diff --git a/tests/domain/modelling/test_products.py b/tests/domain/modelling/test_products.py
index e4953a3e..e99daedc 100644
--- a/tests/domain/modelling/test_products.py
+++ b/tests/domain/modelling/test_products.py
@@ -111,3 +111,33 @@ def test_decommission_falls_back_for_systems_not_on_the_rate_sheet() -> None:
     assert abs(products.ashp_bundle_cost(_small_no_reuse(AshpExistingSystem.NONE)).total - (base + 0.0)) <= 1e-9
     assert abs(products.ashp_bundle_cost(_small_no_reuse(AshpExistingSystem.ELECTRIC_OTHER)).total - (base + 570.0)) <= 1e-9
     assert abs(products.ashp_bundle_cost(_small_no_reuse(AshpExistingSystem.OTHER)).total - (base + 720.0)) <= 1e-9
+
+
+def _pump_price(products: Products, design_heat_loss_kw: float) -> float:
+    """Isolate the heat-pump line: no-system (decommission 0) + cylinder
+    2382.60 + distribution (7 rads) 3618 = 6000.60 base, so total - base is the
+    pump band price for ``design_heat_loss_kw``."""
+    inputs = AshpCostInputs(
+        existing_system=AshpExistingSystem.NONE,
+        is_small_property=True,
+        design_heat_loss_kw=design_heat_loss_kw,
+        radiator_count=7,
+        has_reusable_wet_system=False,
+    )
+    return products.ashp_bundle_cost(inputs).total - 6000.60
+
+
+def test_heat_pump_rounds_design_heat_loss_up_to_the_next_band() -> None:
+    # Arrange
+    products = Products()
+
+    # Act / Assert — bands {5,8,11,15,16+} kW -> {9720,9840,10200,10680,11400};
+    # a load is rounded UP to the smallest band that covers it.
+    assert abs(_pump_price(products, 5.0) - 9720.0) <= 1e-9   # at the 5 kW edge
+    assert abs(_pump_price(products, 5.01) - 9840.0) <= 1e-9  # just over -> 8 kW
+    assert abs(_pump_price(products, 8.0) - 9840.0) <= 1e-9
+    assert abs(_pump_price(products, 8.01) - 10200.0) <= 1e-9
+    assert abs(_pump_price(products, 11.0) - 10200.0) <= 1e-9
+    assert abs(_pump_price(products, 15.0) - 10680.0) <= 1e-9
+    assert abs(_pump_price(products, 15.01) - 11400.0) <= 1e-9  # above largest
+    assert abs(_pump_price(products, 25.0) - 11400.0) <= 1e-9