diff --git a/secmod/optimization_MILP.py b/secmod/optimization_MILP.py
index c9d577ddcc1412e042f4d1a27926b90cb67d019f..8423648d6556d46ff201d14fe5c8489775abbe43 100644
--- a/secmod/optimization_MILP.py
+++ b/secmod/optimization_MILP.py
@@ -1674,8 +1674,7 @@ class Optimization_MILP(abc.ABC):
         """
         return(
             # sum of the current storage level, the deposits and withdrawals
-            (model.storage_level[node, process_storage, year, year_construction, time_slice]*(1-model.rel_storage_loss_factor[process_storage,year_construction])
-             # multiplication of the duration of the time slice with...
+            (model.storage_level[node, process_storage, year, year_construction, time_slice]*(1-model.rel_storage_loss_factor[process_storage,year_construction])**model.time_slice_duration[time_slice]             # multiplication of the duration of the time slice with...
              + (model.time_slice_duration[time_slice]
              # ... the sum of all deposits and withdrawals from the storage
              * sum(model.storage_level_factor[direction_storage]