolerance=1):
@@ -349,12 +349,12 @@ def fit_pvsyst_iec61853_sandia(effective_irradiance, temp_cell,
         Temperature coefficient of maximum power voltage.  If not specified,
         it will be estimated using the ``v_mp`` values at irradiance of
         1000 W/m2. [1/K]
-    r_sh_coeff : float, default 0.12
-        Shunt resistance fitting coefficient.  The default value is taken
-        from [1]_.
     R_s : float, optional
         Series resistance value.  If not provided, a value will be estimated
         from the input measurements. [ohm]
+    r_sh_coeff : float, default 0.12
+        Shunt resistance fitting coefficient.  The default value is taken
+        from [1]_.
     min_Rsh_irradiance : float, optional
         Irradiance threshold below which values are excluded when estimating
         shunt resistance parameter values.  May be useful for modules

From bb86a53345fd7252036b2fcaea864dbac3b99882 Mon Sep 17 00:00:00 2001
From: Kevin Anderson <kevin.anderso@gmail.com>
Date: Thu, 10 Apr 2025 11:52:36 -0400
Subject: [PATCH 12/17] replace statsmodels with np.linalg.lstsq

---
 pvlib/ivtools/sdm/pvsyst.py      | 44 +++++++++++++-------------------
 tests/ivtools/sdm/test_pvsyst.py |  3 ---
 2 files changed, 18 insertions(+), 29 deletions(-)

diff --git a/pvlib/ivtools/sdm/pvsyst.py b/pvlib/ivtools/sdm/pvsyst.py
index ef3173c0bf..2399faf921 100644
--- a/pvlib/ivtools/sdm/pvsyst.py
+++ b/pvlib/ivtools/sdm/pvsyst.py
@@ -414,11 +414,6 @@ def fit_pvsyst_iec61853_sandia(effective_irradiance, temp_cell,
        of Photovoltaics, vol. 15, 3, 2025. :doi:`10.1109/JPHOTOV.2025.3554338`
     """
 
-    try:
-        import statsmodels.api as sm
-    except ImportError:
-        raise ImportError('fit_pvsyst_iec61853_sandia requires statsmodels')
-
     is_g_stc = np.isclose(effective_irradiance, 1000, rtol=0,
                           atol=irradiance_tolerance)
     is_t_stc = np.isclose(temp_cell, 25, rtol=0,
@@ -440,11 +435,10 @@ def fit_pvsyst_iec61853_sandia(effective_irradiance, temp_cell,
     )
 
     if R_s is None:
-        R_s = _fit_series_resistance_pvsyst_iec61853_sandia(sm, v_oc,
-                                                            i_mp, v_mp)
+        R_s = _fit_series_resistance_pvsyst_iec61853_sandia(v_oc, i_mp, v_mp)
 
     gamma_ref, mu_gamma = _fit_diode_ideality_factor_pvsyst_iec61853_sandia(
-        sm, i_sc[is_t_stc], v_oc[is_t_stc], i_mp[is_t_stc], v_mp[is_t_stc],
+        i_sc[is_t_stc], v_oc[is_t_stc], i_mp[is_t_stc], v_mp[is_t_stc],
         effective_irradiance[is_t_stc], temp_cell[is_t_stc],
         R_sh_ref, R_sh_0, R_sh_exp, R_s, cells_in_series
     )
@@ -455,14 +449,14 @@ def fit_pvsyst_iec61853_sandia(effective_irradiance, temp_cell,
     )
 
     I_L_ref = _fit_photocurrent_pvsyst_iec61853_sandia(
-        sm, i_sc, effective_irradiance, temp_cell, alpha_sc,
+        i_sc, effective_irradiance, temp_cell, alpha_sc,
         gamma_ref, mu_gamma,
         I_o_ref, R_sh_ref, R_sh_0, R_sh_exp, R_s, cells_in_series, EgRef
     )
 
     gamma_ref, mu_gamma = \
         _fit_diode_ideality_factor_post_pvsyst_iec61853_sandia(
-            sm, i_mp, v_mp, effective_irradiance, temp_cell, alpha_sc, I_L_ref,
+            i_mp, v_mp, effective_irradiance, temp_cell, alpha_sc, I_L_ref,
             I_o_ref, R_sh_ref, R_sh_0, R_sh_exp, R_s, cells_in_series, EgRef)
 
     fitted_params = dict(
@@ -524,21 +518,20 @@ def _fit_shunt_resistances_pvsyst_iec61853_sandia(
     return Rshref, Rsh0, Rshexp
 
 
-def _fit_series_resistance_pvsyst_iec61853_sandia(sm, v_oc, i_mp, v_mp):
+def _fit_series_resistance_pvsyst_iec61853_sandia(v_oc, i_mp, v_mp):
     # Stein et al 2014, https://doi.org/10.1109/PVSC.2014.6925326
 
     # Eq 13
-    x = np.array([i_mp, np.log(i_mp), v_mp]).T
-    x = sm.add_constant(x)
+    x = np.array([np.ones(len(i_mp)), i_mp, np.log(i_mp), v_mp]).T
     y = v_oc
 
-    results = sm.OLS(endog=y, exog=x).fit()
-    R_s = results.params['x1']
+    coeff, _, _, _ = np.linalg.lstsq(x, y, rcond=None)
+    R_s = coeff[1]
     return R_s
 
 
 def _fit_diode_ideality_factor_pvsyst_iec61853_sandia(
-        sm, i_sc, v_oc, i_mp, v_mp, effective_irradiance, temp_cell,
+        i_sc, v_oc, i_mp, v_mp, effective_irradiance, temp_cell,
         R_sh_ref, R_sh_0, R_sh_exp, R_s, cells_in_series):
 
     NsVth = _pvsyst_nNsVth(temp_cell, gamma=1, cells_in_series=cells_in_series)
@@ -552,8 +545,8 @@ def _fit_diode_ideality_factor_pvsyst_iec61853_sandia(
     x = np.array([x1]).T
     y = v_mp + i_mp*R_s - v_oc
 
-    results = sm.OLS(endog=y, exog=x).fit()
-    gamma_ref = results.params['x1']
+    coeff, _, _, _ = np.linalg.lstsq(x, y, rcond=None)
+    gamma_ref = coeff[0]
     return gamma_ref, 0
 
 
@@ -576,7 +569,7 @@ def _fit_saturation_current_pvsyst_iec61853_sandia(
 
 
 def _fit_photocurrent_pvsyst_iec61853_sandia(
-        sm, i_sc, effective_irradiance, temp_cell, alpha_sc, gamma_ref,
+        i_sc, effective_irradiance, temp_cell, alpha_sc, gamma_ref,
         mu_gamma, I_o_ref, R_sh_ref, R_sh_0, R_sh_exp, R_s, cells_in_series,
         EgRef):
     R_sh = _pvsyst_Rsh(effective_irradiance, R_sh_ref, R_sh_0, R_sh_exp)
@@ -590,13 +583,13 @@ def _fit_photocurrent_pvsyst_iec61853_sandia(
     # Eq 20
     x = np.array([effective_irradiance / 1000]).T
     y = I_L_est - effective_irradiance / 1000 * alpha_sc * (temp_cell - 25)
-    results = sm.OLS(endog=y, exog=x).fit()
-    I_L_ref = results.params['x1']
+    coeff, _, _, _ = np.linalg.lstsq(x, y, rcond=None)
+    I_L_ref = coeff[0]
     return I_L_ref
 
 
 def _fit_diode_ideality_factor_post_pvsyst_iec61853_sandia(
-        sm, i_mp, v_mp, effective_irradiance, temp_cell, alpha_sc, I_L_ref,
+        i_mp, v_mp, effective_irradiance, temp_cell, alpha_sc, I_L_ref,
         I_o_ref, R_sh_ref, R_sh_0, R_sh_exp, R_s, cells_in_series, EgRef):
 
     Rsh = _pvsyst_Rsh(effective_irradiance, R_sh_ref, R_sh_0, R_sh_exp)
@@ -620,10 +613,9 @@ def _fit_diode_ideality_factor_post_pvsyst_iec61853_sandia(
     gamma_est = numerator / denominator
 
     # Eq 22
-    x = np.array([temp_cell - 25]).T
-    x = sm.add_constant(x)
+    x = np.array([np.ones(len(i_mp)), temp_cell - 25]).T
     y = gamma_est
 
-    results = sm.OLS(endog=y, exog=x).fit()
-    gamma_ref, mu_gamma = results.params
+    coeff, _, _, _ = np.linalg.lstsq(x, y, rcond=None)
+    gamma_ref, mu_gamma = coeff
     return gamma_ref, mu_gamma
diff --git a/tests/ivtools/sdm/test_pvsyst.py b/tests/ivtools/sdm/test_pvsyst.py
index 19fc9ddbec..7d2d7797c9 100644
--- a/tests/ivtools/sdm/test_pvsyst.py
+++ b/tests/ivtools/sdm/test_pvsyst.py
@@ -238,7 +238,6 @@ def iec61853_conditions():
     return ee, tc
 
 
-@requires_statsmodels
 def test_fit_pvsyst_iec61853_sandia(pvsyst_iec61853_table3,
                                     iec61853_conditions):
     ee, tc = iec61853_conditions
@@ -258,7 +257,6 @@ def test_fit_pvsyst_iec61853_sandia(pvsyst_iec61853_table3,
                               atol=0, rtol=1e-3)
 
 
-@requires_statsmodels
 def test_fit_pvsyst_iec61853_sandia_optional(pvsyst_iec61853_table3,
                                              iec61853_conditions):
     # verify that modifying each optional parameter results in different output
@@ -306,7 +304,6 @@ def test_fit_pvsyst_iec61853_sandia_optional(pvsyst_iec61853_table3,
                           atol=0, rtol=1e-3)
 
 
-@requires_statsmodels
 def test_fit_pvsyst_iec61853_sandia_tolerance(pvsyst_iec61853_table3,
                                               iec61853_conditions):
     # verify that the *_tolerance parameters allow non-"perfect" irradiance

From fe146b61132b4330ead5b3c3444d0ae9421fe996 Mon Sep 17 00:00:00 2001
From: Kevin Anderson <kevin.anderso@gmail.com>
Date: Thu, 10 Apr 2025 12:12:55 -0400
Subject: [PATCH 13/17] add _2025 to function name

---
 .../reference/pv_modeling/parameters.rst      |  2 +-
 docs/sphinx/source/whatsnew/v0.12.1.rst       |  2 +-
 pvlib/ivtools/sdm/__init__.py                 |  2 +-
 pvlib/ivtools/sdm/pvsyst.py                   | 64 ++++++++++---------
 tests/ivtools/sdm/test_pvsyst.py              | 45 ++++++-------
 5 files changed, 60 insertions(+), 55 deletions(-)

diff --git a/docs/sphinx/source/reference/pv_modeling/parameters.rst b/docs/sphinx/source/reference/pv_modeling/parameters.rst
index 69e8ba942f..1f146eaccc 100644
--- a/docs/sphinx/source/reference/pv_modeling/parameters.rst
+++ b/docs/sphinx/source/reference/pv_modeling/parameters.rst
@@ -12,7 +12,7 @@ Functions for fitting single diode models
     ivtools.sdm.fit_cec_sam
     ivtools.sdm.fit_desoto
     ivtools.sdm.fit_pvsyst_sandia
-    ivtools.sdm.fit_pvsyst_iec61853_sandia
+    ivtools.sdm.fit_pvsyst_iec61853_sandia_2025
     ivtools.sdm.fit_desoto_sandia
 
 Functions for fitting the single diode equation
diff --git a/docs/sphinx/source/whatsnew/v0.12.1.rst b/docs/sphinx/source/whatsnew/v0.12.1.rst
index a58d792471..b7fffd5996 100644
--- a/docs/sphinx/source/whatsnew/v0.12.1.rst
+++ b/docs/sphinx/source/whatsnew/v0.12.1.rst
@@ -16,7 +16,7 @@ Enhancements
 ~~~~~~~~~~~~
 * :py:mod:`pvlib.ivtools.sdm` is now a subpackage. (:issue:`2252`, :pull:`2256`)
 * Add a function for estimating PVsyst SDM parameters from IEC 61853-1 matrix
-  data (:py:func:`~pvlib.ivtools.sdm.fit_pvsyst_iec61853_sandia`). (:issue:`2185`, :pull:`2429`)
+  data (:py:func:`~pvlib.ivtools.sdm.fit_pvsyst_iec61853_sandia_2025`). (:issue:`2185`, :pull:`2429`)
 
 Documentation
 ~~~~~~~~~~~~~
diff --git a/pvlib/ivtools/sdm/__init__.py b/pvlib/ivtools/sdm/__init__.py
index d0b658c198..8535f1f5e6 100644
--- a/pvlib/ivtools/sdm/__init__.py
+++ b/pvlib/ivtools/sdm/__init__.py
@@ -15,6 +15,6 @@
 
 from pvlib.ivtools.sdm.pvsyst import (  # noqa: F401
     fit_pvsyst_sandia,
-    fit_pvsyst_iec61853_sandia,
+    fit_pvsyst_iec61853_sandia_2025,
     pvsyst_temperature_coeff,
 )
diff --git a/pvlib/ivtools/sdm/pvsyst.py b/pvlib/ivtools/sdm/pvsyst.py
index 2399faf921..5be4654200 100644
--- a/pvlib/ivtools/sdm/pvsyst.py
+++ b/pvlib/ivtools/sdm/pvsyst.py
@@ -310,14 +310,14 @@ def maxp(temp_cell, irrad_ref, alpha_sc, gamma_ref, mu_gamma, I_L_ref,
     return gamma_pdc / pmp
 
 
-def fit_pvsyst_iec61853_sandia(effective_irradiance, temp_cell,
-                               i_sc, v_oc, i_mp, v_mp,
-                               cells_in_series, EgRef=1.121,
-                               alpha_sc=None, beta_mp=None,
-                               R_s=None, r_sh_coeff=0.12,
-                               min_Rsh_irradiance=None,
-                               irradiance_tolerance=20,
-                               temperature_tolerance=1):
+def fit_pvsyst_iec61853_sandia_2025(effective_irradiance, temp_cell,
+                                    i_sc, v_oc, i_mp, v_mp,
+                                    cells_in_series, EgRef=1.121,
+                                    alpha_sc=None, beta_mp=None,
+                                    R_s=None, r_sh_coeff=0.12,
+                                    min_Rsh_irradiance=None,
+                                    irradiance_tolerance=20,
+                                    temperature_tolerance=1):
     """
     Estimate parameters for the PVsyst module performance model using
     IEC 61853-1 matrix measurements.
@@ -420,42 +420,45 @@ def fit_pvsyst_iec61853_sandia(effective_irradiance, temp_cell,
                           atol=temperature_tolerance)
 
     if alpha_sc is None:
-        mu_i_sc = _fit_tempco_pvsyst_iec61853_sandia(i_sc[is_g_stc],
-                                                     temp_cell[is_g_stc])
+        mu_i_sc = _fit_tempco_pvsyst_iec61853_sandia_2025(i_sc[is_g_stc],
+                                                          temp_cell[is_g_stc])
         i_sc_ref = float(i_sc[is_g_stc & is_t_stc].item())
         alpha_sc = mu_i_sc * i_sc_ref
 
     if beta_mp is None:
-        beta_mp = _fit_tempco_pvsyst_iec61853_sandia(v_mp[is_g_stc],
-                                                     temp_cell[is_g_stc])
+        beta_mp = _fit_tempco_pvsyst_iec61853_sandia_2025(v_mp[is_g_stc],
+                                                          temp_cell[is_g_stc])
 
-    R_sh_ref, R_sh_0, R_sh_exp = _fit_shunt_resistances_pvsyst_iec61853_sandia(
-        i_sc, i_mp, v_mp, effective_irradiance, temp_cell, beta_mp,
-        coeff=r_sh_coeff, min_irradiance=min_Rsh_irradiance
+    R_sh_ref, R_sh_0, R_sh_exp = \
+        _fit_shunt_resistances_pvsyst_iec61853_sandia_2025(
+            i_sc, i_mp, v_mp, effective_irradiance, temp_cell, beta_mp,
+            coeff=r_sh_coeff, min_irradiance=min_Rsh_irradiance
     )
 
     if R_s is None:
-        R_s = _fit_series_resistance_pvsyst_iec61853_sandia(v_oc, i_mp, v_mp)
+        R_s = _fit_series_resistance_pvsyst_iec61853_sandia_2025(v_oc, i_mp,
+                                                                 v_mp)
 
-    gamma_ref, mu_gamma = _fit_diode_ideality_factor_pvsyst_iec61853_sandia(
-        i_sc[is_t_stc], v_oc[is_t_stc], i_mp[is_t_stc], v_mp[is_t_stc],
-        effective_irradiance[is_t_stc], temp_cell[is_t_stc],
-        R_sh_ref, R_sh_0, R_sh_exp, R_s, cells_in_series
+    gamma_ref, mu_gamma = \
+        _fit_diode_ideality_factor_pvsyst_iec61853_sandia_2025(
+            i_sc[is_t_stc], v_oc[is_t_stc], i_mp[is_t_stc], v_mp[is_t_stc],
+            effective_irradiance[is_t_stc], temp_cell[is_t_stc],
+            R_sh_ref, R_sh_0, R_sh_exp, R_s, cells_in_series
     )
 
-    I_o_ref = _fit_saturation_current_pvsyst_iec61853_sandia(
+    I_o_ref = _fit_saturation_current_pvsyst_iec61853_sandia_2025(
         i_sc, v_oc, effective_irradiance, temp_cell, gamma_ref, mu_gamma,
         R_sh_ref, R_sh_0, R_sh_exp, R_s, cells_in_series, EgRef
     )
 
-    I_L_ref = _fit_photocurrent_pvsyst_iec61853_sandia(
+    I_L_ref = _fit_photocurrent_pvsyst_iec61853_sandia_2025(
         i_sc, effective_irradiance, temp_cell, alpha_sc,
         gamma_ref, mu_gamma,
         I_o_ref, R_sh_ref, R_sh_0, R_sh_exp, R_s, cells_in_series, EgRef
     )
 
     gamma_ref, mu_gamma = \
-        _fit_diode_ideality_factor_post_pvsyst_iec61853_sandia(
+        _fit_diode_ideality_factor_post_pvsyst_iec61853_sandia_2025(
             i_mp, v_mp, effective_irradiance, temp_cell, alpha_sc, I_L_ref,
             I_o_ref, R_sh_ref, R_sh_0, R_sh_exp, R_s, cells_in_series, EgRef)
 
@@ -475,14 +478,15 @@ def fit_pvsyst_iec61853_sandia(effective_irradiance, temp_cell,
     return fitted_params
 
 
-def _fit_tempco_pvsyst_iec61853_sandia(values, temp_cell, temp_cell_ref=25):
+def _fit_tempco_pvsyst_iec61853_sandia_2025(values, temp_cell,
+                                            temp_cell_ref=25):
     fit = np.polynomial.polynomial.Polynomial.fit(temp_cell, values, deg=1)
     intercept, slope = fit.convert().coef
     value_ref = intercept + slope*temp_cell_ref
     return slope / value_ref
 
 
-def _fit_shunt_resistances_pvsyst_iec61853_sandia(
+def _fit_shunt_resistances_pvsyst_iec61853_sandia_2025(
         i_sc, i_mp, v_mp, effective_irradiance, temp_cell,
         beta_v_mp, coeff=0.2, min_irradiance=None):
     if min_irradiance is None:
@@ -518,7 +522,7 @@ def _fit_shunt_resistances_pvsyst_iec61853_sandia(
     return Rshref, Rsh0, Rshexp
 
 
-def _fit_series_resistance_pvsyst_iec61853_sandia(v_oc, i_mp, v_mp):
+def _fit_series_resistance_pvsyst_iec61853_sandia_2025(v_oc, i_mp, v_mp):
     # Stein et al 2014, https://doi.org/10.1109/PVSC.2014.6925326
 
     # Eq 13
@@ -530,7 +534,7 @@ def _fit_series_resistance_pvsyst_iec61853_sandia(v_oc, i_mp, v_mp):
     return R_s
 
 
-def _fit_diode_ideality_factor_pvsyst_iec61853_sandia(
+def _fit_diode_ideality_factor_pvsyst_iec61853_sandia_2025(
         i_sc, v_oc, i_mp, v_mp, effective_irradiance, temp_cell,
         R_sh_ref, R_sh_0, R_sh_exp, R_s, cells_in_series):
 
@@ -550,7 +554,7 @@ def _fit_diode_ideality_factor_pvsyst_iec61853_sandia(
     return gamma_ref, 0
 
 
-def _fit_saturation_current_pvsyst_iec61853_sandia(
+def _fit_saturation_current_pvsyst_iec61853_sandia_2025(
         i_sc, v_oc, effective_irradiance, temp_cell, gamma_ref, mu_gamma,
         R_sh_ref, R_sh_0, R_sh_exp, R_s, cells_in_series, EgRef):
     R_sh = _pvsyst_Rsh(effective_irradiance, R_sh_ref, R_sh_0, R_sh_exp)
@@ -568,7 +572,7 @@ def _fit_saturation_current_pvsyst_iec61853_sandia(
     return I_o_ref
 
 
-def _fit_photocurrent_pvsyst_iec61853_sandia(
+def _fit_photocurrent_pvsyst_iec61853_sandia_2025(
         i_sc, effective_irradiance, temp_cell, alpha_sc, gamma_ref,
         mu_gamma, I_o_ref, R_sh_ref, R_sh_0, R_sh_exp, R_s, cells_in_series,
         EgRef):
@@ -588,7 +592,7 @@ def _fit_photocurrent_pvsyst_iec61853_sandia(
     return I_L_ref
 
 
-def _fit_diode_ideality_factor_post_pvsyst_iec61853_sandia(
+def _fit_diode_ideality_factor_post_pvsyst_iec61853_sandia_2025(
         i_mp, v_mp, effective_irradiance, temp_cell, alpha_sc, I_L_ref,
         I_o_ref, R_sh_ref, R_sh_0, R_sh_exp, R_s, cells_in_series, EgRef):
 
diff --git a/tests/ivtools/sdm/test_pvsyst.py b/tests/ivtools/sdm/test_pvsyst.py
index 7d2d7797c9..2a1793633b 100644
--- a/tests/ivtools/sdm/test_pvsyst.py
+++ b/tests/ivtools/sdm/test_pvsyst.py
@@ -238,8 +238,8 @@ def iec61853_conditions():
     return ee, tc
 
 
-def test_fit_pvsyst_iec61853_sandia(pvsyst_iec61853_table3,
-                                    iec61853_conditions):
+def test_fit_pvsyst_iec61853_sandia_2025(pvsyst_iec61853_table3,
+                                         iec61853_conditions):
     ee, tc = iec61853_conditions
     for _, case in pvsyst_iec61853_table3.items():
         true_params = case['true']
@@ -248,7 +248,7 @@ def test_fit_pvsyst_iec61853_sandia(pvsyst_iec61853_table3,
         sde_params = pvsystem.calcparams_pvsyst(ee, tc, **true_params)
         iv = pvsystem.singlediode(*sde_params)
 
-        fitted_params = sdm.fit_pvsyst_iec61853_sandia(
+        fitted_params = sdm.fit_pvsyst_iec61853_sandia_2025(
             ee, tc, iv['i_sc'], iv['v_oc'], iv['i_mp'], iv['v_mp'],
             true_params['cells_in_series'], EgRef=true_params['EgRef'],
         )
@@ -257,8 +257,8 @@ def test_fit_pvsyst_iec61853_sandia(pvsyst_iec61853_table3,
                               atol=0, rtol=1e-3)
 
 
-def test_fit_pvsyst_iec61853_sandia_optional(pvsyst_iec61853_table3,
-                                             iec61853_conditions):
+def test_fit_pvsyst_iec61853_sandia_2025_optional(pvsyst_iec61853_table3,
+                                                  iec61853_conditions):
     # verify that modifying each optional parameter results in different output
     ee, tc = iec61853_conditions
     case = pvsyst_iec61853_table3['high current']
@@ -273,39 +273,39 @@ def test_fit_pvsyst_iec61853_sandia_optional(pvsyst_iec61853_table3,
         cells_in_series=true_params['cells_in_series']
     )
 
-    fitted_params = sdm.fit_pvsyst_iec61853_sandia(**main_inputs,
-                                                   EgRef=1.0)
+    fitted_params = sdm.fit_pvsyst_iec61853_sandia_2025(**main_inputs,
+                                                        EgRef=1.0)
     assert not np.isclose(fitted_params['I_o_ref'], expected['I_o_ref'],
                           atol=0, rtol=1e-3)
 
-    fitted_params = sdm.fit_pvsyst_iec61853_sandia(**main_inputs,
-                                                   alpha_sc=0.5e-3)
+    fitted_params = sdm.fit_pvsyst_iec61853_sandia_2025(**main_inputs,
+                                                        alpha_sc=0.5e-3)
     assert not np.isclose(fitted_params['alpha_sc'], expected['alpha_sc'],
                           atol=0, rtol=1e-3)
 
-    fitted_params = sdm.fit_pvsyst_iec61853_sandia(**main_inputs,
-                                                   beta_mp=-1e-4)
+    fitted_params = sdm.fit_pvsyst_iec61853_sandia_2025(**main_inputs,
+                                                        beta_mp=-1e-4)
     assert not np.isclose(fitted_params['R_sh_ref'], expected['R_sh_ref'],
                           atol=0, rtol=1e-3)
 
-    fitted_params = sdm.fit_pvsyst_iec61853_sandia(**main_inputs,
-                                                   r_sh_coeff=0.3)
+    fitted_params = sdm.fit_pvsyst_iec61853_sandia_2025(**main_inputs,
+                                                        r_sh_coeff=0.3)
     assert not np.isclose(fitted_params['R_sh_ref'], expected['R_sh_ref'],
                           atol=0, rtol=1e-3)
 
-    fitted_params = sdm.fit_pvsyst_iec61853_sandia(**main_inputs,
-                                                   R_s=0.5)
+    fitted_params = sdm.fit_pvsyst_iec61853_sandia_2025(**main_inputs,
+                                                        R_s=0.5)
     assert not np.isclose(fitted_params['R_s'], expected['R_s'],
                           atol=0, rtol=1e-3)
 
-    fitted_params = sdm.fit_pvsyst_iec61853_sandia(**main_inputs,
-                                                   min_Rsh_irradiance=500)
+    fitted_params = sdm.fit_pvsyst_iec61853_sandia_2025(**main_inputs,
+                                                        min_Rsh_irradiance=500)
     assert not np.isclose(fitted_params['R_sh_ref'], expected['R_sh_ref'],
                           atol=0, rtol=1e-3)
 
 
-def test_fit_pvsyst_iec61853_sandia_tolerance(pvsyst_iec61853_table3,
-                                              iec61853_conditions):
+def test_fit_pvsyst_iec61853_sandia_2025_tolerance(pvsyst_iec61853_table3,
+                                                   iec61853_conditions):
     # verify that the *_tolerance parameters allow non-"perfect" irradiance
     # and temperature values
     ee, tc = iec61853_conditions
@@ -322,15 +322,16 @@ def test_fit_pvsyst_iec61853_sandia_tolerance(pvsyst_iec61853_table3,
         v_oc=iv['v_oc'], i_mp=iv['i_mp'], v_mp=iv['v_mp'],
         cells_in_series=true_params['cells_in_series']
     )
-    fitted_params = sdm.fit_pvsyst_iec61853_sandia(**inputs)
+    fitted_params = sdm.fit_pvsyst_iec61853_sandia_2025(**inputs)
     # still get approximately the expected values
     for key in expected.keys():
         assert np.isclose(fitted_params[key], expected[key], atol=0, rtol=1e-2)
 
     # but if the changes exceed the specified tolerance, then error:
     with pytest.raises(ValueError, match='Coefficient array is empty'):
-        sdm.fit_pvsyst_iec61853_sandia(**inputs, irradiance_tolerance=0.1)
+        sdm.fit_pvsyst_iec61853_sandia_2025(**inputs, irradiance_tolerance=0.1)
 
     with pytest.raises(ValueError,
                        match='can only convert an array of size 1'):
-        sdm.fit_pvsyst_iec61853_sandia(**inputs, temperature_tolerance=0.1)
+        sdm.fit_pvsyst_iec61853_sandia_2025(**inputs,
+                                            temperature_tolerance=0.1)

From d34705cff28368c203858dabf4a13976f3e8d11c Mon Sep 17 00:00:00 2001
From: Kevin Anderson <kevin.anderso@gmail.com>
Date: Thu, 10 Apr 2025 12:14:57 -0400
Subject: [PATCH 14/17] add note regarding array shape

---
 pvlib/ivtools/sdm/pvsyst.py | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/pvlib/ivtools/sdm/pvsyst.py b/pvlib/ivtools/sdm/pvsyst.py
index 5be4654200..d44c2d3795 100644
--- a/pvlib/ivtools/sdm/pvsyst.py
+++ b/pvlib/ivtools/sdm/pvsyst.py
@@ -402,6 +402,9 @@ def fit_pvsyst_iec61853_sandia_2025(effective_irradiance, temp_cell,
 
     Notes
     -----
+    Input arrays of operating conditions and electrical measurements must be
+    1-D with equal lengths.
+    
     This method is non-iterative.  In some cases, it may be desirable to
     refine the estimated parameter values using a numerical optimizer like
     those provided in ``scipy.optimize``.

From 6684b2ed43eaacf755e389aa0e68f4ce839cc0a6 Mon Sep 17 00:00:00 2001
From: Kevin Anderson <kevin.anderso@gmail.com>
Date: Thu, 10 Apr 2025 12:36:32 -0400
Subject: [PATCH 15/17] lint

---
 pvlib/ivtools/sdm/pvsyst.py | 8 +++-----
 1 file changed, 3 insertions(+), 5 deletions(-)

diff --git a/pvlib/ivtools/sdm/pvsyst.py b/pvlib/ivtools/sdm/pvsyst.py
index d44c2d3795..11886db44b 100644
--- a/pvlib/ivtools/sdm/pvsyst.py
+++ b/pvlib/ivtools/sdm/pvsyst.py
@@ -404,7 +404,7 @@ def fit_pvsyst_iec61853_sandia_2025(effective_irradiance, temp_cell,
     -----
     Input arrays of operating conditions and electrical measurements must be
     1-D with equal lengths.
-    
+
     This method is non-iterative.  In some cases, it may be desirable to
     refine the estimated parameter values using a numerical optimizer like
     those provided in ``scipy.optimize``.
@@ -435,8 +435,7 @@ def fit_pvsyst_iec61853_sandia_2025(effective_irradiance, temp_cell,
     R_sh_ref, R_sh_0, R_sh_exp = \
         _fit_shunt_resistances_pvsyst_iec61853_sandia_2025(
             i_sc, i_mp, v_mp, effective_irradiance, temp_cell, beta_mp,
-            coeff=r_sh_coeff, min_irradiance=min_Rsh_irradiance
-    )
+            coeff=r_sh_coeff, min_irradiance=min_Rsh_irradiance)
 
     if R_s is None:
         R_s = _fit_series_resistance_pvsyst_iec61853_sandia_2025(v_oc, i_mp,
@@ -446,8 +445,7 @@ def fit_pvsyst_iec61853_sandia_2025(effective_irradiance, temp_cell,
         _fit_diode_ideality_factor_pvsyst_iec61853_sandia_2025(
             i_sc[is_t_stc], v_oc[is_t_stc], i_mp[is_t_stc], v_mp[is_t_stc],
             effective_irradiance[is_t_stc], temp_cell[is_t_stc],
-            R_sh_ref, R_sh_0, R_sh_exp, R_s, cells_in_series
-    )
+            R_sh_ref, R_sh_0, R_sh_exp, R_s, cells_in_series)
 
     I_o_ref = _fit_saturation_current_pvsyst_iec61853_sandia_2025(
         i_sc, v_oc, effective_irradiance, temp_cell, gamma_ref, mu_gamma,

From 773e809aeb3441a619d19c92d6844778fa849720 Mon Sep 17 00:00:00 2001
From: Kevin Anderson <kevin.anderso@gmail.com>
Date: Thu, 10 Apr 2025 13:11:37 -0400
Subject: [PATCH 16/17] Update pvsyst.py

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

diff --git a/pvlib/ivtools/sdm/pvsyst.py b/pvlib/ivtools/sdm/pvsyst.py
index 11886db44b..3e5a485fa8 100644
--- a/pvlib/ivtools/sdm/pvsyst.py
+++ b/pvlib/ivtools/sdm/pvsyst.py
@@ -406,8 +406,8 @@ def fit_pvsyst_iec61853_sandia_2025(effective_irradiance, temp_cell,
     1-D with equal lengths.
 
     This method is non-iterative.  In some cases, it may be desirable to
-    refine the estimated parameter values using a numerical optimizer like
-    those provided in ``scipy.optimize``.
+    refine the estimated parameter values using a numerical optimizer such as
+    the default method in ``scipy.optimize.minimize``.
 
     References
     ----------

From 7e64e8eead3cce187182a884199d2f5e6bcb9d62 Mon Sep 17 00:00:00 2001
From: Kevin Anderson <kevin.anderso@gmail.com>
Date: Fri, 11 Apr 2025 08:22:26 -0400
Subject: [PATCH 17/17] add note regarding temp coeff consistency

---
 pvlib/ivtools/sdm/pvsyst.py | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/pvlib/ivtools/sdm/pvsyst.py b/pvlib/ivtools/sdm/pvsyst.py
index 3e5a485fa8..7e1f8c71ab 100644
--- a/pvlib/ivtools/sdm/pvsyst.py
+++ b/pvlib/ivtools/sdm/pvsyst.py
@@ -405,6 +405,10 @@ def fit_pvsyst_iec61853_sandia_2025(effective_irradiance, temp_cell,
     Input arrays of operating conditions and electrical measurements must be
     1-D with equal lengths.
 
+    Values supplied for ``alpha_sc``, ``beta_mp``, and ``R_s`` must be
+    consistent with the matrix data, as these values are used when estimating
+    other model parameters.
+
     This method is non-iterative.  In some cases, it may be desirable to
     refine the estimated parameter values using a numerical optimizer such as
     the default method in ``scipy.optimize.minimize``.