implement inverter.pvwatts_multi (#1106)

* implement inverter.pvwatts_multi

* fix text, formatting

* fix docstring

* remove mistaken .T

* review

* pdc not p_dc in docstring

Author: cwhanse

docs/sphinx/source/api.rst

@@ -295,6 +295,7 @@ Inverter models (DC to AC conversion)
    inverter.sandia_multi
    inverter.adr
    inverter.pvwatts
+   inverter.pvwatts_multi
 
 Functions for fitting inverter models
 

docs/sphinx/source/whatsnew/v0.8.1.rst

@@ -17,8 +17,8 @@ Enhancements
   option to :py:class:`~pvlib.modelchain.ModelChain`. (:pull:`1042`) (:issue:`1073`)
 * Added :py:func:`pvlib.temperature.ross` for cell temperature modeling using
   only NOCT. (:pull:`1045`)
-* Added :py:func:`pvlib.inverter.sandia_multi` for modeling inverters with
-  multiple MPPTs (:issue:`457`, :pull:`1085`)
+* Added :py:func:`pvlib.inverter.sandia_multi` and :py:func:`pvlib.inverter.pvwatts_multi`
+  for modeling inverters with multiple MPPTs (:issue:`457`, :pull:`1085`, :pull:`1106`)
 * Added optional ``attributes`` parameter to :py:func:`pvlib.iotools.get_psm3`
   and added the option of fetching 5- and 15-minute PSM3 data. (:pull:`1086`)
 * Added :py:func:`pvlib.irradiance.campbell_norman` for estimating DNI, DHI and GHI

pvlib/inverter.py

@@ -328,7 +328,7 @@ def adr(v_dc, p_dc, inverter, vtol=0.10):
 
 def pvwatts(pdc, pdc0, eta_inv_nom=0.96, eta_inv_ref=0.9637):
     r"""
-    Implements NREL's PVWatts inverter model.
+    NREL's PVWatts inverter model.
 
     The PVWatts inverter model [1]_ calculates inverter efficiency :math:`\eta`
     as a function of input DC power
@@ -348,7 +348,7 @@ def pvwatts(pdc, pdc0, eta_inv_nom=0.96, eta_inv_ref=0.9637):
 
     Parameters
     ----------
-    pdc: numeric
+    pdc : numeric
         DC power. Same unit as ``pdc0``.
     pdc0: numeric
         DC input limit of the inverter.  Same unit as ``pdc``.
@@ -371,6 +371,10 @@ def pvwatts(pdc, pdc0, eta_inv_nom=0.96, eta_inv_ref=0.9637):
     :py:func:`pvlib.pvsystem.pvwatts_dc` refers to the DC power of the modules
     at reference conditions.
 
+    See Also
+    --------
+    pvlib.inverter.pvwatts_multi
+
     References
     ----------
     .. [1] A. P. Dobos, "PVWatts Version 5 Manual,"
@@ -396,6 +400,39 @@ def pvwatts(pdc, pdc0, eta_inv_nom=0.96, eta_inv_ref=0.9637):
     return power_ac
 
 
+def pvwatts_multi(pdc, pdc0, eta_inv_nom=0.96, eta_inv_ref=0.9637):
+    r"""
+    Extend NREL's PVWatts inverter model for multiple MPP inputs.
+
+    DC input power is summed over MPP inputs to obtain the DC power
+    input to the PVWatts inverter model. See :py:func:`pvlib.inverter.pvwatts`
+    for details.
+
+    Parameters
+    ----------
+    pdc : tuple, list or array of numeric
+        DC power on each MPPT input of the inverter. If type is array, must
+        be 2d with axis 0 being the MPPT inputs. Same unit as ``pdc0``.
+    pdc0: numeric
+        DC input limit of the inverter.  Same unit as ``pdc``.
+    eta_inv_nom: numeric, default 0.96
+        Nominal inverter efficiency. [unitless]
+    eta_inv_ref: numeric, default 0.9637
+        Reference inverter efficiency. PVWatts defines it to be 0.9637
+        and is included here for flexibility. [unitless]
+
+    Returns
+    -------
+    power_ac: numeric
+        AC power.  Same unit as ``pdc0``.
+
+    See Also
+    --------
+    pvlib.inverter.pvwatts
+    """
+    return pvwatts(sum(pdc), pdc0, eta_inv_nom, eta_inv_ref)
+
+
 def fit_sandia(ac_power, dc_power, dc_voltage, dc_voltage_level, p_ac_0, p_nt):
     r'''
     Determine parameters for the Sandia inverter model.

pvlib/tests/test_inverter.py

@@ -171,6 +171,25 @@ def test_pvwatts_series():
     assert_series_equal(expected, out)
 
 
+def test_pvwatts_multi():
+    pdc = np.array([np.nan, 0, 50, 100]) / 2
+    pdc0 = 100
+    expected = np.array([np.nan, 0., 47.608436, 95.])
+    out = inverter.pvwatts_multi((pdc, pdc), pdc0, 0.95)
+    assert_allclose(expected, out)
+    # with 2D array
+    pdc_2d = np.array([pdc, pdc])
+    out = inverter.pvwatts_multi(pdc_2d, pdc0, 0.95)
+    assert_allclose(expected, out)
+    # with Series
+    pdc = pd.Series(pdc)
+    out = inverter.pvwatts_multi((pdc, pdc), pdc0, 0.95)
+    assert_series_equal(expected, out)
+    # with list instead of tuple
+    out = inverter.pvwatts_multi([pdc, pdc], pdc0, 0.95)
+    assert_series_equal(expected, out)
+
+
 INVERTER_TEST_MEAS = DATA_DIR / 'inverter_fit_snl_meas.csv'
 INVERTER_TEST_SIM = DATA_DIR / 'inverter_fit_snl_sim.csv'