Added irradiance_loss_pvsyst function to pvsystem

docs/sphinx/source/api.rst

@@ -369,6 +369,13 @@ Other
 Effects on PV System Output
 ===========================
 
+Loss models
+^^^^^^^^^^^
+.. autosummary::
+   :toctree: generated/
+
+   pvsystem.combine_loss_factors
+
 .. autosummary::
    :toctree: generated/
 

docs/sphinx/source/whatsnew/v0.8.0.rst

@@ -92,6 +92,9 @@ Enhancements
   PVSystem, LocalizedPVSystem, SingleAxisTracker, and
   LocalizedSingleAxisTracker repr methods. (:issue:`1027`)
 * Added ability for :py:func:`pvlib.soiling.hsu` to accept arbitrary time intervals. (:pull:`980`)
+* Add :py:func:`pvlib.pvsystem.combine_loss_factors` as general purpose
+  function to combine loss factors with a common index.
+  Partialy addresses :issue:`988`. Contributed by Brock Taute :ghuser:`btaute`
 
 Bug fixes
 ~~~~~~~~~

pvlib/pvsystem.py

@@ -2310,6 +2310,48 @@ def pvwatts_losses(soiling=2, shading=3, snow=0, mismatch=2, wiring=2,
     return losses
 
 
+def combine_loss_factors(index, *losses, fill_method='ffill'):
+    r"""
+    Combines Series loss fractions while setting a common index.
+
+    The separate losses are compounded using the following equation:
+
+    .. math::
+
+        L_{total} = 1 - [ 1 - \Pi_i ( 1 - L_i ) ]
+
+    :math:`L_{total}` is the total loss returned
+    :math:`L_i` is each individual loss factor input
+
+    Note the losses must each be a series with a DatetimeIndex.
+    All losses will be resampled to match the index parameter using
+    the fill method specified (defaults to "fill forward").
+
+    Parameters
+    ----------
+    index : DatetimeIndex
+        The index of the returned loss factors
+
+    *losses : Series
+        One or more Series of fractions to be compounded
+
+    fill_method : {'ffill', 'bfill', 'nearest'}, default 'ffill'
+        Method to use for filling holes in reindexed DataFrame
+
+    Returns
+    -------
+    Series
+        Fractions resulting from the combination of each loss factor
+    """
+    combined_factor = 1
+
+    for loss in losses:
+        loss = loss.reindex(index, method=fill_method)
+        combined_factor *= (1 - loss)
+
+    return 1 - combined_factor
+
+
 snlinverter = deprecated('0.8', alternative='inverter.sandia',
                          name='snlinverter', removal='0.9')(inverter.sandia)
 

pvlib/tests/test_pvsystem.py

@@ -1299,6 +1299,18 @@ def test_PVSystem_pvwatts_ac_kwargs(mocker):
     assert out < pdc
 
 
+def test_combine_loss_factors():
+    test_index = pd.date_range(start='1990/01/01T12:00', periods=365, freq='D')
+    loss_1 = pd.Series(.10, index=test_index)
+    loss_2 = pd.Series(.05, index=pd.date_range(start='1990/01/01T12:00',
+                                                periods=365*2, freq='D'))
+    loss_3 = pd.Series(.02, index=pd.date_range(start='1990/01/01',
+                                                periods=12, freq='MS'))
+    expected = pd.Series(.1621, index=test_index)
+    out = pvsystem.combine_loss_factors(test_index, loss_1, loss_2, loss_3)
+    assert_series_equal(expected, out)
+
+
 @fail_on_pvlib_version('0.9')
 def test_deprecated_09(cec_inverter_parameters, adr_inverter_parameters):
     # deprecated function pvsystem.snlinverter