diff --git a/doc/release_notes.rst b/doc/release_notes.rst
index 3849cafd..8326f615 100644
--- a/doc/release_notes.rst
+++ b/doc/release_notes.rst
@@ -172,6 +172,8 @@ Upcoming Release
 
 * Fix error with `symbol` of `buses` in `simplify_network`.
 
+* Fix index of existing capacities in `add_power_capacities_installed_before_baseyear` with `m` option.
+
 PyPSA-Eur 0.10.0 (19th February 2024)
 =====================================
 
diff --git a/scripts/add_existing_baseyear.py b/scripts/add_existing_baseyear.py
index 9c35819f..f445efe3 100644
--- a/scripts/add_existing_baseyear.py
+++ b/scripts/add_existing_baseyear.py
@@ -259,13 +259,21 @@ def add_power_capacities_installed_before_baseyear(n, grouping_years, costs, bas
 
                     # for offshore the splitting only includes coastal regions
                     inv_ind = [
-                        i for i in inv_ind if (i + name_suffix) in n.generators.index
+                        i
+                        for i in inv_ind
+                        if (i + name_suffix)
+                        in n.generators.index.str.replace(
+                            str(baseyear), str(grouping_year)
+                        )
                     ]
 
                     p_max_pu = n.generators_t.p_max_pu[
                         [i + name_suffix for i in inv_ind]
                     ]
-                    p_max_pu.columns = [i + name_suffix for i in inv_ind]
+                    p_max_pu.columns = [
+                        i + name_suffix.replace(str(grouping_year), str(baseyear))
+                        for i in inv_ind
+                    ]
 
                     n.madd(
                         "Generator",