diff --git a/eeco/costs.py b/eeco/costs.py
index c575bb8..ef483fa 100644
--- a/eeco/costs.py
+++ b/eeco/costs.py
@@ -1061,7 +1061,7 @@ def get_converted_consumption_data(
 
 
 def get_charge_array_duration(key):
-    """Parse a charge array key to determine the duration of the charge period.
+    """Parse a charge array key to determine the duration of the charge period in days.
 
     Parameters
     ----------
@@ -1093,7 +1093,7 @@ def get_charge_array_duration(key):
         try:
             start_date = dt.datetime.strptime(start_date_str, date_format)
             end_date = dt.datetime.strptime(end_date_str, date_format)
-            return (end_date - start_date).days
+            return (end_date - start_date).days + 1
         except ValueError:
             continue
 
diff --git a/eeco/tests/test_costs.py b/eeco/tests/test_costs.py
index af37d80..bb15259 100644
--- a/eeco/tests/test_costs.py
+++ b/eeco/tests/test_costs.py
@@ -2105,11 +2105,11 @@ def test_calculate_export_revenue(
     "key, expected",
     [
         # YYYYMMDD format
-        ("electric_demand_peak_20240710_20240710_100", 0),
-        ("electric_demand_peak_20240710_20240731_100", 21),
+        ("electric_demand_peak_20240710_20240710_100", 1),
+        ("electric_demand_peak_20240710_20240731_100", 22),
         # YYYY-MM-DD format
-        ("electric_energy_0_2024-07-10_2024-07-10_0", 0),
-        ("electric_energy_0_2024-07-10_2024-07-31_0", 21),
+        ("electric_energy_0_2024-07-10_2024-07-10_0", 1),
+        ("electric_energy_0_2024-07-10_2024-07-31_0", 22),
     ],
 )
 def test_get_charge_array_duration(key, expected):