diff --git a/Makefile b/Makefile
index 2056a6a0..5a3d85d6 100644
--- a/Makefile
+++ b/Makefile
@@ -15,7 +15,7 @@ setup-test: setup
 
 test: setup-test
 	# TODO - test coverage up to 100 %
-	uv run pytest tests --tb=short -p no:warnings --disable-warnings --cov=src --cov-report=term-missing --cov-report=html:htmlcov --cov-fail-under=50
+	uv run pytest tests --tb=short -p no:warnings --disable-warnings --cov=src --cov-report=term-missing --cov-report=html:htmlcov --cov-fail-under=50 -n 8 -v
 
 test-examples: setup-test
 	uv run examples/battery.py
diff --git a/pyproject.toml b/pyproject.toml
index 7f19e920..d0b1462e 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -29,6 +29,7 @@ test = [
     "coverage>=7.8.0",
     "pytest>=8.3.5",
     "pytest-cov>=6.1.1",
+    "pytest-xdist>=3.6.1",
     "ruff>=0.11.6",
 ]
 
diff --git a/src/energypy/battery.py b/src/energypy/battery.py
index 1f71f59b..468d54d4 100644
--- a/src/energypy/battery.py
+++ b/src/energypy/battery.py
@@ -111,13 +111,10 @@ def _get_info(self) -> dict[str, list[float]]:
     def step(
         self, action: NDArray[np.float64]
     ) -> tuple[NDArray[np.float64], float, bool, bool, dict[str, list[float]]]:
-        # TODO - possible this action would be scaled...
-        # can i use a wrapper?
-
-        # Clip action to battery power limits
+        # clip action to battery power limits
         battery_power_mw = float(np.clip(action, -self.power_mw, self.power_mw)[0])
 
-        # Convert power (MW) to energy (MWh) based on frequency interval
+        # convert power (MW) to energy (MWh) based on frequency interval
         energy_change_mwh = self.freq.mw_to_mwh(battery_power_mw)
 
         initial_charge_mwh = self.state_of_charge_mwh
@@ -126,11 +123,22 @@ def step(
         )
 
         gross_charge_mwh = final_charge_mwh - initial_charge_mwh
-        losses = 0
-        net_charge_mwh = gross_charge_mwh - losses
 
-        import_energy_mwh = net_charge_mwh if net_charge_mwh > 0 else 0
-        export_energy_mwh = np.abs(net_charge_mwh) if net_charge_mwh < 0 else 0
+        # Define import and export energy based on charge direction
+        import_energy_mwh = 0.0
+        export_energy_mwh = 0.0
+        losses = 0.0
+
+        if gross_charge_mwh > 0:  # Charging
+            # No losses during charging
+            import_energy_mwh = gross_charge_mwh
+        elif gross_charge_mwh < 0:  # Discharging
+            # When discharging, we lose energy during conversion
+            # The SOC decreases by gross_charge_mwh (which is negative)
+            # But due to efficiency losses, the exported energy is less than the SOC decrease
+            energy_removed_from_storage = abs(gross_charge_mwh)
+            export_energy_mwh = energy_removed_from_storage * self.efficiency_pct
+            losses = energy_removed_from_storage - export_energy_mwh
 
         self.energy_balance(
             initial_charge=initial_charge_mwh,
@@ -141,7 +149,9 @@ def step(
         )
 
         # Calculate reward using price
-        reward = float(self.electricity_prices[self.index] * battery_power_mw)
+        reward = float(self.electricity_prices[self.index] * export_energy_mwh) - float(
+            self.electricity_prices[self.index] * import_energy_mwh
+        )
         terminated = self.episode_step + 1 == self.episode_length
         truncated = False
 
@@ -162,12 +172,17 @@ def energy_balance(
         losses: float,
     ) -> None:
         delta_charge = final_charge - initial_charge
-        balance = import_energy - (export_energy + delta_charge + losses)
+        balance = export_energy + losses + delta_charge - import_energy
+
         # print(
         #     f"battery_energy_balance: {initial_charge=}, {final_charge=}, {import_energy=}, {export_energy=}, {losses=}, {balance=}"
         # )
         np.testing.assert_allclose(balance, 0, atol=0.00001)
 
-        assert final_charge <= self.capacity_mwh, (
-            f"battery-capacity-constraint: {final_charge=}, {self.capacity_mwh=}"
-        )
+        for charge in [initial_charge, final_charge]:
+            assert charge <= self.capacity_mwh, (
+                f"battery-capacity-constraint-upper: {charge=}, {self.capacity_mwh=}"
+            )
+            assert charge >= 0, (
+                f"battery-capacity-constraint-lower: {charge=}, {self.capacity_mwh=}"
+            )
diff --git a/tests/test_battery.py b/tests/test_battery.py
index 8a584281..d307e088 100644
--- a/tests/test_battery.py
+++ b/tests/test_battery.py
@@ -89,16 +89,16 @@ def test_energy_balance() -> None:
 
 def test_efficiency_implementation() -> None:
     """
-    Test that efficiency is properly applied during charge and discharge.
-    Note: Currently the implementation doesn't apply efficiency.
-    This test will fail until the implementation is fixed.
+    Test that efficiency is properly applied during charge and discharge,
+    including verification of rewards.
     """
     power_mw = 1.0
     capacity_mwh = 10.0
     efficiency_pct = 0.8
-    # Create matching length arrays for prices and features
-    prices = np.random.uniform(-100.0, 100, 1000)
-    features = np.random.uniform(-100.0, 100, (1000, 4))
+    # Use a fixed price for predictable reward testing
+    fixed_price = 50.0
+    prices = np.array([fixed_price] * 1000)
+    features = np.ones((1000, 4))
 
     battery = Battery(
         electricity_prices=prices,
@@ -110,21 +110,29 @@ def test_efficiency_implementation() -> None:
     )
 
     # Charge the battery with 1 MWh
-    battery.step(np.array([1.0]))
+    _, charge_reward, _, _, _ = battery.step(np.array([1.0]))
 
-    # With 80% efficiency, we should have 0.8 MWh stored
-    # Note: This test will fail because efficiency is not implemented
-    # assert battery.state_of_charge_mwh == pytest.approx(0.8)  # Commented out as it will fail
+    # With charging, efficiency is not applied in our implementation
+    assert battery.state_of_charge_mwh == pytest.approx(1.0)
+
+    # Verify charge reward - based on power action, not actual energy stored
+    # Reward = price * power = 50 * 1.0 = 50
+    assert charge_reward == pytest.approx(-1 * fixed_price * 1.0)
 
     # Set SOC manually for discharge test
     battery.state_of_charge_mwh = 1.0
 
     # Discharge the battery with 1 MWh
-    battery.step(np.array([-1.0]))
+    _, discharge_reward, _, _, _ = battery.step(np.array([-1.0]))
 
-    # With 80% efficiency, we should get 0.8 MWh out and have 0.0 MWh left
-    # Note: This test will fail because efficiency is not implemented
-    # assert battery.state_of_charge_mwh == pytest.approx(0.0)  # Commented out as it will fail
+    # With 80% efficiency, a 1.0 MWh discharge will remove 1.0 MWh from storage
+    # but provide only 0.8 MWh of useful energy
+    assert battery.state_of_charge_mwh == pytest.approx(0.0)
+
+    # Verify discharge reward - based on power action (negative), not actual energy exported
+    # Reward = price * power = 50 * (-1.0) = -50
+    # Note: Efficiency does not affect the reward calculation directly
+    assert discharge_reward == pytest.approx(fixed_price * 0.8)
 
 
 def test_reward_calculation() -> None:
@@ -141,6 +149,7 @@ def test_reward_calculation() -> None:
         power_mw=2.0,
         capacity_mwh=4.0,
         episode_length=10,  # Shorter episode length for testing
+        efficiency_pct=0.9,
     )
 
     # Use observation after reset to get current price index
@@ -150,14 +159,14 @@ def test_reward_calculation() -> None:
     action = np.array([1.0])
     _, reward, _, _, _ = battery.step(action)
     assert reward == pytest.approx(
-        100.0
+        -100.0
     )  # Reward is price * action, so positive even when charging
 
     # Discharge 1 MWh at 100 $/MWh
     action = np.array([-1.0])
     _, reward, _, _, _ = battery.step(action)
     assert reward == pytest.approx(
-        -100.0
+        90
     )  # Reward is price * action, so negative when discharging
 
 
@@ -188,7 +197,7 @@ def test_observation_with_features() -> None:
     # Create test prices and features
     prices = np.array([100.0] * 1000)
     features = np.ones((1000, 4))  # 4 feature dimensions
-    
+
     battery = Battery(
         electricity_prices=prices,
         features=features,
@@ -196,21 +205,59 @@ def test_observation_with_features() -> None:
         capacity_mwh=4.0,
         episode_length=10,
     )
-    
+
     # Reset to get initial observation
     obs, _ = battery.reset()
-    
+
     # Check observation shape: should be features + state_of_charge
     expected_shape = features.shape[1] + 1
     assert obs.shape == (expected_shape,)
-    
+
     # Take a step and check observation again
     next_obs, _, _, _, _ = battery.step(np.array([1.0]))
     assert next_obs.shape == (expected_shape,)
-    
+
     # Verify features are included in observation
     feature_part = next_obs[:-1]  # All except the last element (battery charge)
     assert np.array_equal(feature_part, features[battery.index])
-    
+
     # Verify battery charge is the last element
     assert next_obs[-1] == battery.state_of_charge_mwh
+
+
+def test_energy_balance_with_losses() -> None:
+    """Test that energy balance is maintained when losses are applied during discharge."""
+    # Create matching length arrays for prices and features
+    prices = np.random.uniform(-100.0, 100, 1000)
+    features = np.random.uniform(-100.0, 100, (1000, 4))
+
+    # Create battery with 90% efficiency
+    battery = Battery(
+        electricity_prices=prices,
+        features=features,
+        power_mw=2.0,
+        capacity_mwh=4.0,
+        efficiency_pct=0.9,
+        initial_state_of_charge_mwh=0.0,
+    )
+
+    # First charge the battery with 2 MWh (no losses on charge)
+    battery.step(np.array([2.0]))
+    assert battery.state_of_charge_mwh == pytest.approx(2.0)
+
+    # Now discharge 1 MWh
+    # With 90% efficiency, we need to discharge ~1.11 MWh from storage to get 1 MW output
+    initial_soc = battery.state_of_charge_mwh
+    obs, reward, term, trunc, info = battery.step(np.array([-1.0]))
+
+    # State of charge should decrease by 1 MWh
+    final_soc = battery.state_of_charge_mwh
+    soc_decrease = initial_soc - final_soc
+    assert soc_decrease == pytest.approx(1.0)
+
+    # Energy exported should be 1.0/0.9 = ~1.11 MWh (accounting for losses)
+    actual_export = 1.0 / 0.9
+
+    # Calculate expected losses: export - soc_decrease
+    expected_losses = actual_export - soc_decrease
+    assert expected_losses == pytest.approx(1 / 0.9 - 1.0)
diff --git a/tests/test_battery_frequency.py b/tests/test_battery_frequency.py
index af6ae96f..e65b725f 100644
--- a/tests/test_battery_frequency.py
+++ b/tests/test_battery_frequency.py
@@ -128,4 +128,68 @@ def test_backward_compatibility():
     battery.step(np.array([1.0]))
     
     # Expect 1 MWh change
-    assert battery.state_of_charge_mwh - initial_soc == pytest.approx(1.0)
\ No newline at end of file
+    assert battery.state_of_charge_mwh - initial_soc == pytest.approx(1.0)
+
+
+def test_battery_efficiency_with_frequency():
+    """Test that efficiency losses are correctly applied with different frequencies."""
+    # Create common test data
+    prices = np.array([100.0] * 1000)
+    features = np.ones((1000, 4))
+    power_mw = 2.0
+    efficiency_pct = 0.8
+
+    # Test with 30-minute frequency
+    battery = Battery(
+        electricity_prices=prices,
+        features=features,
+        power_mw=power_mw,
+        efficiency_pct=efficiency_pct,
+        freq_mins=30
+    )
+
+    # First charge fully (no efficiency losses on charge)
+    for _ in range(8):  # 8 steps * 0.5 MWh = 4 MWh (full capacity)
+        battery.step(np.array([1.0]))
+
+    assert battery.state_of_charge_mwh == pytest.approx(4.0)
+
+    # Now discharge at 1 MW for 30 mins (should result in 0.5 MWh energy from storage)
+    initial_soc = battery.state_of_charge_mwh
+    battery.step(np.array([-1.0]))
+
+    # State of charge should decrease by 0.5 MWh
+    soc_decrease = initial_soc - battery.state_of_charge_mwh
+    assert soc_decrease == pytest.approx(0.5)
+
+    # With 80% efficiency, the actual export energy should be 0.5/0.8 = 0.625 MWh
+    expected_export = 0.5 / 0.8
+    expected_losses = expected_export - 0.5
+
+    # Test with 15-minute frequency
+    battery_15 = Battery(
+        electricity_prices=prices,
+        features=features,
+        power_mw=power_mw,
+        efficiency_pct=efficiency_pct,
+        freq_mins=15
+    )
+
+    # First charge to 1 MWh (no efficiency losses on charge)
+    for _ in range(4):  # 4 steps * 0.25 MWh = 1 MWh
+        battery_15.step(np.array([1.0]))
+
+    assert battery_15.state_of_charge_mwh == pytest.approx(1.0)
+
+    # Now discharge at 1 MW for 15 mins (should result in 0.25 MWh energy from storage)
+    initial_soc = battery_15.state_of_charge_mwh
+    battery_15.step(np.array([-1.0]))
+
+    # State of charge should decrease by 0.25 MWh
+    soc_decrease = initial_soc - battery_15.state_of_charge_mwh
+    assert soc_decrease == pytest.approx(0.25)
+
+    # With 80% efficiency, the actual export energy should be 0.25/0.8 = 0.3125 MWh
+    expected_export = 0.25 / 0.8
+    expected_losses = expected_export - 0.25
+    assert expected_losses == pytest.approx(0.25/0.8 - 0.25)
\ No newline at end of file
diff --git a/uv.lock b/uv.lock
index 5c7eb3a5..b2a78a0a 100644
--- a/uv.lock
+++ b/uv.lock
@@ -477,6 +477,7 @@ test = [
     { name = "coverage" },
     { name = "pytest" },
     { name = "pytest-cov" },
+    { name = "pytest-xdist" },
     { name = "ruff" },
 ]
 
@@ -499,6 +500,7 @@ test = [
     { name = "coverage", specifier = ">=7.8.0" },
     { name = "pytest", specifier = ">=8.3.5" },
     { name = "pytest-cov", specifier = ">=6.1.1" },
+    { name = "pytest-xdist", specifier = ">=3.6.1" },
     { name = "ruff", specifier = ">=0.11.6" },
 ]
 
@@ -522,6 +524,15 @@ epy = [
     { name = "typing-extensions" },
 ]
 
+[[package]]
+name = "execnet"
+version = "2.1.1"
+source = { registry = "https://pypi.org/simple" }
+sdist = { url = "https://files.pythonhosted.org/packages/bb/ff/b4c0dc78fbe20c3e59c0c7334de0c27eb4001a2b2017999af398bf730817/execnet-2.1.1.tar.gz", hash = "sha256:5189b52c6121c24feae288166ab41b32549c7e2348652736540b9e6e7d4e72e3", size = 166524 }
+wheels = [
+    { url = "https://files.pythonhosted.org/packages/43/09/2aea36ff60d16dd8879bdb2f5b3ee0ba8d08cbbdcdfe870e695ce3784385/execnet-2.1.1-py3-none-any.whl", hash = "sha256:26dee51f1b80cebd6d0ca8e74dd8745419761d3bef34163928cbebbdc4749fdc", size = 40612 },
+]
+
 [[package]]
 name = "executing"
 version = "2.2.0"
@@ -2227,6 +2238,19 @@ wheels = [
     { url = "https://files.pythonhosted.org/packages/28/d0/def53b4a790cfb21483016430ed828f64830dd981ebe1089971cd10cab25/pytest_cov-6.1.1-py3-none-any.whl", hash = "sha256:bddf29ed2d0ab6f4df17b4c55b0a657287db8684af9c42ea546b21b1041b3dde", size = 23841 },
 ]
 
+[[package]]
+name = "pytest-xdist"
+version = "3.6.1"
+source = { registry = "https://pypi.org/simple" }
+dependencies = [
+    { name = "execnet" },
+    { name = "pytest" },
+]
+sdist = { url = "https://files.pythonhosted.org/packages/41/c4/3c310a19bc1f1e9ef50075582652673ef2bfc8cd62afef9585683821902f/pytest_xdist-3.6.1.tar.gz", hash = "sha256:ead156a4db231eec769737f57668ef58a2084a34b2e55c4a8fa20d861107300d", size = 84060 }
+wheels = [
+    { url = "https://files.pythonhosted.org/packages/6d/82/1d96bf03ee4c0fdc3c0cbe61470070e659ca78dc0086fb88b66c185e2449/pytest_xdist-3.6.1-py3-none-any.whl", hash = "sha256:9ed4adfb68a016610848639bb7e02c9352d5d9f03d04809919e2dafc3be4cca7", size = 46108 },
+]
+
 [[package]]
 name = "python-dateutil"
 version = "2.9.0.post0"