make optional in PVSystem and ModelChain as well

Author: kandersolar

pvlib/modelchain.py

@@ -611,7 +611,7 @@ def infer_dc_model(self):
         """Infer DC power model from Array module parameters."""
         params = _common_keys(
             tuple(array.module_parameters for array in self.system.arrays))
-        if {'A0', 'A1', 'C7'} <= params:
+        if {'A0', 'A1', 'C3'} <= params:
             return self.sapm, 'sapm'
         elif {'a_ref', 'I_L_ref', 'I_o_ref', 'R_sh_ref', 'R_s',
               'Adjust'} <= params:

pvlib/pvsystem.py

@@ -29,7 +29,7 @@
 # a dict of required parameter names for each DC power model
 _DC_MODEL_PARAMS = {
     'sapm': {
-        'C0', 'C1', 'C2', 'C3', 'C4', 'C5', 'C6', 'C7',
+        'C0', 'C1', 'C2', 'C3',  # C4-C7 (i_x and i_xx params) not required
         'Isco', 'Impo', 'Voco', 'Vmpo', 'Aisc', 'Aimp', 'Bvoco',
         'Mbvoc', 'Bvmpo', 'Mbvmp', 'N', 'Cells_in_Series',
         'IXO', 'IXXO'},

tests/test_modelchain.py

@@ -1762,6 +1762,16 @@ def test_invalid_dc_model_params(sapm_dc_snl_ac_system, cec_dc_snl_ac_system,
         ModelChain(pvwatts_dc_pvwatts_ac_system, location, **kwargs)
 
 
+def test_sapm_optional_params(sapm_dc_snl_ac_system, location):
+    # inference works when the optional (i_x, i_xx) SAPM parameters are missing
+    for array in sapm_dc_snl_ac_system.arrays:
+        for key in ['C4', 'C5', 'C6', 'C7']:
+            array.module_parameters.pop(key)
+        
+    # no error:
+    ModelChain(sapm_dc_snl_ac_system, location)
+
+
 @pytest.mark.parametrize('model', [
     'dc_model', 'ac_model', 'aoi_model', 'spectral_model',
     'temperature_model', 'losses_model'