diff --git a/CHANGELOG.md b/CHANGELOG.md
index 171b46c5..d5ead2de 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,8 @@
 
 ### User-facing changes
 
+|fixed| Contribution of `cost_om_annual_investment_fraction` to total investment costs, to not apply to depreciated costs (#645).
+
 |new| Piecewise constraints added to the YAML math with its own unique syntax (#107).
 These constraints will be added to the optimisation problem using Special Ordered Sets of Type 2 (SOS2) variables.
 
diff --git a/src/calliope/math/base.yaml b/src/calliope/math/base.yaml
index 1d9d6fc4..ea1f35e8 100644
--- a/src/calliope/math/base.yaml
+++ b/src/calliope/math/base.yaml
@@ -884,13 +884,13 @@ global_expressions:
     equations:
       - expression: >
           $annualisation_weight * (
-            $depreciation_rate * (
+            ($depreciation_rate + cost_om_annual_investment_fraction) * (
               sum(default_if_empty(cost_investment_flow_cap, 0), over=carriers) +
               default_if_empty(cost_investment_storage_cap, 0) +
               default_if_empty(cost_investment_source_cap, 0) +
               default_if_empty(cost_investment_area_use, 0) +
               default_if_empty(cost_investment_purchase, 0)
-            ) * (1 + cost_om_annual_investment_fraction)
+            )
             + sum(cost_om_annual * flow_cap, over=carriers)
           )
     sub_expressions: