diff --git a/docs/usage.rst b/docs/usage.rst
index 4e39cbee4..cb25937d9 100644
--- a/docs/usage.rst
+++ b/docs/usage.rst
@@ -75,7 +75,7 @@ Both code blocks will create an hourly datetime index for 2011:
 .. code-block:: python
 
     import pandas as pd
-    my_index = pd.date_range('1/1/2011', periods=8761, freq='H')
+    my_index = pd.date_range('1/1/2011', periods=8761, freq='h')
 
 This index can be used to define the EnergySystem:
 
@@ -816,7 +816,7 @@ This small example of PV, grid and SinkDSM shows how to use the component
     data = pd.DataFrame.from_dict(data_dict)
 
     # Do timestamp stuff
-    datetimeindex = pd.date_range(start='1/1/2013', periods=len(data.index), freq='H')
+    datetimeindex = pd.date_range(start='1/1/2013', periods=len(data.index), freq='h')
     data['timestamp'] = datetimeindex
     data.set_index('timestamp', inplace=True)
 
@@ -1030,10 +1030,10 @@ First, you start by defining your energy system as you might have done before, b
     import pandas as pd
     import oemof.solph as solph
 
-    my_index = pd.date_range('1/1/2013', periods=17520, freq='H')
+    my_index = pd.date_range('1/1/2013', periods=17520, freq='h')
     periods = [
-        pd.date_range('1/1/2013', periods=8760, freq='H'),
-        pd.date_range('1/1/2014', periods=8760, freq='H'),
+        pd.date_range('1/1/2013', periods=8760, freq='h'),
+        pd.date_range('1/1/2014', periods=8760, freq='h'),
     ]
     my_energysystem = solph.EnergySystem(timeindex=my_index, periods=periods)
 
@@ -1084,7 +1084,7 @@ So if you want to use this, the above would simplify to:
     def determine_periods(datetimeindex):
         ...
 
-    my_index = pd.date_range('1/1/2013', periods=17520, freq='H')
+    my_index = pd.date_range('1/1/2013', periods=17520, freq='h')
     periods = determine_periods(my_index)  # Make use of method
     my_energysystem = solph.EnergySystem(timeindex=my_index, periods=periods)
 
diff --git a/examples/activity_costs/activity_costs.py b/examples/activity_costs/activity_costs.py
index 950304d21..9f129c015 100644
--- a/examples/activity_costs/activity_costs.py
+++ b/examples/activity_costs/activity_costs.py
@@ -52,7 +52,7 @@ def main():
     # Calculate parameters and initialize the energy system and
     ##########################################################################
     periods = 24
-    time = pd.date_range("1/1/2018", periods=periods, freq="H")
+    time = pd.date_range("1/1/2018", periods=periods, freq="h")
 
     demand_heat = np.full(periods, 5)
     demand_heat[:4] = 0
diff --git a/examples/electrical/transshipment.py b/examples/electrical/transshipment.py
index 8f6349c72..d5e0e53ef 100644
--- a/examples/electrical/transshipment.py
+++ b/examples/electrical/transshipment.py
@@ -129,7 +129,7 @@ def draw_graph(
 
 
 def main():
-    datetimeindex = pd.date_range("1/1/2017", periods=3, freq="H")
+    datetimeindex = pd.date_range("1/1/2017", periods=3, freq="h")
 
     es = EnergySystem(timeindex=datetimeindex, infer_last_interval=False)
 
diff --git a/examples/emission_constraint/emission_constraint.py b/examples/emission_constraint/emission_constraint.py
index 4945eaa28..73537ead4 100644
--- a/examples/emission_constraint/emission_constraint.py
+++ b/examples/emission_constraint/emission_constraint.py
@@ -39,7 +39,7 @@
 def main():
     # create energy system
     energysystem = solph.EnergySystem(
-        timeindex=pd.date_range("1/1/2012", periods=3, freq="H")
+        timeindex=pd.date_range("1/1/2012", periods=3, freq="h")
     )
 
     # create gas bus
diff --git a/examples/flexible_modelling/add_constraints.py b/examples/flexible_modelling/add_constraints.py
index 824da815a..dda9e3639 100644
--- a/examples/flexible_modelling/add_constraints.py
+++ b/examples/flexible_modelling/add_constraints.py
@@ -56,7 +56,7 @@ def run_add_constraints_example(solver="cbc", nologg=False):
     # ##### creating an oemof solph optimization model, nothing special here ##
     # create an energy system object for the oemof solph nodes
     es = EnergySystem(
-        timeindex=pd.date_range("1/1/2017", periods=5, freq="H"),
+        timeindex=pd.date_range("1/1/2017", periods=5, freq="h"),
         infer_last_interval=False,
     )
     # add some nodes
diff --git a/examples/flexible_modelling/saturating_storage.py b/examples/flexible_modelling/saturating_storage.py
index 08efa43ab..46f9d7730 100644
--- a/examples/flexible_modelling/saturating_storage.py
+++ b/examples/flexible_modelling/saturating_storage.py
@@ -40,7 +40,7 @@
 
 def saturating_storage_example():
     # create an energy system
-    idx = pd.date_range("1/1/2023", periods=100, freq="H")
+    idx = pd.date_range("1/1/2023", periods=100, freq="h")
     es = solph.EnergySystem(timeindex=idx, infer_last_interval=False)
 
     # power bus
diff --git a/examples/flow_count_limit/flow_count_limit.py b/examples/flow_count_limit/flow_count_limit.py
index 0ba707e74..210d85052 100644
--- a/examples/flow_count_limit/flow_count_limit.py
+++ b/examples/flow_count_limit/flow_count_limit.py
@@ -50,7 +50,7 @@
 
 def main():
     energy_system = solph.EnergySystem(
-        timeindex=pd.date_range("1/1/2012", periods=4, freq="H")
+        timeindex=pd.date_range("1/1/2012", periods=4, freq="h")
     )
 
     bel = solph.Bus(label="bel")
diff --git a/examples/gradient_example/gradient_example.py b/examples/gradient_example/gradient_example.py
index c1e153668..c1f7f58cb 100644
--- a/examples/gradient_example/gradient_example.py
+++ b/examples/gradient_example/gradient_example.py
@@ -50,7 +50,7 @@ def main():
     # used.
     gradient = 0.01
 
-    date_time_index = pd.date_range("1/1/2012", periods=48, freq="H")
+    date_time_index = pd.date_range("1/1/2012", periods=48, freq="h")
     print(date_time_index)
     energysystem = EnergySystem(
         timeindex=date_time_index, infer_last_interval=True
diff --git a/examples/invest_nonconvex_flow_examples/diesel_genset_nonconvex_investment.py b/examples/invest_nonconvex_flow_examples/diesel_genset_nonconvex_investment.py
index 8ecaa15af..9ea6dfa30 100644
--- a/examples/invest_nonconvex_flow_examples/diesel_genset_nonconvex_investment.py
+++ b/examples/invest_nonconvex_flow_examples/diesel_genset_nonconvex_investment.py
@@ -89,7 +89,7 @@ def main():
 
     # Change the index of data to be able to select data based on the time
     # range.
-    data.index = pd.date_range(start="2022-01-01", periods=len(data), freq="H")
+    data.index = pd.date_range(start="2022-01-01", periods=len(data), freq="h")
 
     # Choose the range of the solar potential and demand
     # based on the selected simulation period.
diff --git a/examples/offset_converter_example/offset_diesel_genset_nonconvex_investment.py b/examples/offset_converter_example/offset_diesel_genset_nonconvex_investment.py
index e56a013e5..061c77c78 100644
--- a/examples/offset_converter_example/offset_diesel_genset_nonconvex_investment.py
+++ b/examples/offset_converter_example/offset_diesel_genset_nonconvex_investment.py
@@ -89,7 +89,7 @@ def offset_converter_example():
     data = pd.read_csv(filepath_or_buffer=filename)
 
     # Change the index of data to be able to select data based on the time range.
-    data.index = pd.date_range(start="2022-01-01", periods=len(data), freq="H")
+    data.index = pd.date_range(start="2022-01-01", periods=len(data), freq="h")
 
     # Choose the range of the solar potential and demand
     # based on the selected simulation period.
@@ -100,7 +100,7 @@ def offset_converter_example():
 
     # Create the energy system.
     date_time_index = pd.date_range(
-        start=start_date, periods=n_days * 24, freq="H"
+        start=start_date, periods=n_days * 24, freq="h"
     )
     energy_system = solph.EnergySystem(timeindex=date_time_index)
 
diff --git a/examples/storage_balanced_unbalanced/storage.py b/examples/storage_balanced_unbalanced/storage.py
index 396e96bb1..29c8069c6 100644
--- a/examples/storage_balanced_unbalanced/storage.py
+++ b/examples/storage_balanced_unbalanced/storage.py
@@ -66,7 +66,7 @@ def storage_example():
     )
 
     # create an energy system
-    idx = pd.date_range("1/1/2017", periods=len(timeseries), freq="H")
+    idx = pd.date_range("1/1/2017", periods=len(timeseries), freq="h")
     es = solph.EnergySystem(timeindex=idx)
 
     for data_set in DATA:
diff --git a/examples/storage_costs/storage_costs.py b/examples/storage_costs/storage_costs.py
index d82068860..312c5ab49 100644
--- a/examples/storage_costs/storage_costs.py
+++ b/examples/storage_costs/storage_costs.py
@@ -42,7 +42,7 @@
 
 def storage_costs_example():
     # create an energy system
-    idx = pd.date_range("1/1/2023", periods=13, freq="H")
+    idx = pd.date_range("1/1/2023", periods=13, freq="h")
     es = solph.EnergySystem(timeindex=idx, infer_last_interval=False)
 
     # power bus
diff --git a/pyproject.toml b/pyproject.toml
index d2bd463d8..059f2ba56 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -58,7 +58,7 @@ dependencies = [
     "blinker",
     "dill",
     "numpy",
-    "pandas >= 1.5.3",
+    "pandas >= 2.0.0",
     "pyomo >= 6.6.0, < 7.0",
     "networkx",
     "oemof.tools >= 0.4.3",
diff --git a/src/oemof/solph/_energy_system.py b/src/oemof/solph/_energy_system.py
index c089b0ef6..79681ad04 100644
--- a/src/oemof/solph/_energy_system.py
+++ b/src/oemof/solph/_energy_system.py
@@ -302,4 +302,13 @@ def create_time_index(
         number = round(hoy / interval)
     if start is None:
         start = f"1/1/{year}"
-    return pd.date_range(start, periods=number + 1, freq=f"{interval}h")
+    try:
+        time_index = pd.date_range(
+            start, periods=number + 1, freq=f"{interval}h"
+        )
+    except ValueError:
+        # Pandas <2.2 compatibility
+        time_index = pd.date_range(
+            start, periods=number + 1, freq=f"{interval}H"
+        )
+    return time_index
diff --git a/src/oemof/solph/components/_sink.py b/src/oemof/solph/components/_sink.py
index a48c6c188..7b5c4f829 100644
--- a/src/oemof/solph/components/_sink.py
+++ b/src/oemof/solph/components/_sink.py
@@ -64,9 +64,7 @@ def __init__(self, label=None, inputs=None, custom_attributes=None):
             )
 
         super().__init__(
-            label=label,
-            inputs=inputs,
-            custom_properties=custom_attributes
+            label=label, inputs=inputs, custom_properties=custom_attributes
         )
 
     def constraint_group(self):
diff --git a/src/oemof/solph/components/_source.py b/src/oemof/solph/components/_source.py
index beb9b4f72..13af1576f 100644
--- a/src/oemof/solph/components/_source.py
+++ b/src/oemof/solph/components/_source.py
@@ -74,9 +74,7 @@ def __init__(self, label=None, outputs=None, custom_attributes=None):
             )
 
         super().__init__(
-            label=label,
-            outputs=outputs,
-            custom_properties=custom_attributes
+            label=label, outputs=outputs, custom_properties=custom_attributes
         )
 
     def constraint_group(self):
diff --git a/tests/constraint_tests.py b/tests/constraint_tests.py
index 3b759d627..80bfdcaa1 100644
--- a/tests/constraint_tests.py
+++ b/tests/constraint_tests.py
@@ -29,7 +29,7 @@ def setup_class(cls):
             r"^objective.*(?=s\.t\.)", re.DOTALL | re.MULTILINE
         )
 
-        cls.date_time_index = pd.date_range("1/1/2012", periods=3, freq="H")
+        cls.date_time_index = pd.date_range("1/1/2012", periods=3, freq="h")
 
         cls.tmppath = solph.helpers.extend_basic_path("tmp")
         logging.info(cls.tmppath)
@@ -1903,7 +1903,7 @@ def test_nonequidistant_storage(self):
         """Constraint test of an energy system
         with non-equidistant time index
         """
-        idxh = pd.date_range("1/1/2017", periods=3, freq="H")
+        idxh = pd.date_range("1/1/2017", periods=3, freq="h")
         idx2h = pd.date_range("1/1/2017 03:00:00", periods=2, freq="2H")
         idx30m = pd.date_range("1/1/2017 07:00:00", periods=4, freq="30min")
         timeindex = idxh.append([idx2h, idx30m])
diff --git a/tests/multi_period_constraint_tests.py b/tests/multi_period_constraint_tests.py
index 991efeb86..ca2053202 100644
--- a/tests/multi_period_constraint_tests.py
+++ b/tests/multi_period_constraint_tests.py
@@ -45,9 +45,9 @@ def setup_class(cls):
             r"^objective.*(?=s\.t\.)", re.DOTALL | re.MULTILINE
         )
 
-        timeindex1 = pd.date_range("1/1/2012", periods=2, freq="H")
-        timeindex2 = pd.date_range("1/1/2013", periods=2, freq="H")
-        timeindex3 = pd.date_range("1/1/2014", periods=2, freq="H")
+        timeindex1 = pd.date_range("1/1/2012", periods=2, freq="h")
+        timeindex2 = pd.date_range("1/1/2013", periods=2, freq="h")
+        timeindex3 = pd.date_range("1/1/2014", periods=2, freq="h")
         cls.date_time_index = timeindex1.append(timeindex2).append(timeindex3)
         cls.periods = [timeindex1, timeindex2, timeindex3]
 
@@ -2316,7 +2316,7 @@ def test_multi_period_varying_period_length(self):
 
         # Create a list of timeindex for each period
         periods = [
-            pd.date_range(f"1/1/{i}", periods=3, freq="H") for i in years
+            pd.date_range(f"1/1/{i}", periods=3, freq="h") for i in years
         ]
 
         # Create an overall timeindex
diff --git a/tests/regression_tests.py b/tests/regression_tests.py
index 8ec62c759..f233d8fbf 100644
--- a/tests/regression_tests.py
+++ b/tests/regression_tests.py
@@ -24,7 +24,7 @@ def test_version_metadata():
 
 
 def test_wrong_logging_level():
-    datetimeindex = pd.date_range("1/1/2012", periods=12, freq="H")
+    datetimeindex = pd.date_range("1/1/2012", periods=12, freq="h")
     es = solph.EnergySystem(timeindex=datetimeindex, infer_last_interval=True)
     tools.logger.define_logging()
     my_logger = logging.getLogger()
diff --git a/tests/test_constraints_module.py b/tests/test_constraints_module.py
index e28889a55..fbf4fc15b 100644
--- a/tests/test_constraints_module.py
+++ b/tests/test_constraints_module.py
@@ -4,7 +4,7 @@
 
 
 def test_special():
-    date_time_index = pd.date_range("1/1/2012", periods=5, freq="H")
+    date_time_index = pd.date_range("1/1/2012", periods=5, freq="h")
     energysystem = solph.EnergySystem(
         timeindex=date_time_index,
         infer_last_interval=True,
@@ -28,7 +28,7 @@ def test_special():
 
 
 def test_something_else():
-    date_time_index = pd.date_range("1/1/2012", periods=5, freq="H")
+    date_time_index = pd.date_range("1/1/2012", periods=5, freq="h")
     energysystem = solph.EnergySystem(
         timeindex=date_time_index, infer_last_interval=True
     )
diff --git a/tests/test_energy_system.py b/tests/test_energy_system.py
index d4a9b0cf4..77833087b 100644
--- a/tests/test_energy_system.py
+++ b/tests/test_energy_system.py
@@ -22,20 +22,20 @@
 )
 def test_add_periods():
     """test method _add_periods of energy system"""
-    timeindex = pd.date_range(start="2012-01-01", periods=10000, freq="H")
+    timeindex = pd.date_range(start="2012-01-01", periods=10000, freq="h")
     periods = [
-        pd.date_range(start="2012-01-01", periods=8784, freq="H"),
-        pd.date_range(start="2013-01-01", periods=1217, freq="H"),
+        pd.date_range(start="2012-01-01", periods=8784, freq="h"),
+        pd.date_range(start="2013-01-01", periods=1217, freq="h"),
     ]
     es = EnergySystem(
         timeindex=timeindex, periods=periods, infer_last_interval=True
     )
     assert len(es.periods) == 2
     assert es.periods[0].equals(
-        pd.date_range(start="2012-01-01", periods=8784, freq="H")
+        pd.date_range(start="2012-01-01", periods=8784, freq="h")
     )
     assert es.periods[1].equals(
-        pd.date_range(start="2013-01-01", periods=1217, freq="H")
+        pd.date_range(start="2013-01-01", periods=1217, freq="h")
     )
 
 
@@ -48,9 +48,9 @@ def test_add_periods():
 )
 def test_extract_periods_years():
     """test method _extract_periods_years of energy system"""
-    t_idx_1 = pd.date_range("1/1/2020", periods=3, freq="H").to_series()
-    t_idx_2 = pd.date_range("1/1/2041", periods=3, freq="H").to_series()
-    t_idx_3 = pd.date_range("1/1/2050", periods=3, freq="H").to_series()
+    t_idx_1 = pd.date_range("1/1/2020", periods=3, freq="h").to_series()
+    t_idx_2 = pd.date_range("1/1/2041", periods=3, freq="h").to_series()
+    t_idx_3 = pd.date_range("1/1/2050", periods=3, freq="h").to_series()
     timeindex = pd.concat([t_idx_1, t_idx_2, t_idx_3]).index
     periods = [t_idx_1, t_idx_2, t_idx_3]
     es = EnergySystem(
diff --git a/tests/test_models.py b/tests/test_models.py
index d450d887a..95086ba5c 100644
--- a/tests/test_models.py
+++ b/tests/test_models.py
@@ -112,7 +112,7 @@ def test_multi_period_default_discount_rate():
 
 def test_cellular_structure_detection():
     """Test flag creation if list is passed as energysystem to model"""
-    timeindex = pd.date_range(start="2020-01-01", periods=1, freq="H")
+    timeindex = pd.date_range(start="2020-01-01", periods=1, freq="h")
     es = solph.EnergySystem(timeindex=timeindex, infer_last_interval=True)
     ec_1 = solph.EnergySystem(timeindex=timeindex, infer_last_interval=True)
     ec_2 = solph.EnergySystem(timeindex=timeindex, infer_last_interval=True)
@@ -125,7 +125,7 @@ def test_sub_cell_node_consideration():
     Test if the nodes of sub-cells are considered for cellular
     energysystems.
     """
-    timeindex = pd.date_range(start="2020-01-01", periods=1, freq="H")
+    timeindex = pd.date_range(start="2020-01-01", periods=1, freq="h")
     es = solph.EnergySystem(timeindex=timeindex, infer_last_interval=True)
     ec_1 = solph.EnergySystem(timeindex=timeindex, infer_last_interval=True)
     bus_es = solph.buses.Bus(label="bus_es")
@@ -141,7 +141,7 @@ def test_sub_cell_flow_consideration():
     Test if the flows of sub-cells are considered for cellular
     energysystems.
     """
-    timeindex = pd.date_range(start="2020-01-01", periods=1, freq="H")
+    timeindex = pd.date_range(start="2020-01-01", periods=1, freq="h")
     es = solph.EnergySystem(timeindex=timeindex, infer_last_interval=True)
     ec_1 = solph.EnergySystem(timeindex=timeindex, infer_last_interval=True)
     bus_es = solph.buses.Bus(label="bus_es")
diff --git a/tests/test_non_equidistant_time_index.py b/tests/test_non_equidistant_time_index.py
index d4f3592f3..1b68df738 100644
--- a/tests/test_non_equidistant_time_index.py
+++ b/tests/test_non_equidistant_time_index.py
@@ -24,7 +24,7 @@
 class TestParameterResult:
     @classmethod
     def setup_class(cls):
-        dtindex1 = pd.date_range("1/1/2012", periods=24, freq="H")
+        dtindex1 = pd.date_range("1/1/2012", periods=24, freq="h")
         dtindex2 = pd.date_range("1/2/2012", periods=49, freq="30min")
         dtindex = dtindex1.union(dtindex2)
         es = EnergySystem(timeindex=dtindex, infer_last_interval=False)
diff --git a/tests/test_outputlib/__init__.py b/tests/test_outputlib/__init__.py
index 752119194..5e86927e3 100644
--- a/tests/test_outputlib/__init__.py
+++ b/tests/test_outputlib/__init__.py
@@ -122,7 +122,7 @@
     conversion_factors={bel: 1 / 3, b_heat_source: (cop - 1) / cop},
 )
 
-datetimeindex = pd.date_range("1/1/2012", periods=24, freq="H")
+datetimeindex = pd.date_range("1/1/2012", periods=24, freq="h")
 energysystem = EnergySystem(timeindex=datetimeindex, infer_last_interval=True)
 energysystem.add(
     bcoal,
diff --git a/tests/test_processing.py b/tests/test_processing.py
index c7d9d5842..b35c0e9e3 100644
--- a/tests/test_processing.py
+++ b/tests/test_processing.py
@@ -34,7 +34,7 @@ def setup_class(cls):
             timeindex=pandas.date_range(
                 "2016-01-01",
                 periods=cls.period,
-                freq="H",
+                freq="h",
             ),
             infer_last_interval=True,
         )
diff --git a/tests/test_scripts/test_solph/test_connect_invest/test_connect_invest.py b/tests/test_scripts/test_solph/test_connect_invest/test_connect_invest.py
index 498261204..a409177fb 100644
--- a/tests/test_scripts/test_solph/test_connect_invest/test_connect_invest.py
+++ b/tests/test_scripts/test_solph/test_connect_invest/test_connect_invest.py
@@ -28,7 +28,7 @@
 
 
 def test_connect_invest():
-    date_time_index = pd.date_range("1/1/2012", periods=24 * 7, freq="H")
+    date_time_index = pd.date_range("1/1/2012", periods=24 * 7, freq="h")
 
     es = EnergySystem(timeindex=date_time_index, infer_last_interval=True)
 
diff --git a/tests/test_scripts/test_solph/test_flexible_modelling/test_add_constraints.py b/tests/test_scripts/test_solph/test_flexible_modelling/test_add_constraints.py
index 7851f9d52..9919eb735 100644
--- a/tests/test_scripts/test_solph/test_flexible_modelling/test_add_constraints.py
+++ b/tests/test_scripts/test_solph/test_flexible_modelling/test_add_constraints.py
@@ -32,7 +32,7 @@ def test_add_constraints_example(solver="cbc", nologg=False):
     # ##### creating an oemof solph optimization model, nothing special here ##
     # create an energy system object for the oemof solph nodes
     es = EnergySystem(
-        timeindex=pd.date_range("1/1/2012", periods=4, freq="H"),
+        timeindex=pd.date_range("1/1/2012", periods=4, freq="h"),
         infer_last_interval=True,
     )
 
diff --git a/tests/test_scripts/test_solph/test_generic_caes/test_generic_caes.py b/tests/test_scripts/test_solph/test_generic_caes/test_generic_caes.py
index 335e30964..dc40286cb 100644
--- a/tests/test_scripts/test_solph/test_generic_caes/test_generic_caes.py
+++ b/tests/test_scripts/test_solph/test_generic_caes/test_generic_caes.py
@@ -37,7 +37,7 @@ def test_gen_caes():
     periods = len(data) - 1
 
     # create an energy system
-    idx = pd.date_range("1/1/2017", periods=periods, freq="H")
+    idx = pd.date_range("1/1/2017", periods=periods, freq="h")
     es = EnergySystem(timeindex=idx, infer_last_interval=True)
 
     # resources
diff --git a/tests/test_scripts/test_solph/test_generic_chp/test_generic_chp.py b/tests/test_scripts/test_solph/test_generic_chp/test_generic_chp.py
index 910c83532..6c9f8c423 100644
--- a/tests/test_scripts/test_solph/test_generic_chp/test_generic_chp.py
+++ b/tests/test_scripts/test_solph/test_generic_chp/test_generic_chp.py
@@ -31,7 +31,7 @@ def test_gen_chp():
     periods = len(data) - 1
 
     # create an energy system
-    idx = pd.date_range("1/1/2017", periods=periods, freq="H")
+    idx = pd.date_range("1/1/2017", periods=periods, freq="h")
     es = solph.EnergySystem(timeindex=idx, infer_last_interval=True)
 
     # resources
diff --git a/tests/test_scripts/test_solph/test_invest_fix_flow/test_simple_invest_fixed.py b/tests/test_scripts/test_solph/test_invest_fix_flow/test_simple_invest_fixed.py
index 8f2eb1d71..1a025b9a6 100644
--- a/tests/test_scripts/test_solph/test_invest_fix_flow/test_simple_invest_fixed.py
+++ b/tests/test_scripts/test_solph/test_invest_fix_flow/test_simple_invest_fixed.py
@@ -60,7 +60,7 @@ def test_dispatch_fix_example(solver="cbc", periods=10):
         inputs={bel: Flow(nominal_value=85, fix=data["demand_el"])},
     )
 
-    datetimeindex = pd.date_range("1/1/2012", periods=periods, freq="H")
+    datetimeindex = pd.date_range("1/1/2012", periods=periods, freq="h")
 
     energysystem = EnergySystem(
         timeindex=datetimeindex, infer_last_interval=True
diff --git a/tests/test_scripts/test_solph/test_lopf/test_lopf.py b/tests/test_scripts/test_solph/test_lopf/test_lopf.py
index 26b2a1fa6..e831734a4 100644
--- a/tests/test_scripts/test_solph/test_lopf/test_lopf.py
+++ b/tests/test_scripts/test_solph/test_lopf/test_lopf.py
@@ -34,7 +34,7 @@ def test_lopf(solver="cbc"):
     logging.info("Initialize the energy system")
 
     # create time index for 192 hours in May.
-    date_time_index = pd.date_range("5/5/2012", periods=1, freq="H")
+    date_time_index = pd.date_range("5/5/2012", periods=1, freq="h")
     es = EnergySystem(timeindex=date_time_index, infer_last_interval=True)
 
     ##########################################################################
diff --git a/tests/test_scripts/test_solph/test_multi_period_model/test_multi_period_dispatch_model.py b/tests/test_scripts/test_solph/test_multi_period_model/test_multi_period_dispatch_model.py
index 9b1ceef27..c61952720 100644
--- a/tests/test_scripts/test_solph/test_multi_period_model/test_multi_period_dispatch_model.py
+++ b/tests/test_scripts/test_solph/test_multi_period_model/test_multi_period_dispatch_model.py
@@ -36,9 +36,9 @@
 def test_multi_period_dispatch_model(solver="cbc"):
     """Test a simple multi_period dispatch model"""
 
-    t_idx_1 = pd.date_range("1/1/2020", periods=3, freq="H")
-    t_idx_2 = pd.date_range("1/1/2030", periods=3, freq="H")
-    t_idx_3 = pd.date_range("1/1/2040", periods=3, freq="H")
+    t_idx_1 = pd.date_range("1/1/2020", periods=3, freq="h")
+    t_idx_2 = pd.date_range("1/1/2030", periods=3, freq="h")
+    t_idx_3 = pd.date_range("1/1/2040", periods=3, freq="h")
 
     # Create an overall timeindex
     t_idx_1_series = pd.Series(index=t_idx_1, dtype="float64")
diff --git a/tests/test_scripts/test_solph/test_multi_period_model/test_multi_period_investment_model.py b/tests/test_scripts/test_solph/test_multi_period_model/test_multi_period_investment_model.py
index e8d36f7ce..43c45834b 100644
--- a/tests/test_scripts/test_solph/test_multi_period_model/test_multi_period_investment_model.py
+++ b/tests/test_scripts/test_solph/test_multi_period_model/test_multi_period_investment_model.py
@@ -39,9 +39,9 @@ def test_multi_period_investment_model(solver="cbc"):
     for multiple SinkDSM approaches"""
 
     for approach in ["oemof", "DLR", "DIW"]:
-        t_idx_1 = pd.date_range("1/1/2020", periods=3, freq="H")
-        t_idx_2 = pd.date_range("1/1/2030", periods=3, freq="H")
-        t_idx_3 = pd.date_range("1/1/2040", periods=3, freq="H")
+        t_idx_1 = pd.date_range("1/1/2020", periods=3, freq="h")
+        t_idx_2 = pd.date_range("1/1/2030", periods=3, freq="h")
+        t_idx_3 = pd.date_range("1/1/2040", periods=3, freq="h")
 
         # Create an overall timeindex
         t_idx_1_series = pd.Series(index=t_idx_1, dtype="float64")
diff --git a/tests/test_scripts/test_solph/test_piecewiselineartransformer/test_piecewiselineartransformer.py b/tests/test_scripts/test_solph/test_piecewiselineartransformer/test_piecewiselineartransformer.py
index 41b7a60b1..64823a6b9 100644
--- a/tests/test_scripts/test_solph/test_piecewiselineartransformer/test_piecewiselineartransformer.py
+++ b/tests/test_scripts/test_solph/test_piecewiselineartransformer/test_piecewiselineartransformer.py
@@ -26,7 +26,7 @@
 def test_pwltf():
     # Set timeindex and create data
     periods = 20
-    datetimeindex = pd.date_range("1/1/2019", periods=periods, freq="H")
+    datetimeindex = pd.date_range("1/1/2019", periods=periods, freq="h")
     step = 5
     demand = np.arange(0, step * periods, step)
 
diff --git a/tests/test_scripts/test_solph/test_simple_model/test_simple_dispatch.py b/tests/test_scripts/test_solph/test_simple_model/test_simple_dispatch.py
index d7fc978ec..e8d12b264 100644
--- a/tests/test_scripts/test_solph/test_simple_model/test_simple_dispatch.py
+++ b/tests/test_scripts/test_solph/test_simple_model/test_simple_dispatch.py
@@ -125,7 +125,7 @@ def test_dispatch_example(solver="cbc", periods=24 * 5):
         conversion_factors={bel: 1 / 3, b_heat_source: (cop - 1) / cop},
     )
 
-    datetimeindex = pd.date_range("1/1/2012", periods=periods, freq="H")
+    datetimeindex = pd.date_range("1/1/2012", periods=periods, freq="h")
     energysystem = EnergySystem(
         timeindex=datetimeindex, infer_last_interval=True
     )
diff --git a/tests/test_scripts/test_solph/test_simple_model/test_simple_invest.py b/tests/test_scripts/test_solph/test_simple_model/test_simple_invest.py
index dcefb11e8..3634406f3 100644
--- a/tests/test_scripts/test_solph/test_simple_model/test_simple_invest.py
+++ b/tests/test_scripts/test_solph/test_simple_model/test_simple_invest.py
@@ -141,7 +141,7 @@ def test_dispatch_example(solver="cbc", periods=24 * 5):
         conversion_factors={bel: 1 / 3, b_heat_source: (cop - 1) / cop},
     )
 
-    datetimeindex = pd.date_range("1/1/2012", periods=periods, freq="H")
+    datetimeindex = pd.date_range("1/1/2012", periods=periods, freq="h")
     energysystem = EnergySystem(
         timeindex=datetimeindex, infer_last_interval=True
     )
diff --git a/tests/test_scripts/test_solph/test_storage_investment/test_invest_storage_regression.py b/tests/test_scripts/test_solph/test_storage_investment/test_invest_storage_regression.py
index 1faa13677..b33f915e5 100644
--- a/tests/test_scripts/test_solph/test_storage_investment/test_invest_storage_regression.py
+++ b/tests/test_scripts/test_solph/test_storage_investment/test_invest_storage_regression.py
@@ -23,7 +23,7 @@ def test_regression_investment_storage(solver="cbc"):
     """
 
     logging.info("Initialize the energy system")
-    date_time_index = pd.date_range("1/1/2012", periods=4, freq="H")
+    date_time_index = pd.date_range("1/1/2012", periods=4, freq="h")
 
     energysystem = solph.EnergySystem(
         timeindex=date_time_index, infer_last_interval=True
diff --git a/tests/test_scripts/test_solph/test_storage_investment/test_storage_investment.py b/tests/test_scripts/test_solph/test_storage_investment/test_storage_investment.py
index 3d89bde91..e67bec253 100644
--- a/tests/test_scripts/test_solph/test_storage_investment/test_storage_investment.py
+++ b/tests/test_scripts/test_solph/test_storage_investment/test_storage_investment.py
@@ -54,7 +54,7 @@ def test_optimise_storage_size(
     global PP_GAS
 
     logging.info("Initialize the energy system")
-    date_time_index = pd.date_range("1/1/2012", periods=400, freq="H")
+    date_time_index = pd.date_range("1/1/2012", periods=400, freq="h")
 
     es = solph.EnergySystem(
         timeindex=date_time_index,
diff --git a/tests/test_scripts/test_solph/test_storage_investment/test_storage_with_tuple_label.py b/tests/test_scripts/test_solph/test_storage_investment/test_storage_with_tuple_label.py
index 998373fac..2b9654fd1 100644
--- a/tests/test_scripts/test_solph/test_storage_investment/test_storage_with_tuple_label.py
+++ b/tests/test_scripts/test_solph/test_storage_investment/test_storage_with_tuple_label.py
@@ -63,7 +63,7 @@ def test_tuples_as_labels_example(
     filename="storage_investment.csv", solver="cbc"
 ):
     logging.info("Initialize the energy system")
-    date_time_index = pd.date_range("1/1/2012", periods=40, freq="H")
+    date_time_index = pd.date_range("1/1/2012", periods=40, freq="h")
 
     energysystem = solph.EnergySystem(
         timeindex=date_time_index,
diff --git a/tests/test_scripts/test_solph/test_variable_chp/test_variable_chp.py b/tests/test_scripts/test_solph/test_variable_chp/test_variable_chp.py
index 9753e3684..eec3f7267 100644
--- a/tests/test_scripts/test_solph/test_variable_chp/test_variable_chp.py
+++ b/tests/test_scripts/test_solph/test_variable_chp/test_variable_chp.py
@@ -26,7 +26,7 @@ def test_variable_chp(filename="variable_chp.csv", solver="cbc"):
     logging.info("Initialize the energy system")
 
     # create time index for 192 hours in May.
-    date_time_index = pd.date_range("5/5/2012", periods=5, freq="H")
+    date_time_index = pd.date_range("5/5/2012", periods=5, freq="h")
     energysystem = solph.EnergySystem(
         timeindex=date_time_index, infer_last_interval=True
     )
diff --git a/tests/test_time_index.py b/tests/test_time_index.py
index f5a7aaf03..08d2b56cb 100644
--- a/tests/test_time_index.py
+++ b/tests/test_time_index.py
@@ -16,28 +16,28 @@
 
 def test_energysystem_with_datetimeindex_infer_last_interval():
     """Test EnergySystem with DatetimeIndex (equidistant)"""
-    datetimeindex = pd.date_range("1/1/2012", periods=24, freq="H")
+    datetimeindex = pd.date_range("1/1/2012", periods=24, freq="h")
     es = solph.EnergySystem(timeindex=datetimeindex, infer_last_interval=True)
     assert es.timeincrement[1] == 1.0
     assert es.timeincrement.sum() == 24
 
 
 def test_energysystem_with_datetimeindex():
-    datetimeindex = pd.date_range("1/1/2012", periods=24, freq="H")
+    datetimeindex = pd.date_range("1/1/2012", periods=24, freq="h")
     es = solph.EnergySystem(timeindex=datetimeindex, infer_last_interval=False)
     assert es.timeincrement[1] == 1.0
     assert es.timeincrement.sum() == 23
 
 
 def test_energysystem_interval_inference_warning():
-    datetimeindex = pd.date_range("1/1/2012", periods=24, freq="H")
+    datetimeindex = pd.date_range("1/1/2012", periods=24, freq="h")
     with pytest.warns(FutureWarning):
         _ = solph.EnergySystem(timeindex=datetimeindex)
 
 
 def test_energysystem_with_datetimeindex_non_equidistant_infer_last_interval():
     """Test EnergySystem with DatetimeIndex (non-equidistant)"""
-    dtindex1 = pd.date_range("1/1/2012", periods=24, freq="H")
+    dtindex1 = pd.date_range("1/1/2012", periods=24, freq="h")
     dtindex2 = pd.date_range("1/2/2012", periods=49, freq="30min")
     dtindex = dtindex1.union(dtindex2)
     msg = (
@@ -50,7 +50,7 @@ def test_energysystem_with_datetimeindex_non_equidistant_infer_last_interval():
 
 def test_energysystem_with_datetimeindex_non_equidistant():
     """Test EnergySystem with DatetimeIndex (non-equidistant)"""
-    dtindex1 = pd.date_range("1/1/2012", periods=24, freq="H")
+    dtindex1 = pd.date_range("1/1/2012", periods=24, freq="h")
     dtindex2 = pd.date_range("1/2/2012", periods=49, freq="30min")
     dtindex = dtindex1.union(dtindex2)
     es = solph.EnergySystem(timeindex=dtindex, infer_last_interval=False)
@@ -107,7 +107,7 @@ def test_energysystem_with_numeric_index_non_equidistant():
 
 
 def test_model_timeincrement_with_valid_timeindex():
-    datetimeindex = pd.date_range("1/1/2012", periods=5, freq="H")
+    datetimeindex = pd.date_range("1/1/2012", periods=5, freq="h")
     es = solph.EnergySystem(timeindex=datetimeindex, infer_last_interval=True)
     m = solph._models.BaseModel(es)
     assert es.timeincrement.sum() == 5
@@ -129,7 +129,7 @@ def test_conflicting_time_index():
     )
     with pytest.raises(AttributeError, match=msg):
         solph.EnergySystem(
-            timeindex=pd.date_range("1/1/2012", periods=2, freq="H"),
+            timeindex=pd.date_range("1/1/2012", periods=2, freq="h"),
             timeincrement=[1, 2, 3, 4],
             infer_last_interval=False,
         )
@@ -147,7 +147,7 @@ def test_missing_timeincrement():
 
 def test_overwrite_timeincrement():
     es = solph.EnergySystem(
-        timeindex=pd.date_range("1/1/2012", periods=2, freq="H"),
+        timeindex=pd.date_range("1/1/2012", periods=2, freq="h"),
         infer_last_interval=True,
     )
     assert es.timeincrement[0] == 1
@@ -163,8 +163,8 @@ def test_model_timeincrement_list():
 
 def test_nonequ_inconsistent_timeindex():
     # with pytest.raises(IndexError):
-    timeindex_one = pd.date_range("1/1/2019", periods=1, freq="H")
-    timeindex_hourly = pd.date_range("1/1/2019", periods=3, freq="H")
+    timeindex_one = pd.date_range("1/1/2019", periods=1, freq="h")
+    timeindex_hourly = pd.date_range("1/1/2019", periods=3, freq="h")
     timeindex_45mins = pd.date_range("1/1/2019", periods=2, freq="45min")
     timeindex1 = timeindex_one.append(timeindex_hourly)
     timeindex2 = timeindex_hourly.append([timeindex_45mins])
diff --git a/tests/test_warnings.py b/tests/test_warnings.py
index 446739277..7619aaf79 100644
--- a/tests/test_warnings.py
+++ b/tests/test_warnings.py
@@ -173,7 +173,7 @@ def test_link_raise_key_error_in_Linkblock(warning_fixture):
     """Link raises KeyError if conversion factors don't match the connected
     busses."""
 
-    date_time_index = pd.date_range("1/1/2012", periods=3, freq="H")
+    date_time_index = pd.date_range("1/1/2012", periods=3, freq="h")
     energysystem = solph.EnergySystem(
         timeindex=date_time_index,
         infer_last_interval=True,
diff --git a/tox.ini b/tox.ini
index eb75a35a0..bbbaac9c0 100644
--- a/tox.ini
+++ b/tox.ini
@@ -113,6 +113,7 @@ deps =
     {[testenv]deps}
     pytest-cov
 
+
 [testenv:py39]
 basepython = {env:TOXPYTHON:python3.9}
 setenv =
@@ -124,5 +125,16 @@ deps =
     {[testenv]deps}
     pytest-cov
 
+[testenv:py38]
+basepython = {env:TOXPYTHON:python3.8}
+setenv =
+    {[testenv]setenv}
+usedevelop = true
+commands =
+    {posargs:pytest --cov --cov-report=term-missing -vv}
+deps =
+    {[testenv]deps}
+    pytest-cov
+
 [testenv:py3-nocov]
 basepython = {env:TOXPYTHON:python3}