Add to error message

Author: SabineHaas

windpowerlib/turbine_cluster_modelchain.py

@@ -159,7 +159,8 @@ def wind_farm_power_curve(self, wind_farm, **kwargs):
                         raise ValueError(
                             "`roughness_length` must be defined for using " +
                             "'turbulence_intensity' as " +
-                            "`standard_deviation_method`")
+                            "`standard_deviation_method` if " +
+                            "`turbulence_intensity` is not given")
             if self. density_correction:
                 pass # TODO: restrictions (density needed)
             if self.wake_losses_method is not None: