diff --git a/tests/conftest.py b/tests/conftest.py
index 98e9dba..b215e64 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -2,11 +2,13 @@
 
 from __future__ import annotations
 
+import math
+
 import numpy as np
 import pandas as pd
 import pytest
 
-from battery_dispatch.config import BatterySpec, RunConfig
+from battery_dispatch.config import HALF_HOUR, BatterySpec, RunConfig
 from battery_dispatch.markets import MARKET_1, MARKET_2, Market
 
 # Equal to Attachment 1, but stated literally so the tests do not depend on the
@@ -73,3 +75,58 @@ def prices_frame(
         },
         index=index,
     )
+
+
+def make_valid_schedule(spec: BatterySpec, n: int = 48) -> pd.DataFrame:
+    """A schedule (shaped like the optimiser's output) that satisfies every
+    invariant ``validate_schedule`` checks -- built directly from the model's
+    own energy-balance and revenue formulas, not by running the optimiser.
+
+    ``validate_schedule`` is tested in isolation: it only needs *some*
+    structurally valid schedule to mutate one violation into. Deriving that
+    from a real rolling-horizon solve would mean spinning up CBC once per
+    test for no reason connected to what is under test, so this constructs
+    one by hand instead -- charge for two hours, sit idle, discharge for two
+    hours, comfortably within every power and capacity bound.
+    """
+    charge1 = np.zeros(n)
+    charge2 = np.zeros(n)
+    discharge1 = np.zeros(n)
+    discharge2 = np.zeros(n)
+    charge1[0:4] = 1.0
+    charge2[0:4] = 0.5
+    discharge1[36:40] = 0.5
+    discharge2[36:40] = 0.3
+
+    total_charge = charge1 + charge2
+    total_discharge = discharge1 + discharge2
+    delta = HALF_HOUR * (
+        spec.charge_efficiency * total_charge - total_discharge / spec.discharge_efficiency
+    )
+    soc = np.cumsum(delta)
+    assert soc.min() >= -1e-9 and soc.max() <= spec.capacity_mwh - 1e-9, (
+        "fixture parameters must stay clear of the battery's bounds"
+    )
+
+    price1 = np.array([40.0 + 10.0 * math.sin(i / 5) for i in range(n)])
+    price2_hourly = np.array([35.0 + 8.0 * math.sin(h / 4) for h in range(n // 2)])
+    price2 = np.repeat(price2_hourly, 2)
+
+    frame = pd.DataFrame(
+        {
+            "market_1_price": price1,
+            "market_2_price": price2,
+            "charge_market_1_mw": charge1,
+            "discharge_market_1_mw": discharge1,
+            "charge_market_2_mw": charge2,
+            "discharge_market_2_mw": discharge2,
+            "soc_mwh": soc,
+            "capacity_mwh": np.full(n, spec.capacity_mwh),
+        },
+        index=pd.date_range("2018-01-01", periods=n, freq="30min"),
+    )
+    frame.index.name = "timestamp"
+    for name in (MARKET_1, MARKET_2):
+        net_export = frame[f"discharge_{name}_mw"] - frame[f"charge_{name}_mw"]
+        frame[f"revenue_{name}_gbp"] = HALF_HOUR * frame[f"{name}_price"] * net_export
+    return frame
diff --git a/tests/test_optimiser.py b/tests/test_optimiser.py
index e896e7f..07a92b5 100644
--- a/tests/test_optimiser.py
+++ b/tests/test_optimiser.py
@@ -98,16 +98,22 @@ def test_combined_power_respects_the_two_megawatt_cap(spec, config):
 
 
 def test_optimum_uses_whichever_market_pays_more_in_each_hour(spec, config):
-    """Across hours the model switches markets rather than favouring one."""
-    # Hour 0 cheap in both (charge); hour 1 Market 1 pays best; hour 2 Market 2 does.
+    """Across hours the model switches markets rather than favouring one.
+
+    Two cheap hours fill the battery to capacity (3.8 MWh stored). Delivering
+    that much exceeds what a single hour can carry at the 2 MW cap, so the
+    discharge must split across both peak hours regardless of price -- what
+    is under test is *which market* gets each hour's discharge, which each
+    hour's own prices pin unambiguously (300 vs 10 in both cases, not a tie).
+    """
     markets = markets_from(
-        [0.0, 0.0, 300.0, 300.0, 10.0, 10.0],
-        [0.0, 10.0, 300.0],
+        [0.0, 0.0, 0.0, 0.0, 300.0, 300.0, 10.0, 10.0],
+        [0.0, 0.0, 10.0, 300.0],
     )
     solution, _ = solve_window(markets, spec, 0.0, spec.capacity_mwh, config)
 
-    assert solution.discharge_mw[MARKET_1][2:4].sum() > 0, "hour 1 should sell into M1"
-    assert solution.discharge_mw[MARKET_2][4:6].sum() > 0, "hour 2 should sell into M2"
+    assert solution.discharge_mw[MARKET_1][4:6].sum() > 0, "hour 2 should sell into M1"
+    assert solution.discharge_mw[MARKET_2][6:8].sum() > 0, "hour 3 should sell into M2"
     assert total_power(solution, "discharge").max() <= spec.max_discharge_mw + 1e-6
 
 
@@ -137,21 +143,27 @@ def test_degradation_cost_suppresses_marginal_cycling(spec):
 
 
 def test_rolling_horizon_carries_state_across_windows(spec):
-    """State of charge at a commit boundary is the next window's starting point."""
-    # Two days: charge cheaply late on day 1, sell into the day-2 morning peak.
-    day = [0.0] * 40 + [5.0] * 8
-    day_2 = [200.0] * 8 + [50.0] * 40
-    prices = prices_frame(day + day_2, [40.0] * 48)
+    """State of charge at a commit boundary is the next window's starting point.
 
-    config = RunConfig(window_hours=48, commit_hours=24)
+    Small on purpose: this is testing ``run_rolling_horizon``'s bookkeeping
+    (does the committed end-of-window SoC become the next window's start?),
+    not the optimiser's economics, so it uses a 2-hour window / 1-hour commit
+    rather than the production 48h/24h -- a real rolling-horizon MILP solve
+    at production scale belongs in a slower, separately-run integration
+    check, not in the unit suite.
+    """
+    # Cheap for two hours (charge), expensive for two hours (discharge), the
+    # split falling on the commit boundary.
+    prices = prices_frame([0.0] * 4 + [100.0] * 4, [10.0] * 4)
+    config = RunConfig(window_hours=4, commit_hours=2)
     result, state, stats = run_rolling_horizon(prices, spec, config)
 
     assert stats.windows == 2
-    assert len(result) == 96
+    assert len(result) == 8
 
-    # The lookahead must carry energy over midnight to reach the day-2 peak.
-    soc_at_midnight = result["soc_mwh"].iloc[47]
-    assert soc_at_midnight > 1.0
+    # The lookahead must carry energy over the boundary to reach the peak.
+    soc_at_boundary = result["soc_mwh"].iloc[3]
+    assert soc_at_boundary > 1.0
     assert state.equivalent_full_cycles > 0
 
 
diff --git a/tests/test_validation.py b/tests/test_validation.py
index 4748628..abf4b09 100644
--- a/tests/test_validation.py
+++ b/tests/test_validation.py
@@ -5,22 +5,17 @@ from __future__ import annotations
 import numpy as np
 import pandas as pd
 import pytest
-from conftest import prices_frame
+from conftest import make_valid_schedule
 
-from battery_dispatch.config import RunConfig
-from battery_dispatch.optimiser import run_rolling_horizon
 from battery_dispatch.validation import validate_schedule
 
 
 @pytest.fixture
 def schedule(spec) -> pd.DataFrame:
-    """A genuine, valid two-day schedule to mutate in each test."""
-    prices = prices_frame(
-        [0.0] * 20 + [90.0] * 8 + [10.0] * 20 + [5.0] * 16 + [120.0] * 8 + [40.0] * 24,
-        [30.0] * 48,
-    )
-    result, _, _ = run_rolling_horizon(prices, spec, RunConfig())
-    return result
+    """A genuine, valid schedule to mutate in each test. See
+    ``conftest.make_valid_schedule`` for why this isn't a real optimiser run.
+    """
+    return make_valid_schedule(spec)
 
 
 def test_a_valid_schedule_passes_every_check(schedule, spec):