diff --git a/scripts/add_existing_baseyear.py b/scripts/add_existing_baseyear.py
index 75d83e38..01870a26 100644
--- a/scripts/add_existing_baseyear.py
+++ b/scripts/add_existing_baseyear.py
@@ -189,6 +189,15 @@ def add_power_capacities_installed_before_baseyear(n, grouping_years, costs, bas
     phased_out = df_agg[df_agg["DateOut"] < baseyear].index
     df_agg.drop(phased_out, inplace=True)
 
+    older_assets = (df_agg.DateIn<min(grouping_years)).sum()
+    if older_assets:
+        logger.warning(f"There are {older_assets} assets with build year "
+                       f"before first power grouping year {min(grouping_years)}. "
+                       "These assets are dropped and not considered."
+                       "Consider to redefine the grouping years to keep them.")
+        to_drop = df_agg[df_agg.DateIn<min(grouping_years)].index
+        df_agg.drop(to_drop, inplace=True)
+        
     df_agg["grouping_year"] = np.take(
         grouping_years[::-1],
         np.digitize(df_agg.DateIn, grouping_years[::-1]))