diff --git a/app/integrations/expose.py b/app/integrations/expose.py
index 258d641..7a5b169 100644
--- a/app/integrations/expose.py
+++ b/app/integrations/expose.py
@@ -58,6 +58,21 @@ class DeviceInfo:
     name: str
     """Human-readable device name (may change; triggers HA discovery re-publish)."""
 
+    provides_availability: bool = True
+    """Whether this device publishes an availability ("online"/"offline") heartbeat.
+
+    When True (the default — e.g. Modbus devices, which have an ``online``
+    binary_sensor driving availability), the HA Discovery config for each entity
+    declares an ``availability`` topic and HA marks the entity unavailable until
+    "online" is published.
+
+    When False (e.g. the energy-cost device, which has only sensors and no
+    heartbeat source), the config omits ``availability`` so HA treats the entity
+    as *always available* and shows its state as soon as one arrives.  Without
+    this, such entities would stay perpetually ``unavailable`` in HA even though
+    their state is being published.
+    """
+
 
 @dataclass
 class ExposableEntity:
@@ -422,9 +437,13 @@ def _energy_cost_provider(session: Session) -> list[ExposableEntity]:
         currency = latest_period.currency
 
     # --- Shared DeviceInfo (2-element identifiers — required by ha_discovery.py [1] access) ---
+    # provides_availability=False: the energy-cost device has only sensors and no
+    # online/offline heartbeat, so its entities must be "always available" in HA.
+    # (Otherwise HA shows them unavailable despite state being published.)
     device_info = DeviceInfo(
         identifiers=("energy-cost", "energy-cost"),
         name="Energy Cost",
+        provides_availability=False,
     )
 
     # --- value_getter: current buy price ---
diff --git a/app/services/ha_discovery.py b/app/services/ha_discovery.py
index b317a4b..a99f834 100644
--- a/app/services/ha_discovery.py
+++ b/app/services/ha_discovery.py
@@ -133,14 +133,21 @@ def build_discovery_payload(
         "unique_id": _unique_id(entity),
         "name": entity.name,
         "state_topic": state_t,
-        "availability": [{"topic": avail_topic}],
-        "availability_mode": "all",
         "device": {
             "identifiers": list(entity.device.identifiers),
             "name": entity.device.name,
         },
     }
 
+    # Only declare an availability topic for devices that actually publish an
+    # online/offline heartbeat (e.g. Modbus, via its "online" binary_sensor).
+    # Devices without a heartbeat (e.g. energy-cost) omit availability so HA
+    # treats their entities as always-available — otherwise HA would mark them
+    # ``unavailable`` forever even though their state is being published.
+    if entity.device.provides_availability:
+        config["availability"] = [{"topic": avail_topic}]
+        config["availability_mode"] = "all"
+
     # Component-specific fields
     if entity.component == "binary_sensor":
         # "online" binary sensor: payload ON/OFF
diff --git a/tests/test_energy_expose.py b/tests/test_energy_expose.py
index 624bccb..b860387 100644
--- a/tests/test_energy_expose.py
+++ b/tests/test_energy_expose.py
@@ -648,6 +648,29 @@ def test_build_discovery_payload_no_index_error_for_energy_entities(energy_db) -
         assert "energy-cost" in config["device"]["identifiers"]
 
 
+def test_energy_cost_entities_omit_availability_so_ha_shows_them(energy_db) -> None:
+    """The energy-cost device has no online/offline heartbeat, so its discovery
+    config must NOT declare an availability topic — otherwise HA marks the
+    entities ``unavailable`` forever even though state is being published.
+    """
+    from app.integrations.expose import build_catalog
+    from app.services.ha_discovery import build_discovery_payload
+
+    with Session(energy_db) as session:
+        catalog = build_catalog(session)
+
+    energy_entries = [e for e in catalog if e.entity.key.startswith("energy.")]
+    assert len(energy_entries) == 4
+
+    for entry in energy_entries:
+        assert entry.entity.device.provides_availability is False
+        _, config = build_discovery_payload(entry.entity, "homeassistant")
+        assert "availability" not in config, (
+            f"energy entity {entry.entity.key} must omit availability (always-available)"
+        )
+        assert "availability_mode" not in config
+
+
 def test_energy_entity_discovery_topics_contain_correct_node_id() -> None:
     """Discovery topic node_id for energy entities must be 'energy-cost' (hyphens → underscores)."""
     from app.integrations.expose import DeviceInfo, ExposableEntity