diff --git a/config/config.default.yaml b/config/config.default.yaml
index d438c51f..0d40bafc 100644
--- a/config/config.default.yaml
+++ b/config/config.default.yaml
@@ -361,7 +361,7 @@ solar_thermal:
 # docs in https://pypsa-eur.readthedocs.io/en/latest/configuration.html#existing-capacities
 existing_capacities:
   grouping_years_power: [1960, 1965, 1970, 1975, 1980, 1985, 1990, 1995, 2000, 2005, 2010, 2015, 2020, 2025, 2030]
-  grouping_years_heat: [1980, 1985, 1990, 1995, 2000, 2005, 2010, 2015, 2019] # these should not extend 2020
+  grouping_years_heat: [1980, 1985, 1990, 1995, 2000, 2005, 2010, 2015, 2020] # heat grouping years >= baseyear will be ignored
   threshold_capacity: 10
   default_heating_lifetime: 20
   conventional_carriers:
diff --git a/doc/release_notes.rst b/doc/release_notes.rst
index d42b149f..cdad3fa0 100644
--- a/doc/release_notes.rst
+++ b/doc/release_notes.rst
@@ -9,6 +9,7 @@ Release Notes
 
 Upcoming Release
 ================
+* Group existing capacities to the earlier grouping_year for consistency with optimized capacities
 
 * Include gas and oil fields and saline aquifers in estimation of CO2 sequestration potential.
 
diff --git a/scripts/add_existing_baseyear.py b/scripts/add_existing_baseyear.py
index f445efe3..75d83e38 100644
--- a/scripts/add_existing_baseyear.py
+++ b/scripts/add_existing_baseyear.py
@@ -190,8 +190,8 @@ def add_power_capacities_installed_before_baseyear(n, grouping_years, costs, bas
     df_agg.drop(phased_out, inplace=True)
 
     df_agg["grouping_year"] = np.take(
-        grouping_years, np.digitize(df_agg.DateIn, grouping_years, right=True)
-    )
+        grouping_years[::-1],
+        np.digitize(df_agg.DateIn, grouping_years[::-1]))
 
     # calculate (adjusted) remaining lifetime before phase-out (+1 because assuming
     # phase out date at the end of the year)
@@ -444,12 +444,16 @@ def add_heating_capacities_installed_before_baseyear(
         else:
             efficiency = costs.at[costs_name, "efficiency"]
 
-        for i, grouping_year in enumerate(grouping_years):
-            if int(grouping_year) + default_lifetime <= int(baseyear):
-                continue
-
-            # installation is assumed to be linear for the past default_lifetime years
-            ratio = (int(grouping_year) - int(grouping_years[i - 1])) / default_lifetime
+        valid_grouping_years = pd.Series(
+            [int(gy) for gy in grouping_years if
+            int(baseyear) - default_lifetime <= int(gy) < int(baseyear)])
+        # Installation is assumed to be linear for the past
+        _intervals = pd.concat(
+            [valid_grouping_years[1:], pd.Series(baseyear)],
+            ignore_index=True) - valid_grouping_years
+        ratios = _intervals / _intervals.sum()
+        
+        for ratio, grouping_year in zip(ratios, valid_grouping_years):
 
             n.madd(
                 "Link",