diff --git a/docs/user-guide/mathematical-notation/features/InvestParameters.md b/docs/user-guide/mathematical-notation/features/InvestParameters.md
index 21d071dc8..14fe02c79 100644
--- a/docs/user-guide/mathematical-notation/features/InvestParameters.md
+++ b/docs/user-guide/mathematical-notation/features/InvestParameters.md
@@ -49,16 +49,16 @@ This uses the **bounds with state** pattern described in [Bounds and States](../
 
 ### Optional vs. Mandatory Investment
 
-The `optional` parameter controls whether investment is required:
+The `mandatory` parameter controls whether investment is required:
 
-**Optional Investment** (`optional=True`):
+**Optional Investment** (`mandatory=False`, default):
 $$\label{eq:invest_optional}
 s_\text{invest} \in \{0, 1\}
 $$
 
 The optimization can freely choose to invest or not.
 
-**Mandatory Investment** (`optional=False`):
+**Mandatory Investment** (`mandatory=True`):
 $$\label{eq:invest_mandatory}
 s_\text{invest} = 1
 $$
@@ -228,7 +228,7 @@ $$
 **Key Parameters:**
 - `fixed_size`: For binary investments (mutually exclusive with continuous sizing)
 - `minimum_size`, `maximum_size`: For continuous sizing
-- `optional`: Whether investment can be skipped
+- `mandatory`: Whether investment is required (default: `False`)
 - `effects_of_investment`: Fixed effects incurred when investing (replaces deprecated `fix_effects`)
 - `effects_of_investment_per_size`: Per-unit effects proportional to size (replaces deprecated `specific_effects`)
 - `piecewise_effects_of_investment`: Non-linear effect modeling (replaces deprecated `piecewise_effects`)
@@ -250,7 +250,7 @@ See the [`InvestParameters`][flixopt.interface.InvestParameters] API documentati
 ```python
 solar_investment = InvestParameters(
     fixed_size=100,  # 100 kW system
-    optional=True,
+    mandatory=False,  # Optional investment (default)
     effects_of_investment={'cost': 25000},  # Installation costs
     effects_of_investment_per_size={'cost': 1200},  # €1200/kW
 )
@@ -261,7 +261,7 @@ solar_investment = InvestParameters(
 battery_investment = InvestParameters(
     minimum_size=10,  # kWh
     maximum_size=1000,
-    optional=True,
+    mandatory=False,  # Optional investment (default)
     effects_of_investment={'cost': 5000},  # Grid connection
     effects_of_investment_per_size={'cost': 600},  # €600/kWh
 )
@@ -272,7 +272,7 @@ battery_investment = InvestParameters(
 boiler_replacement = InvestParameters(
     minimum_size=50,  # kW
     maximum_size=200,
-    optional=True,
+    mandatory=False,  # Optional investment (default)
     effects_of_investment={'cost': 15000},
     effects_of_investment_per_size={'cost': 400},
     effects_of_retirement={'cost': 8000},  # Demolition if not replaced
diff --git a/examples/01_Simple/simple_example.py b/examples/01_Simple/simple_example.py
index 924d165d5..a6db595ef 100644
--- a/examples/01_Simple/simple_example.py
+++ b/examples/01_Simple/simple_example.py
@@ -64,7 +64,7 @@
         label='Storage',
         charging=fx.Flow('Q_th_load', bus='Fernwärme', size=1000),
         discharging=fx.Flow('Q_th_unload', bus='Fernwärme', size=1000),
-        capacity_in_flow_hours=fx.InvestParameters(effects_of_investment=20, fixed_size=30, optional=False),
+        capacity_in_flow_hours=fx.InvestParameters(effects_of_investment=20, fixed_size=30, mandatory=True),
         initial_charge_state=0,  # Initial storage state: empty
         relative_maximum_charge_state=1 / 100 * np.array([80, 70, 80, 80, 80, 80, 80, 80, 80]),
         relative_maximum_final_charge_state=0.8,
diff --git a/examples/02_Complex/complex_example.py b/examples/02_Complex/complex_example.py
index 5c46f650e..81d5035f6 100644
--- a/examples/02_Complex/complex_example.py
+++ b/examples/02_Complex/complex_example.py
@@ -59,7 +59,7 @@
             size=fx.InvestParameters(
                 effects_of_investment=1000,  # Fixed investment costs
                 fixed_size=50,  # Fixed size
-                optional=False,  # Forced investment
+                mandatory=True,  # Forced investment
                 effects_of_investment_per_size={Costs.label: 10, PE.label: 2},  # Specific costs
             ),
             load_factor_max=1.0,  # Maximum load factor (50 kW)
@@ -131,7 +131,7 @@
         discharging=fx.Flow('Q_th_unload', bus='Fernwärme', size=1e4),
         capacity_in_flow_hours=fx.InvestParameters(
             piecewise_effects_of_investment=segmented_investment_effects,  # Investment effects
-            optional=False,  # Forced investment
+            mandatory=True,  # Forced investment
             minimum_size=0,
             maximum_size=1000,  # Optimizing between 0 and 1000 kWh
         ),
diff --git a/examples/04_Scenarios/scenario_example.py b/examples/04_Scenarios/scenario_example.py
index c48ee56a8..eeb97005c 100644
--- a/examples/04_Scenarios/scenario_example.py
+++ b/examples/04_Scenarios/scenario_example.py
@@ -77,7 +77,7 @@
         label='Storage',
         charging=fx.Flow('Q_th_load', bus='Fernwärme', size=1000),
         discharging=fx.Flow('Q_th_unload', bus='Fernwärme', size=1000),
-        capacity_in_flow_hours=fx.InvestParameters(effects_of_investment=20, fixed_size=30, optional=False),
+        capacity_in_flow_hours=fx.InvestParameters(effects_of_investment=20, fixed_size=30, mandatory=True),
         initial_charge_state=0,  # Initial storage state: empty
         relative_maximum_charge_state=np.array([80, 70, 80, 80, 80, 80, 80, 80, 80]) * 0.01,
         relative_maximum_final_charge_state=0.8,
diff --git a/flixopt/elements.py b/flixopt/elements.py
index f911b6646..4de6fa041 100644
--- a/flixopt/elements.py
+++ b/flixopt/elements.py
@@ -489,11 +489,6 @@ def size_is_fixed(self) -> bool:
         # Wenn kein InvestParameters existiert --> True; Wenn Investparameter, den Wert davon nehmen
         return False if (isinstance(self.size, InvestParameters) and self.size.fixed_size is None) else True
 
-    @property
-    def invest_is_optional(self) -> bool:
-        # Wenn kein InvestParameters existiert: # Investment ist nicht optional -> Keine Variable --> False
-        return False if (isinstance(self.size, InvestParameters) and not self.size.optional) else True
-
 
 class FlowModel(ElementModel):
     element: Flow  # Type hint
@@ -678,8 +673,8 @@ def absolute_flow_rate_bounds(self) -> tuple[TemporalData, TemporalData]:
             if not self.with_investment:
                 # Basic case without investment and without OnOff
                 lb = lb_relative * self.element.size
-            elif isinstance(self.element.size, InvestParameters) and not self.element.size.optional:
-                # With non-optional Investment
+            elif self.with_investment and self.element.size.mandatory:
+                # With mandatory Investment
                 lb = lb_relative * self.element.size.minimum_or_fixed_size
 
         if self.with_investment:
diff --git a/flixopt/features.py b/flixopt/features.py
index f266a1946..7be0c5f30 100644
--- a/flixopt/features.py
+++ b/flixopt/features.py
@@ -56,12 +56,13 @@ def _create_variables_and_constraints(self):
         size_min, size_max = (self.parameters.minimum_or_fixed_size, self.parameters.maximum_or_fixed_size)
         self.add_variables(
             short_name='size',
-            lower=0 if self.parameters.optional else size_min,
+            lower=0 if not self.parameters.mandatory else size_min,
             upper=size_max,
             coords=self._model.get_coords(['period', 'scenario']),
         )
 
-        if self.parameters.optional:
+        # Optional (not mandatory)
+        if not self.parameters.mandatory:
             self.add_variables(
                 binary=True,
                 coords=self._model.get_coords(['period', 'scenario']),
@@ -100,7 +101,7 @@ def _add_effects(self):
                 target='periodic',
             )
 
-        if self.parameters.effects_of_retirement and self.parameters.optional:
+        if self.parameters.effects_of_retirement and not self.parameters.mandatory:
             self._model.effects.add_share_to_effects(
                 name=self.label_of_element,
                 expressions={
diff --git a/flixopt/interface.py b/flixopt/interface.py
index b985729e6..fb2f6e915 100644
--- a/flixopt/interface.py
+++ b/flixopt/interface.py
@@ -696,8 +696,9 @@ class InvestParameters(Interface):
             Ignored if fixed_size is specified.
         maximum_size: Upper bound for continuous sizing. Default: CONFIG.modeling.BIG.
             Ignored if fixed_size is specified.
-        optional: If True, can choose not to invest. If False, investment is mandatory.
-            Default: True.
+        mandatory: Controls whether investment is required. When True, forces investment
+            to occur (useful for mandatory upgrades or replacement decisions).
+            When False (default), optimization can choose not to invest.
         effects_of_investment: Fixed costs if investment is made, regardless of size.
             Dict: {'effect_name': value} (e.g., {'cost': 10000}).
         effects_of_investment_per_size: Variable costs proportional to size (per-unit costs).
@@ -716,6 +717,8 @@ class InvestParameters(Interface):
             Will be removed in version 4.0.
         piecewise_effects: **Deprecated**. Use `piecewise_effects_of_investment` instead.
             Will be removed in version 4.0.
+        optional: DEPRECATED. Use `mandatory` instead. Opposite of `mandatory`.
+            Will be removed in version 4.0.
 
     Cost Annualization Requirements:
         All cost values must be properly weighted to match the optimization model's time horizon.
@@ -733,7 +736,7 @@ class InvestParameters(Interface):
         ```python
         solar_investment = InvestParameters(
             fixed_size=100,  # 100 kW system (binary decision)
-            optional=True,
+            mandatory=False,  # Investment is optional
             effects_of_investment={
                 'cost': 25000,  # Installation and permitting costs
                 'CO2': -50000,  # Avoided emissions over lifetime
@@ -751,7 +754,7 @@ class InvestParameters(Interface):
         battery_investment = InvestParameters(
             minimum_size=10,  # Minimum viable system size (kWh)
             maximum_size=1000,  # Maximum installable capacity
-            optional=True,
+            mandatory=False,  # Investment is optional
             effects_of_investment={
                 'cost': 5000,  # Grid connection and control system
                 'installation_time': 2,  # Days for fixed components
@@ -783,7 +786,7 @@ class InvestParameters(Interface):
         boiler_replacement = InvestParameters(
             minimum_size=50,
             maximum_size=200,
-            optional=True,  # Can choose not to replace
+            mandatory=False,  # Can choose not to replace
             effects_of_investment={
                 'cost': 15000,  # Installation costs
                 'disruption': 3,  # Days of downtime
@@ -867,7 +870,7 @@ def __init__(
         fixed_size: PeriodicDataUser | None = None,
         minimum_size: PeriodicDataUser | None = None,
         maximum_size: PeriodicDataUser | None = None,
-        optional: bool = True,  # Investition ist weglassbar
+        mandatory: bool = False,
         effects_of_investment: PeriodicEffectsUser | None = None,
         effects_of_investment_per_size: PeriodicEffectsUser | None = None,
         effects_of_retirement: PeriodicEffectsUser | None = None,
@@ -887,6 +890,16 @@ def __init__(
         piecewise_effects_of_investment = self._handle_deprecated_kwarg(
             kwargs, 'piecewise_effects', 'piecewise_effects_of_investment', piecewise_effects_of_investment
         )
+        # For mandatory parameter with non-None default, disable conflict checking
+        if 'optional' in kwargs:
+            warnings.warn(
+                'Deprecated parameter "optional" used. Check conflicts with new parameter "mandatory" manually!',
+                DeprecationWarning,
+                stacklevel=2,
+            )
+        mandatory = self._handle_deprecated_kwarg(
+            kwargs, 'optional', 'mandatory', mandatory, transform=lambda x: not x, check_conflict=False
+        )
 
         # Validate any remaining unexpected kwargs
         self._validate_kwargs(kwargs)
@@ -898,7 +911,7 @@ def __init__(
             effects_of_retirement if effects_of_retirement is not None else {}
         )
         self.fixed_size = fixed_size
-        self.optional = optional
+        self.mandatory = mandatory
         self.effects_of_investment_per_size: PeriodicEffectsUser = (
             effects_of_investment_per_size if effects_of_investment_per_size is not None else {}
         )
@@ -941,6 +954,20 @@ def transform_data(self, flow_system: FlowSystem, name_prefix: str = '') -> None
                 f'{name_prefix}|fixed_size', self.fixed_size, dims=['period', 'scenario']
             )
 
+    @property
+    def optional(self) -> bool:
+        """DEPRECATED: Use 'mandatory' property instead. Returns the opposite of 'mandatory'."""
+        import warnings
+
+        warnings.warn("Property 'optional' is deprecated. Use 'mandatory' instead.", DeprecationWarning, stacklevel=2)
+        return not self.mandatory
+
+    @optional.setter
+    def optional(self, value: bool):
+        """DEPRECATED: Use 'mandatory' property instead. Sets the opposite of the given value to 'mandatory'."""
+        warnings.warn("Property 'optional' is deprecated. Use 'mandatory' instead.", DeprecationWarning, stacklevel=2)
+        self.mandatory = not value
+
     @property
     def fix_effects(self) -> PeriodicEffectsUser:
         """Deprecated property. Use effects_of_investment instead."""
diff --git a/flixopt/structure.py b/flixopt/structure.py
index 0a4d158ed..3ddfb8bbe 100644
--- a/flixopt/structure.py
+++ b/flixopt/structure.py
@@ -387,13 +387,26 @@ def _handle_deprecated_kwarg(
             new_name: Name of the replacement parameter
             current_value: Current value of the new parameter (if already set)
             transform: Optional callable to transform the old value before returning (e.g., lambda x: [x] to wrap in list)
-            check_conflict: Whether to check if both old and new parameters are specified (default: True)
+            check_conflict: Whether to check if both old and new parameters are specified (default: True).
+                Note: For parameters with non-None default values (e.g., bool parameters with default=False),
+                set check_conflict=False since we cannot distinguish between an explicit value and the default.
 
         Returns:
             The value to use (either from old parameter or current_value)
 
         Raises:
             ValueError: If both old and new parameters are specified and check_conflict is True
+
+        Example:
+            # For parameters where None is the default (conflict checking works):
+            value = self._handle_deprecated_kwarg(kwargs, 'old_param', 'new_param', current_value)
+
+            # For parameters with non-None defaults (disable conflict checking):
+            mandatory = self._handle_deprecated_kwarg(
+                kwargs, 'optional', 'mandatory', mandatory,
+                transform=lambda x: not x,
+                check_conflict=False  # Cannot detect if mandatory was explicitly passed
+            )
         """
         import warnings
 
@@ -404,6 +417,7 @@ def _handle_deprecated_kwarg(
                 DeprecationWarning,
                 stacklevel=3,  # Stack: this method -> __init__ -> caller
             )
+            # Check for conflicts: only raise error if both were explicitly provided
             if check_conflict and current_value is not None:
                 raise ValueError(f'Either {old_name} or {new_name} can be specified, but not both.')
 
@@ -411,6 +425,7 @@ def _handle_deprecated_kwarg(
             if transform is not None:
                 return transform(old_value)
             return old_value
+
         return current_value
 
     def _validate_kwargs(self, kwargs: dict, class_name: str = None) -> None:
diff --git a/tests/conftest.py b/tests/conftest.py
index 9dd8e6d0d..f352dc7cc 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -161,7 +161,7 @@ def complex():
                     size=fx.InvestParameters(
                         effects_of_investment=1000,
                         fixed_size=50,
-                        optional=False,
+                        mandatory=True,
                         effects_of_investment_per_size={'costs': 10, 'PE': 2},
                     ),
                     on_off_parameters=fx.OnOffParameters(
@@ -267,7 +267,7 @@ def simple(timesteps_length=9):
             'Speicher',
             charging=fx.Flow('Q_th_load', bus='Fernwärme', size=1e4),
             discharging=fx.Flow('Q_th_unload', bus='Fernwärme', size=1e4),
-            capacity_in_flow_hours=fx.InvestParameters(effects_of_investment=20, fixed_size=30, optional=False),
+            capacity_in_flow_hours=fx.InvestParameters(effects_of_investment=20, fixed_size=30, mandatory=True),
             initial_charge_state=0,
             relative_maximum_charge_state=1 / 100 * np.array(charge_state_values),
             relative_maximum_final_charge_state=0.8,
@@ -289,7 +289,7 @@ def complex():
                     'PE': fx.Piecewise([fx.Piece(5, 25), fx.Piece(25, 100)]),
                 },
             ),
-            optional=False,
+            mandatory=True,
             effects_of_investment_per_size={'costs': 0.01, 'CO2': 0.01},
             minimum_size=0,
             maximum_size=1000,
diff --git a/tests/test_component.py b/tests/test_component.py
index 98e2ad7db..be1eecf3b 100644
--- a/tests/test_component.py
+++ b/tests/test_component.py
@@ -562,7 +562,7 @@ def test_transmission_unbalanced(self, basic_flow_system, highs_solver):
             in2=fx.Flow(
                 'Rohr2a',
                 'Fernwärme',
-                size=fx.InvestParameters(effects_of_investment_per_size=100, minimum_size=10, optional=False),
+                size=fx.InvestParameters(effects_of_investment_per_size=100, minimum_size=10, mandatory=True),
             ),
             out2=fx.Flow('Rohr2b', bus='Wärme lokal', size=1000),
             balanced=False,
diff --git a/tests/test_effect.py b/tests/test_effect.py
index b855ec8d0..cd3edc537 100644
--- a/tests/test_effect.py
+++ b/tests/test_effect.py
@@ -251,7 +251,7 @@ def test_shares(self, basic_flow_system_linopy_coords, coords_config):
                 Q_th=fx.Flow(
                     'Q_th',
                     bus='Fernwärme',
-                    size=fx.InvestParameters(effects_of_investment_per_size=10, minimum_size=20, optional=False),
+                    size=fx.InvestParameters(effects_of_investment_per_size=10, minimum_size=20, mandatory=True),
                 ),
                 Q_fu=fx.Flow('Q_fu', bus='Gas'),
             ),
diff --git a/tests/test_flow.py b/tests/test_flow.py
index 5f0343bc1..5c8420137 100644
--- a/tests/test_flow.py
+++ b/tests/test_flow.py
@@ -149,7 +149,7 @@ def test_flow_invest(self, basic_flow_system_linopy_coords, coords_config):
         flow = fx.Flow(
             'Wärme',
             bus='Fernwärme',
-            size=fx.InvestParameters(minimum_size=20, maximum_size=100, optional=False),
+            size=fx.InvestParameters(minimum_size=20, maximum_size=100, mandatory=True),
             relative_minimum=np.linspace(0.1, 0.5, timesteps.size),
             relative_maximum=np.linspace(0.5, 1, timesteps.size),
         )
@@ -212,7 +212,7 @@ def test_flow_invest_optional(self, basic_flow_system_linopy_coords, coords_conf
         flow = fx.Flow(
             'Wärme',
             bus='Fernwärme',
-            size=fx.InvestParameters(minimum_size=20, maximum_size=100, optional=True),
+            size=fx.InvestParameters(minimum_size=20, maximum_size=100, mandatory=False),
             relative_minimum=np.linspace(0.1, 0.5, timesteps.size),
             relative_maximum=np.linspace(0.5, 1, timesteps.size),
         )
@@ -287,7 +287,7 @@ def test_flow_invest_optional_wo_min_size(self, basic_flow_system_linopy_coords,
         flow = fx.Flow(
             'Wärme',
             bus='Fernwärme',
-            size=fx.InvestParameters(maximum_size=100, optional=True),
+            size=fx.InvestParameters(maximum_size=100, mandatory=False),
             relative_minimum=np.linspace(0.1, 0.5, timesteps.size),
             relative_maximum=np.linspace(0.5, 1, timesteps.size),
         )
@@ -362,7 +362,7 @@ def test_flow_invest_wo_min_size_non_optional(self, basic_flow_system_linopy_coo
         flow = fx.Flow(
             'Wärme',
             bus='Fernwärme',
-            size=fx.InvestParameters(maximum_size=100, optional=False),
+            size=fx.InvestParameters(maximum_size=100, mandatory=True),
             relative_minimum=np.linspace(0.1, 0.5, timesteps.size),
             relative_maximum=np.linspace(0.5, 1, timesteps.size),
         )
@@ -420,7 +420,7 @@ def test_flow_invest_fixed_size(self, basic_flow_system_linopy_coords, coords_co
         flow = fx.Flow(
             'Wärme',
             bus='Fernwärme',
-            size=fx.InvestParameters(fixed_size=75, optional=False),
+            size=fx.InvestParameters(fixed_size=75, mandatory=True),
             relative_minimum=0.2,
             relative_maximum=0.9,
         )
@@ -458,7 +458,7 @@ def test_flow_invest_with_effects(self, basic_flow_system_linopy_coords, coords_
             size=fx.InvestParameters(
                 minimum_size=20,
                 maximum_size=100,
-                optional=True,
+                mandatory=False,
                 effects_of_investment={'costs': 1000, 'CO2': 5},  # Fixed investment effects
                 effects_of_investment_per_size={'costs': 500, 'CO2': 0.1},  # Specific investment effects
             ),
@@ -496,7 +496,7 @@ def test_flow_invest_divest_effects(self, basic_flow_system_linopy_coords, coord
             size=fx.InvestParameters(
                 minimum_size=20,
                 maximum_size=100,
-                optional=True,
+                mandatory=False,
                 effects_of_retirement={'costs': 500},  # Cost incurred when NOT investing
             ),
         )
@@ -1076,7 +1076,7 @@ def test_flow_on_invest_optional(self, basic_flow_system_linopy_coords, coords_c
         flow = fx.Flow(
             'Wärme',
             bus='Fernwärme',
-            size=fx.InvestParameters(minimum_size=20, maximum_size=200, optional=True),
+            size=fx.InvestParameters(minimum_size=20, maximum_size=200, mandatory=False),
             relative_minimum=0.2,
             relative_maximum=0.8,
             on_off_parameters=fx.OnOffParameters(),
@@ -1177,7 +1177,7 @@ def test_flow_on_invest_non_optional(self, basic_flow_system_linopy_coords, coor
         flow = fx.Flow(
             'Wärme',
             bus='Fernwärme',
-            size=fx.InvestParameters(minimum_size=20, maximum_size=200, optional=False),
+            size=fx.InvestParameters(minimum_size=20, maximum_size=200, mandatory=True),
             relative_minimum=0.2,
             relative_maximum=0.8,
             on_off_parameters=fx.OnOffParameters(),
@@ -1304,7 +1304,7 @@ def test_fixed_profile_with_investment(self, basic_flow_system_linopy_coords, co
         flow = fx.Flow(
             'Wärme',
             bus='Fernwärme',
-            size=fx.InvestParameters(minimum_size=50, maximum_size=200, optional=True),
+            size=fx.InvestParameters(minimum_size=50, maximum_size=200, mandatory=False),
             fixed_relative_profile=profile,
         )
 
diff --git a/tests/test_functional.py b/tests/test_functional.py
index 94172eccf..cd872e84c 100644
--- a/tests/test_functional.py
+++ b/tests/test_functional.py
@@ -270,7 +270,7 @@ def test_optional_invest(solver_fixture, time_steps_fixture):
                 'Q_th',
                 bus='Fernwärme',
                 size=fx.InvestParameters(
-                    optional=True, minimum_size=40, effects_of_investment=10, effects_of_investment_per_size=1
+                    mandatory=False, minimum_size=40, effects_of_investment=10, effects_of_investment_per_size=1
                 ),
             ),
         ),
@@ -282,7 +282,7 @@ def test_optional_invest(solver_fixture, time_steps_fixture):
                 'Q_th',
                 bus='Fernwärme',
                 size=fx.InvestParameters(
-                    optional=True, minimum_size=50, effects_of_investment=10, effects_of_investment_per_size=1
+                    mandatory=False, minimum_size=50, effects_of_investment=10, effects_of_investment_per_size=1
                 ),
             ),
         ),
diff --git a/tests/test_invest_parameters_deprecation.py b/tests/test_invest_parameters_deprecation.py
index 5d7cf49b9..438d7f4b8 100644
--- a/tests/test_invest_parameters_deprecation.py
+++ b/tests/test_invest_parameters_deprecation.py
@@ -281,3 +281,64 @@ def test_unexpected_keyword_arguments(self):
             TypeError, match="InvestParameters.__init__\\(\\) got unexpected keyword argument\\(s\\): 'typo'"
         ):
             InvestParameters(effects_of_investment={'cost': 100}, typo='value')
+
+    def test_optional_parameter_deprecation(self):
+        """Test that optional parameter triggers deprecation warning and maps to mandatory."""
+        # Test optional=True (should map to mandatory=False)
+        with pytest.warns(DeprecationWarning, match='optional.*deprecated.*mandatory'):
+            params = InvestParameters(optional=True)
+            assert params.mandatory is False
+
+        # Test optional=False (should map to mandatory=True)
+        with pytest.warns(DeprecationWarning, match='optional.*deprecated.*mandatory'):
+            params = InvestParameters(optional=False)
+            assert params.mandatory is True
+
+    def test_mandatory_parameter_no_warning(self):
+        """Test that mandatory parameter doesn't trigger warnings."""
+        with warnings.catch_warnings():
+            warnings.simplefilter('error', DeprecationWarning)
+            # Test mandatory=True
+            params = InvestParameters(mandatory=True)
+            assert params.mandatory is True
+
+            # Test mandatory=False (explicit)
+            params = InvestParameters(mandatory=False)
+            assert params.mandatory is False
+
+    def test_mandatory_default_value(self):
+        """Test that default value of mandatory is False when neither optional nor mandatory is specified."""
+        params = InvestParameters()
+        assert params.mandatory is False
+
+    def test_both_optional_and_mandatory_no_error(self):
+        """Test that specifying both optional and mandatory doesn't raise error.
+
+        Note: Conflict checking is disabled for mandatory/optional because mandatory has
+        a non-None default value (False), making it impossible to distinguish between
+        an explicit mandatory=False and the default value. The deprecated optional
+        parameter will take precedence when both are specified.
+        """
+        # When both are specified, optional takes precedence (with deprecation warning)
+        with pytest.warns(DeprecationWarning, match='optional.*deprecated.*mandatory'):
+            params = InvestParameters(optional=True, mandatory=False)
+            # optional=True should result in mandatory=False
+            assert params.mandatory is False
+
+        with pytest.warns(DeprecationWarning, match='optional.*deprecated.*mandatory'):
+            params = InvestParameters(optional=False, mandatory=True)
+            # optional=False should result in mandatory=True (optional takes precedence)
+            assert params.mandatory is True
+
+    def test_optional_property_deprecation(self):
+        """Test that accessing optional property triggers deprecation warning."""
+        params = InvestParameters(mandatory=True)
+
+        # Reading the property triggers warning
+        with pytest.warns(DeprecationWarning, match="Property 'optional' is deprecated"):
+            assert params.optional is False
+
+        # Setting the property triggers warning
+        with pytest.warns(DeprecationWarning, match="Property 'optional' is deprecated"):
+            params.optional = True
+        assert params.mandatory is False
diff --git a/tests/test_scenarios.py b/tests/test_scenarios.py
index b8f8bc45e..1ff9e9cea 100644
--- a/tests/test_scenarios.py
+++ b/tests/test_scenarios.py
@@ -152,7 +152,7 @@ def flow_system_complex_scenarios() -> fx.FlowSystem:
             size=fx.InvestParameters(
                 effects_of_investment=1000,
                 fixed_size=50,
-                optional=False,
+                mandatory=True,
                 effects_of_investment_per_size={'costs': 10, 'PE': 2},
             ),
             on_off_parameters=fx.OnOffParameters(
@@ -178,7 +178,7 @@ def flow_system_complex_scenarios() -> fx.FlowSystem:
                 'PE': fx.Piecewise([fx.Piece(5, 25), fx.Piece(25, 100)]),
             },
         ),
-        optional=False,
+        mandatory=True,
         effects_of_investment_per_size={'costs': 0.01, 'CO2': 0.01},
         minimum_size=0,
         maximum_size=1000,
diff --git a/tests/test_storage.py b/tests/test_storage.py
index 252c7d228..02db3f09a 100644
--- a/tests/test_storage.py
+++ b/tests/test_storage.py
@@ -265,7 +265,7 @@ def test_storage_with_investment(self, basic_flow_system_linopy_coords, coords_c
                 effects_of_investment_per_size=10,
                 minimum_size=20,
                 maximum_size=100,
-                optional=True,
+                mandatory=False,
             ),
             initial_charge_state=0,
             eta_charge=0.9,
@@ -425,19 +425,19 @@ def test_simultaneous_charge_discharge(self, basic_flow_system_linopy_coords, co
             )
 
     @pytest.mark.parametrize(
-        'optional,minimum_size,expected_vars,expected_constraints',
+        'mandatory,minimum_size,expected_vars,expected_constraints',
         [
-            (True, None, {'InvestStorage|is_invested'}, {'InvestStorage|size|lb'}),
-            (True, 20, {'InvestStorage|is_invested'}, {'InvestStorage|size|lb'}),
-            (False, None, set(), set()),
-            (False, 20, set(), set()),
+            (False, None, {'InvestStorage|is_invested'}, {'InvestStorage|size|lb'}),
+            (False, 20, {'InvestStorage|is_invested'}, {'InvestStorage|size|lb'}),
+            (True, None, set(), set()),
+            (True, 20, set(), set()),
         ],
     )
     def test_investment_parameters(
         self,
         basic_flow_system_linopy_coords,
         coords_config,
-        optional,
+        mandatory,
         minimum_size,
         expected_vars,
         expected_constraints,
@@ -449,7 +449,7 @@ def test_investment_parameters(
         invest_params = {
             'effects_of_investment': 100,
             'effects_of_investment_per_size': 10,
-            'optional': optional,
+            'mandatory': mandatory,
         }
         if minimum_size is not None:
             invest_params['minimum_size'] = minimum_size
@@ -471,20 +471,20 @@ def test_investment_parameters(
 
         # Check that expected variables exist
         for var_name in expected_vars:
-            if optional:
+            if not mandatory:  # Optional investment (mandatory=False)
                 assert var_name in model.variables, f'Expected variable {var_name} not found'
 
         # Check that expected constraints exist
         for constraint_name in expected_constraints:
-            if optional:
+            if not mandatory:  # Optional investment (mandatory=False)
                 assert constraint_name in model.constraints, f'Expected constraint {constraint_name} not found'
 
-        # If optional is False, is_invested should be fixed to 1
-        if not optional:
+        # If mandatory is True, is_invested should be fixed to 1
+        if mandatory:
             # Check that the is_invested variable exists and is fixed to 1
             if 'InvestStorage|is_invested' in model.variables:
                 var = model.variables['InvestStorage|is_invested']
                 # Check if the lower and upper bounds are both 1
                 assert var.upper == 1 and var.lower == 1, (
-                    'is_invested variable should be fixed to 1 when optional=False'
+                    'is_invested variable should be fixed to 1 when mandatory=True'
                 )