added some comments

Author: williamhobbs

pvlib/pvsystem.py

@@ -2961,7 +2961,7 @@ def pvwatts_dc(effective_irradiance, temp_cell, pdc0, gamma_pdc, temp_ref=25.,
     pdc = (effective_irradiance * 0.001 * pdc0 *
            (1 + gamma_pdc * (temp_cell - temp_ref)))
 
-    # apply Marion's correction if k is anything but zero
+    # apply Marion's correction if k is provided
     if k is not None:
 
         # preserve input types
@@ -2972,14 +2972,18 @@ def pvwatts_dc(effective_irradiance, temp_cell, pdc0, gamma_pdc, temp_ref=25.,
         err_1 = k * (1 - (1 - effective_irradiance / 200)**4)
         err_2 = k * (1000 - effective_irradiance) / (1000 - 200)
         err = np.where(effective_irradiance <= 200, err_1, err_2)
+
+        # cap adjustment, if needed
         if cap_adjustment:
             err = np.where(effective_irradiance >= 1000, 0, err)
 
+        # make error adjustment
         pdc = pdc - pdc0 * err
 
         # set negative power to zero
         pdc = np.where(pdc < 0, 0, pdc)
 
+        # preserve input types
         if index is not None:
             pdc = pd.Series(pdc, index=index)
         elif is_scalar: