Add FAQ entries for CEC vs Sandia module dbs and multi-inverter syste…

…ms (#1662)

* add CEC vs Sandia and multiple inverter FAQs

* Update docs/sphinx/source/user_guide/faq.rst

Co-authored-by: Will Holmgren <william.holmgren@gmail.com>

* incorporate reviewer feedback

Co-Authored-By: Mark Mikofski <bwana.marko@yahoo.com>

* fix whatsnew

---------

Co-authored-by: Will Holmgren <william.holmgren@gmail.com>
Co-authored-by: Mark Mikofski <bwana.marko@yahoo.com>

docs/sphinx/source/user_guide/faq.rst

@@ -138,3 +138,37 @@ so you can check the tables there for more up-to-date tables.
 For modules, if even the SAM files don't include the module you're looking for
 either, you can calculate CEC module model parameters from
 datasheet information using :py:func:`pvlib.ivtools.sdm.fit_cec_sam`.
+
+
+Which should I use, the CEC or the Sandia PV Module database?
+-------------------------------------------------------------
+
+The CEC PV module database contains parameters for significantly more
+modules, and is more up to date, than the Sandia PV module database.
+Therefore, the CEC PV module database is probably the more useful option
+in most cases.  However, finding parameters for the specific module
+being used is usually more important than which database they came from.
+
+Besides which modules each database includes, another consideration is the
+different modeling capabilities each parameter set provides.  The CEC model
+produces a continuous IV curve while the Sandia model calculates only a few
+specific points of interest on the curve.  For typical simulations where
+only the maximum power point is of interest, either model will suffice.
+
+
+How do I model a system with multiple inverters?
+------------------------------------------------
+
+Currently, pvlib's :ref:`modelchaindoc` and :ref:`pvsystemdoc` only support
+simulating one inverter at a time.  To calculate total power for multiple inverters,
+there are two options:
+
+If the modules, mounting, stringing, and inverters are all identical for each
+inverter, then you may simply simulate one inverter and multiply the
+``ModelChainResult.ac`` by the number of inverters to get the total system output.
+
+If the inverters or their arrays are not all identical,
+define one ``PVSystem`` and ``ModelChain`` per inverter and
+run the simulation for each of them individually.  From there you
+can add up the inverter-level outputs to get the total system output.
+

docs/sphinx/source/whatsnew/v0.9.5.rst

@@ -18,18 +18,14 @@ Enhancements
 ~~~~~~~~~~~~
 * Added the optional `string_factor` parameter to
   :py:func:`pvlib.snow.loss_townsend` (:issue:`1636`, :pull:`1653`)
+* Added optional ``n_ar`` parameter to :py:func:`pvlib.iam.physical` to
+  support an anti-reflective coating. (:issue:`1501`, :pull:`1616`)
 
 Bug fixes
 ~~~~~~~~~
 * Added a limit to :py:func:`pvlib.snow.loss_townsend` to guard against
   incorrect loss results for systems that are near the ground (:issue:`1636`,
   :pull:`1653`)
-
-* Added optional ``n_ar`` parameter to :py:func:`pvlib.iam.physical` to
-  support an anti-reflective coating. (:issue:`1501`, :pull:`1616`)
-
-Bug fixes
-~~~~~~~~~
 * Fixed incorrect mapping of requested parameters names when using the ``get_psm3``
   function. Also fixed the random reordering of the dataframe columns.
   (:issue:`1629`, :issue:`1647`, :pull:`1648`)
@@ -67,5 +63,4 @@ Contributors
 * Karel De Brabandere (:ghuser:`kdebrab`)
 * Mark Mikofski (:ghuser:`mikofski`)
 * Anton Driesse (:ghuser:`adriesse`)
-* Adam R. Jensen (:ghuser:`AdamRJensen`)
 * Michael Deceglie (:ghuser:`mdeceglie`)