diff --git a/app/integrations/expose.py b/app/integrations/expose.py
index 8cd512d..258d641 100644
--- a/app/integrations/expose.py
+++ b/app/integrations/expose.py
@@ -348,3 +348,232 @@ def _modbus_provider(session: Session) -> list[ExposableEntity]:
 
 # Register the modbus provider at module load time.
 register_provider(_modbus_provider)
+
+
+# ---------------------------------------------------------------------------
+# Energy Cost provider
+# ---------------------------------------------------------------------------
+
+
+def _energy_cost_provider(session: Session) -> list[ExposableEntity]:
+    """Enumerate ExposableEntity objects for the energy cost subsystem.
+
+    Produces 4 sensor entities grouped under a single HA device "Energy Cost":
+
+    - ``buy_price_now``         — current effective buy price (EUR/kWh or local currency).
+    - ``sell_price_now``        — current effective sell price (EUR/kWh or local currency).
+    - ``import_cost_total``     — cumulative import cost (total_increasing, monetary).
+    - ``export_revenue_total``  — cumulative export revenue (total_increasing, monetary).
+
+    Current-price algorithm (source-agnostic, with fallback)
+    ---------------------------------------------------------
+    Strategy A: read the ``pricing`` snapshot from the most recent non-degraded
+    ``energy_cost_period`` row.
+
+    - For ``kind="tibber"``: the snapshot contains ``"buy"`` and ``"sell"`` keys
+      (per-unit prices in the contract currency).
+    - For ``kind="manual"``: ``"buy_normal"`` and ``"sell_normal"`` are used as
+      representative effective per-unit prices.  (Both tariff-slot prices differ
+      only by the base rate; energy_tax and ODE are the same for both, so
+      buy_normal is the higher/conservative single representative value.)
+
+    When no non-degraded period exists or the pricing snapshot lacks the
+    expected keys, ``value_getter`` returns ``None`` (``publish_states``
+    skips None values automatically).
+
+    Cumulative totals
+    -----------------
+    ``SUM(import_cost)`` and ``SUM(export_revenue)`` over **all non-degraded**
+    ``energy_cost_period`` rows.  Degraded rows carry 0 costs and are excluded
+    to avoid double-counting when they are later overwritten by real values.
+
+    Currency
+    --------
+    Taken from the most recent non-degraded period's ``currency`` column.
+    Falls back to ``"EUR"`` when no such row exists.
+
+    Key convention
+    --------------
+    Fixed stable string keys (not derived from any mutable field or DB id):
+      - ``"energy.buy_price_now"``
+      - ``"energy.sell_price_now"``
+      - ``"energy.import_cost_total"``
+      - ``"energy.export_revenue_total"``
+
+    DeviceInfo identifiers
+    ----------------------
+    **Two-element tuple** ``("energy-cost", "energy-cost")`` so that
+    ``ha_discovery.py``'s ``entity.device.identifiers[1]`` is always valid
+    (the service uses index [1] as the node_id throughout).
+    """
+    from app.models.energy import EnergyCostPeriod  # local import to avoid circular
+
+    # --- Determine currency and representative pricing from the latest non-degraded row ---
+
+    latest_period: EnergyCostPeriod | None = (
+        session.query(EnergyCostPeriod)
+        .filter(EnergyCostPeriod.degraded.is_(False))
+        .order_by(EnergyCostPeriod.period_start.desc())
+        .first()
+    )
+
+    currency: str = "EUR"
+    if latest_period is not None and latest_period.currency:
+        currency = latest_period.currency
+
+    # --- Shared DeviceInfo (2-element identifiers — required by ha_discovery.py [1] access) ---
+    device_info = DeviceInfo(
+        identifiers=("energy-cost", "energy-cost"),
+        name="Energy Cost",
+    )
+
+    # --- value_getter: current buy price ---
+
+    def _make_buy_price_getter() -> Callable[["Session"], Any]:
+        """Return a getter for the current buy price per kWh."""
+        def _getter(sess: "Session") -> Any:
+            from app.models.energy import EnergyCostPeriod as _ECP
+
+            period = (
+                sess.query(_ECP)
+                .filter(_ECP.degraded.is_(False))
+                .order_by(_ECP.period_start.desc())
+                .first()
+            )
+            if period is None or not period.pricing:
+                return None
+            snap = period.pricing
+            kind = snap.get("kind")
+            if kind == "tibber":
+                raw = snap.get("buy")
+            elif kind == "manual":
+                raw = snap.get("buy_normal")
+            else:
+                # Unknown kind — attempt common keys gracefully.
+                raw = snap.get("buy") or snap.get("buy_normal")
+            if raw is None:
+                return None
+            try:
+                return float(raw)
+            except (TypeError, ValueError):
+                return None
+
+        return _getter
+
+    # --- value_getter: current sell price ---
+
+    def _make_sell_price_getter() -> Callable[["Session"], Any]:
+        """Return a getter for the current sell price per kWh."""
+        def _getter(sess: "Session") -> Any:
+            from app.models.energy import EnergyCostPeriod as _ECP
+
+            period = (
+                sess.query(_ECP)
+                .filter(_ECP.degraded.is_(False))
+                .order_by(_ECP.period_start.desc())
+                .first()
+            )
+            if period is None or not period.pricing:
+                return None
+            snap = period.pricing
+            kind = snap.get("kind")
+            if kind == "tibber":
+                raw = snap.get("sell")
+            elif kind == "manual":
+                raw = snap.get("sell_normal")
+            else:
+                raw = snap.get("sell") or snap.get("sell_normal")
+            if raw is None:
+                return None
+            try:
+                return float(raw)
+            except (TypeError, ValueError):
+                return None
+
+        return _getter
+
+    # --- value_getter: cumulative import cost ---
+
+    def _make_import_cost_getter() -> Callable[["Session"], Any]:
+        """Return a getter for the cumulative import cost (non-degraded rows only)."""
+        def _getter(sess: "Session") -> Any:
+            from app.models.energy import EnergyCostPeriod as _ECP
+            from sqlalchemy import func as _func
+
+            total = sess.query(_func.sum(_ECP.import_cost)).filter(
+                _ECP.degraded.is_(False)
+            ).scalar()
+            if total is None:
+                return None
+            return float(total)
+
+        return _getter
+
+    # --- value_getter: cumulative export revenue ---
+
+    def _make_export_revenue_getter() -> Callable[["Session"], Any]:
+        """Return a getter for the cumulative export revenue (non-degraded rows only)."""
+        def _getter(sess: "Session") -> Any:
+            from app.models.energy import EnergyCostPeriod as _ECP
+            from sqlalchemy import func as _func
+
+            total = sess.query(_func.sum(_ECP.export_revenue)).filter(
+                _ECP.degraded.is_(False)
+            ).scalar()
+            if total is None:
+                return None
+            return float(total)
+
+        return _getter
+
+    # Price unit string: "<currency>/kWh"
+    price_unit = f"{currency}/kWh"
+
+    entities: list[ExposableEntity] = [
+        ExposableEntity(
+            key="energy.buy_price_now",
+            component="sensor",
+            device=device_info,
+            device_class=None,
+            unit=price_unit,
+            name="Energy Buy Price Now",
+            value_getter=_make_buy_price_getter(),
+            state_class="measurement",
+        ),
+        ExposableEntity(
+            key="energy.sell_price_now",
+            component="sensor",
+            device=device_info,
+            device_class=None,
+            unit=price_unit,
+            name="Energy Sell Price Now",
+            value_getter=_make_sell_price_getter(),
+            state_class="measurement",
+        ),
+        ExposableEntity(
+            key="energy.import_cost_total",
+            component="sensor",
+            device=device_info,
+            device_class="monetary",
+            unit=currency,
+            name="Energy Import Cost Total",
+            value_getter=_make_import_cost_getter(),
+            state_class="total_increasing",
+        ),
+        ExposableEntity(
+            key="energy.export_revenue_total",
+            component="sensor",
+            device=device_info,
+            device_class="monetary",
+            unit=currency,
+            name="Energy Export Revenue Total",
+            value_getter=_make_export_revenue_getter(),
+            state_class="total_increasing",
+        ),
+    ]
+
+    return entities
+
+
+# Register the energy cost provider at module load time.
+register_provider(_energy_cost_provider)
diff --git a/app/main.py b/app/main.py
index 9a1b0de..28b1b20 100644
--- a/app/main.py
+++ b/app/main.py
@@ -109,6 +109,15 @@ def _run_scheduled_energy_cost() -> None:
     session: Session = session_local()
     try:
         compute_closed_periods(session)
+        # After billing periods are computed, push fresh energy-cost state values
+        # to MQTT/HA.  publish_states is internally guarded by _should_publish
+        # (MQTT disabled / not connected → no-op), so this never raises due to
+        # unconfigured MQTT and does not block the billing job.
+        try:
+            from app.services.ha_discovery import publish_states
+            publish_states(session)
+        except Exception:
+            logger.exception("_run_scheduled_energy_cost: publish_states failed (non-fatal)")
     except Exception:
         logger.exception("_run_scheduled_energy_cost: unexpected error")
     finally:
diff --git a/docs/design/m6-tibber-dynamic-energy.md b/docs/design/m6-tibber-dynamic-energy.md
index 577a4aa..4112a31 100644
--- a/docs/design/m6-tibber-dynamic-energy.md
+++ b/docs/design/m6-tibber-dynamic-energy.md
@@ -393,7 +393,7 @@ Phase D（API + 前端）
 - **Reviewer checklist**: Decimal 算钱；寄存器差为"末−初"；进出口分开；周期边界 UTC 刻钟；快照不可变（重算才覆盖，且显式）；版本选择按 `effective_from≤t0<effective_to`；job 不崩。
 
 ### M6-T08 — energy_cost expose provider + state 发布
-- **Status**: `todo` · **Depends**: M6-T07
+- **Status**: `done` · **Depends**: M6-T07
 - **Context**: 复用 M5 expose/Discovery，发当前买/卖价 + 累计成本/收入。
 - **Files**: `modify app/integrations/expose.py`（`_energy_cost_provider` + 注册）；`modify app/services/energy_cost.py` 或 `app/main.py`（算完顺带 `publish_states`）；`create tests/test_energy_expose.py`
 - **Steps**: provider 产出 §3.7 实体（`buy_price_now`/`sell_price_now`/`import_cost_total`/`export_revenue_total`），累计项 `total_increasing`+`monetary`+currency；`value_getter` 从 `energy_cost_period` 取；计费 job 后 `publish_states`。
diff --git a/tests/test_energy_expose.py b/tests/test_energy_expose.py
new file mode 100644
index 0000000..624bccb
--- /dev/null
+++ b/tests/test_energy_expose.py
@@ -0,0 +1,783 @@
+"""Tests for M6-T08: energy_cost expose provider + state publish.
+
+Coverage:
+  1. build_catalog contains all 4 energy_cost entities.
+  2. Cumulative entities (import_cost_total / export_revenue_total) have
+     state_class="total_increasing" and device_class="monetary".
+  3. All 4 energy_cost entities default to enabled=False (no toggle row).
+  4. value_getter: buy/sell price from pricing snapshot for tibber and manual kinds.
+  5. value_getter: cumulative SUM(import_cost) / SUM(export_revenue) from
+     non-degraded rows; degraded rows excluded.
+  6. value_getter returns None when no non-degraded period exists.
+  7. MQTT not enabled → publish_states is a no-op (no raises, no publish calls).
+  8. Integration: build_discovery_payload on an energy_cost entity does NOT raise
+     IndexError (validates 2-element identifiers).
+  9. Keys are stable fixed strings (not derived from mutable data or DB ids).
+ 10. Provider registered: energy_cost entities appear alongside modbus entities
+     in the full catalog.
+"""
+
+from __future__ import annotations
+
+from datetime import datetime, timezone
+from pathlib import Path
+from typing import Any
+from unittest.mock import MagicMock, patch
+
+import pytest
+from alembic import command
+from alembic.config import Config
+from sqlalchemy import create_engine
+from sqlalchemy.orm import Session
+
+
+# ---------------------------------------------------------------------------
+# Helpers
+# ---------------------------------------------------------------------------
+
+
+def _make_app_alembic_config(database_url: str) -> Config:
+    cfg = Config("alembic_app.ini")
+    cfg.set_main_option("sqlalchemy.url", database_url)
+    return cfg
+
+
+def _make_period(
+    session: Session,
+    *,
+    period_start: datetime,
+    import_cost: float = 0.10,
+    export_revenue: float = 0.05,
+    net_cost: float = 0.05,
+    currency: str = "EUR",
+    pricing: dict | None = None,
+    degraded: bool = False,
+    d1_kwh: float = 0.5,
+    d2_kwh: float = 0.3,
+    r1_kwh: float = 0.1,
+    r2_kwh: float = 0.1,
+) -> Any:
+    """Insert an EnergyCostPeriod row and return it (session not committed)."""
+    from app.models.energy import EnergyCostPeriod
+
+    now = datetime.now(tz=timezone.utc)
+    p = EnergyCostPeriod(
+        period_start=period_start,
+        d1_kwh=d1_kwh,
+        d2_kwh=d2_kwh,
+        r1_kwh=r1_kwh,
+        r2_kwh=r2_kwh,
+        import_cost=import_cost,
+        export_revenue=export_revenue,
+        net_cost=net_cost,
+        currency=currency,
+        pricing=pricing or {},
+        contract_version_id=None,
+        degraded=degraded,
+        computed_at=now,
+    )
+    session.add(p)
+    session.flush()
+    return p
+
+
+def _make_settings(
+    *,
+    mqtt_enabled: bool = True,
+    ha_discovery_enabled: bool = True,
+    ha_discovery_prefix: str = "homeassistant",
+) -> MagicMock:
+    s = MagicMock()
+    s.mqtt_enabled = mqtt_enabled
+    s.ha_discovery_enabled = ha_discovery_enabled
+    s.ha_discovery_prefix = ha_discovery_prefix
+    return s
+
+
+def _make_mock_manager(*, is_connected: bool = True) -> MagicMock:
+    mgr = MagicMock()
+    mgr.is_connected = is_connected
+    return mgr
+
+
+# ---------------------------------------------------------------------------
+# Fixtures
+# ---------------------------------------------------------------------------
+
+
+@pytest.fixture()
+def energy_db(tmp_path: Path):
+    """Temporary SQLite DB at Alembic head for energy expose tests."""
+    db_path = tmp_path / "energy_expose_test.db"
+    db_url = f"sqlite:///{db_path}"
+    command.upgrade(_make_app_alembic_config(db_url), "head")
+    engine = create_engine(db_url, connect_args={"check_same_thread": False})
+    yield engine
+    engine.dispose()
+
+
+# ---------------------------------------------------------------------------
+# 1. build_catalog contains all 4 energy_cost entities
+# ---------------------------------------------------------------------------
+
+
+def test_build_catalog_contains_4_energy_cost_entities(energy_db) -> None:
+    """build_catalog must include all 4 energy_cost sensor entities."""
+    from app.integrations.expose import build_catalog
+
+    with Session(energy_db) as session:
+        catalog = build_catalog(session)
+
+    energy_keys = {e.entity.key for e in catalog if e.entity.key.startswith("energy.")}
+    expected_keys = {
+        "energy.buy_price_now",
+        "energy.sell_price_now",
+        "energy.import_cost_total",
+        "energy.export_revenue_total",
+    }
+    assert expected_keys == energy_keys, (
+        f"Expected energy keys {expected_keys!r}, got {energy_keys!r}"
+    )
+
+
+# ---------------------------------------------------------------------------
+# 2. Cumulative entities have correct state_class and device_class
+# ---------------------------------------------------------------------------
+
+
+def test_cumulative_entities_have_total_increasing_state_class(energy_db) -> None:
+    """import_cost_total and export_revenue_total must have state_class='total_increasing'."""
+    from app.integrations.expose import build_catalog
+
+    with Session(energy_db) as session:
+        catalog = build_catalog(session)
+
+    cumulative_keys = {"energy.import_cost_total", "energy.export_revenue_total"}
+    for entry in catalog:
+        if entry.entity.key in cumulative_keys:
+            assert entry.entity.state_class == "total_increasing", (
+                f"Entity {entry.entity.key!r} must have state_class='total_increasing', "
+                f"got {entry.entity.state_class!r}"
+            )
+
+
+def test_cumulative_entities_have_monetary_device_class(energy_db) -> None:
+    """import_cost_total and export_revenue_total must have device_class='monetary'."""
+    from app.integrations.expose import build_catalog
+
+    with Session(energy_db) as session:
+        catalog = build_catalog(session)
+
+    cumulative_keys = {"energy.import_cost_total", "energy.export_revenue_total"}
+    for entry in catalog:
+        if entry.entity.key in cumulative_keys:
+            assert entry.entity.device_class == "monetary", (
+                f"Entity {entry.entity.key!r} must have device_class='monetary', "
+                f"got {entry.entity.device_class!r}"
+            )
+
+
+def test_all_energy_entities_are_sensors(energy_db) -> None:
+    """All 4 energy_cost entities must have component='sensor'."""
+    from app.integrations.expose import build_catalog
+
+    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.component == "sensor", (
+            f"Expected component='sensor' for {entry.entity.key!r}, "
+            f"got {entry.entity.component!r}"
+        )
+
+
+# ---------------------------------------------------------------------------
+# 3. Default enabled=False (no toggle row)
+# ---------------------------------------------------------------------------
+
+
+def test_energy_cost_entities_default_to_disabled(energy_db) -> None:
+    """All 4 energy_cost entities must default to enabled=False (no toggle row)."""
+    from app.integrations.expose import build_catalog
+
+    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.enabled is False, (
+            f"Entity {entry.entity.key!r} must default to enabled=False "
+            f"when no toggle row exists, got enabled={entry.enabled!r}"
+        )
+
+
+# ---------------------------------------------------------------------------
+# 4. value_getter: current price from tibber pricing snapshot
+# ---------------------------------------------------------------------------
+
+
+def test_buy_price_getter_reads_tibber_snapshot(energy_db) -> None:
+    """buy_price_now value_getter must return the 'buy' price from tibber snapshot."""
+    from app.integrations.expose import build_catalog
+
+    t0 = datetime(2025, 1, 1, 12, 0, tzinfo=timezone.utc)
+    tibber_pricing = {
+        "kind": "tibber",
+        "buy": "0.2850",
+        "sell": "0.1200",
+        "energy_tax": "0.1234",
+        "sell_adjust": "0.0100",
+        "total": "0.2850",
+        "tibber_price_starts_at": t0.isoformat(),
+        "tibber_price_id": 1,
+    }
+
+    with Session(energy_db) as session:
+        _make_period(
+            session,
+            period_start=t0,
+            import_cost=0.10,
+            export_revenue=0.05,
+            currency="EUR",
+            pricing=tibber_pricing,
+            degraded=False,
+        )
+        session.commit()
+
+    with Session(energy_db) as session:
+        catalog = build_catalog(session)
+        buy_entry = next(
+            e for e in catalog if e.entity.key == "energy.buy_price_now"
+        )
+        value = buy_entry.entity.value_getter(session)
+
+    assert value == pytest.approx(0.2850), (
+        f"Expected buy price 0.2850, got {value!r}"
+    )
+
+
+def test_sell_price_getter_reads_tibber_snapshot(energy_db) -> None:
+    """sell_price_now value_getter must return the 'sell' price from tibber snapshot."""
+    from app.integrations.expose import build_catalog
+
+    t0 = datetime(2025, 1, 1, 12, 15, tzinfo=timezone.utc)
+    tibber_pricing = {
+        "kind": "tibber",
+        "buy": "0.3100",
+        "sell": "0.1500",
+        "energy_tax": "0.1234",
+        "sell_adjust": "0.0100",
+        "total": "0.3100",
+        "tibber_price_starts_at": t0.isoformat(),
+        "tibber_price_id": 2,
+    }
+
+    with Session(energy_db) as session:
+        _make_period(
+            session,
+            period_start=t0,
+            import_cost=0.12,
+            export_revenue=0.07,
+            currency="EUR",
+            pricing=tibber_pricing,
+            degraded=False,
+        )
+        session.commit()
+
+    with Session(energy_db) as session:
+        catalog = build_catalog(session)
+        sell_entry = next(
+            e for e in catalog if e.entity.key == "energy.sell_price_now"
+        )
+        value = sell_entry.entity.value_getter(session)
+
+    assert value == pytest.approx(0.1500), (
+        f"Expected sell price 0.1500, got {value!r}"
+    )
+
+
+def test_buy_price_getter_reads_manual_snapshot(energy_db) -> None:
+    """buy_price_now value_getter must return 'buy_normal' from manual pricing snapshot."""
+    from app.integrations.expose import build_catalog
+
+    t0 = datetime(2025, 2, 1, 8, 0, tzinfo=timezone.utc)
+    manual_pricing = {
+        "kind": "manual",
+        "buy_dal": "0.2500",
+        "buy_normal": "0.2700",
+        "sell_dal": "0.0900",
+        "sell_normal": "0.0950",
+        "energy_tax": "0.1234",
+        "ode": "0.0015",
+    }
+
+    with Session(energy_db) as session:
+        _make_period(
+            session,
+            period_start=t0,
+            import_cost=0.08,
+            export_revenue=0.03,
+            currency="EUR",
+            pricing=manual_pricing,
+            degraded=False,
+        )
+        session.commit()
+
+    with Session(energy_db) as session:
+        catalog = build_catalog(session)
+        buy_entry = next(
+            e for e in catalog if e.entity.key == "energy.buy_price_now"
+        )
+        value = buy_entry.entity.value_getter(session)
+
+    # buy_normal is the representative buy price for manual strategy
+    assert value == pytest.approx(0.2700), (
+        f"Expected buy_normal 0.2700, got {value!r}"
+    )
+
+
+def test_sell_price_getter_reads_manual_snapshot(energy_db) -> None:
+    """sell_price_now value_getter must return 'sell_normal' from manual pricing snapshot."""
+    from app.integrations.expose import build_catalog
+
+    t0 = datetime(2025, 2, 1, 8, 15, tzinfo=timezone.utc)
+    manual_pricing = {
+        "kind": "manual",
+        "buy_dal": "0.2500",
+        "buy_normal": "0.2700",
+        "sell_dal": "0.0900",
+        "sell_normal": "0.0950",
+        "energy_tax": "0.1234",
+        "ode": "0.0015",
+    }
+
+    with Session(energy_db) as session:
+        _make_period(
+            session,
+            period_start=t0,
+            import_cost=0.09,
+            export_revenue=0.04,
+            currency="EUR",
+            pricing=manual_pricing,
+            degraded=False,
+        )
+        session.commit()
+
+    with Session(energy_db) as session:
+        catalog = build_catalog(session)
+        sell_entry = next(
+            e for e in catalog if e.entity.key == "energy.sell_price_now"
+        )
+        value = sell_entry.entity.value_getter(session)
+
+    # sell_normal is the representative sell price for manual strategy
+    assert value == pytest.approx(0.0950), (
+        f"Expected sell_normal 0.0950, got {value!r}"
+    )
+
+
+# ---------------------------------------------------------------------------
+# 5. value_getter: cumulative SUM — degraded rows excluded
+# ---------------------------------------------------------------------------
+
+
+def test_import_cost_total_sums_non_degraded_rows(energy_db) -> None:
+    """import_cost_total must return SUM of non-degraded import_cost values only."""
+    from app.integrations.expose import build_catalog
+
+    t0 = datetime(2025, 3, 1, 6, 0, tzinfo=timezone.utc)
+    t1 = datetime(2025, 3, 1, 6, 15, tzinfo=timezone.utc)
+    t2 = datetime(2025, 3, 1, 6, 30, tzinfo=timezone.utc)
+
+    with Session(energy_db) as session:
+        # Two good rows: 0.10 + 0.20 = 0.30
+        _make_period(session, period_start=t0, import_cost=0.10, degraded=False)
+        _make_period(session, period_start=t1, import_cost=0.20, degraded=False)
+        # One degraded row (import_cost=0.0, should be excluded):
+        _make_period(session, period_start=t2, import_cost=0.0, degraded=True)
+        session.commit()
+
+    with Session(energy_db) as session:
+        catalog = build_catalog(session)
+        import_entry = next(
+            e for e in catalog if e.entity.key == "energy.import_cost_total"
+        )
+        value = import_entry.entity.value_getter(session)
+
+    assert value == pytest.approx(0.30), (
+        f"Expected cumulative import_cost 0.30 (non-degraded only), got {value!r}"
+    )
+
+
+def test_export_revenue_total_sums_non_degraded_rows(energy_db) -> None:
+    """export_revenue_total must return SUM of non-degraded export_revenue values only."""
+    from app.integrations.expose import build_catalog
+
+    t0 = datetime(2025, 3, 2, 6, 0, tzinfo=timezone.utc)
+    t1 = datetime(2025, 3, 2, 6, 15, tzinfo=timezone.utc)
+    t2 = datetime(2025, 3, 2, 6, 30, tzinfo=timezone.utc)
+
+    with Session(energy_db) as session:
+        # Two good rows: 0.05 + 0.07 = 0.12
+        _make_period(session, period_start=t0, export_revenue=0.05, degraded=False)
+        _make_period(session, period_start=t1, export_revenue=0.07, degraded=False)
+        # One degraded row (should be excluded):
+        _make_period(session, period_start=t2, export_revenue=0.0, degraded=True)
+        session.commit()
+
+    with Session(energy_db) as session:
+        catalog = build_catalog(session)
+        export_entry = next(
+            e for e in catalog if e.entity.key == "energy.export_revenue_total"
+        )
+        value = export_entry.entity.value_getter(session)
+
+    assert value == pytest.approx(0.12), (
+        f"Expected cumulative export_revenue 0.12 (non-degraded only), got {value!r}"
+    )
+
+
+def test_cumulative_getter_excludes_degraded_import_cost_row(energy_db) -> None:
+    """A degraded row with non-zero import_cost must NOT contribute to the cumulative sum.
+
+    This verifies the exclusion filter on degraded=True rows.
+    (In practice compute_period writes 0.0 for degraded rows; but if a row was
+    previously successful and then set degraded, its import_cost could be non-zero.)
+    """
+    from app.integrations.expose import build_catalog
+
+    t0 = datetime(2025, 4, 1, 10, 0, tzinfo=timezone.utc)
+    t1 = datetime(2025, 4, 1, 10, 15, tzinfo=timezone.utc)
+
+    with Session(energy_db) as session:
+        _make_period(session, period_start=t0, import_cost=0.50, degraded=False)
+        # A row that is degraded but somehow has a non-zero import_cost (edge case):
+        _make_period(session, period_start=t1, import_cost=0.99, degraded=True)
+        session.commit()
+
+    with Session(energy_db) as session:
+        catalog = build_catalog(session)
+        import_entry = next(
+            e for e in catalog if e.entity.key == "energy.import_cost_total"
+        )
+        value = import_entry.entity.value_getter(session)
+
+    # Only the non-degraded row should contribute: 0.50
+    assert value == pytest.approx(0.50), (
+        f"Degraded row must not be included in SUM; expected 0.50, got {value!r}"
+    )
+
+
+# ---------------------------------------------------------------------------
+# 6. value_getter returns None when no non-degraded period exists
+# ---------------------------------------------------------------------------
+
+
+def test_buy_price_getter_returns_none_with_no_periods(energy_db) -> None:
+    """buy_price_now value_getter must return None when no non-degraded periods exist."""
+    from app.integrations.expose import build_catalog
+
+    with Session(energy_db) as session:
+        catalog = build_catalog(session)
+        buy_entry = next(
+            e for e in catalog if e.entity.key == "energy.buy_price_now"
+        )
+        value = buy_entry.entity.value_getter(session)
+
+    assert value is None, f"Expected None with no periods, got {value!r}"
+
+
+def test_sell_price_getter_returns_none_with_no_periods(energy_db) -> None:
+    """sell_price_now value_getter must return None when no non-degraded periods exist."""
+    from app.integrations.expose import build_catalog
+
+    with Session(energy_db) as session:
+        catalog = build_catalog(session)
+        sell_entry = next(
+            e for e in catalog if e.entity.key == "energy.sell_price_now"
+        )
+        value = sell_entry.entity.value_getter(session)
+
+    assert value is None, f"Expected None with no periods, got {value!r}"
+
+
+def test_import_cost_getter_returns_none_with_no_non_degraded_periods(energy_db) -> None:
+    """import_cost_total value_getter must return None when only degraded rows exist."""
+    from app.integrations.expose import build_catalog
+
+    t0 = datetime(2025, 5, 1, 0, 0, tzinfo=timezone.utc)
+
+    with Session(energy_db) as session:
+        # Only a degraded row → SUM returns None (no rows to aggregate)
+        _make_period(session, period_start=t0, import_cost=0.0, degraded=True)
+        session.commit()
+
+    with Session(energy_db) as session:
+        catalog = build_catalog(session)
+        import_entry = next(
+            e for e in catalog if e.entity.key == "energy.import_cost_total"
+        )
+        value = import_entry.entity.value_getter(session)
+
+    assert value is None, (
+        f"Expected None when only degraded rows exist, got {value!r}"
+    )
+
+
+def test_export_revenue_getter_returns_none_with_no_periods(energy_db) -> None:
+    """export_revenue_total value_getter must return None when no periods at all."""
+    from app.integrations.expose import build_catalog
+
+    with Session(energy_db) as session:
+        catalog = build_catalog(session)
+        export_entry = next(
+            e for e in catalog if e.entity.key == "energy.export_revenue_total"
+        )
+        value = export_entry.entity.value_getter(session)
+
+    assert value is None, f"Expected None with no periods, got {value!r}"
+
+
+# ---------------------------------------------------------------------------
+# 7. MQTT not enabled → publish_states is a no-op
+# ---------------------------------------------------------------------------
+
+
+def test_publish_states_noop_when_mqtt_disabled() -> None:
+    """publish_states must be a no-op and not raise when MQTT is disabled."""
+    settings = _make_settings(mqtt_enabled=False, ha_discovery_enabled=True)
+    mock_mgr = _make_mock_manager(is_connected=False)
+
+    with (
+        patch("app.services.ha_discovery.build_runtime_settings", return_value=settings),
+        patch("app.services.ha_discovery.mqtt_manager", mock_mgr),
+    ):
+        from app.services.ha_discovery import publish_states
+        from sqlalchemy import create_engine as _ce
+
+        eng = _ce("sqlite:///:memory:")
+        with Session(eng) as session:
+            publish_states(session)  # must not raise
+
+    mock_mgr.publish.assert_not_called()
+
+
+def test_publish_states_noop_when_ha_discovery_disabled() -> None:
+    """publish_states must be a no-op when ha_discovery_enabled=False."""
+    settings = _make_settings(mqtt_enabled=True, ha_discovery_enabled=False)
+    mock_mgr = _make_mock_manager(is_connected=True)
+
+    with (
+        patch("app.services.ha_discovery.build_runtime_settings", return_value=settings),
+        patch("app.services.ha_discovery.mqtt_manager", mock_mgr),
+    ):
+        from app.services.ha_discovery import publish_states
+        from sqlalchemy import create_engine as _ce
+
+        eng = _ce("sqlite:///:memory:")
+        with Session(eng) as session:
+            publish_states(session)  # must not raise
+
+    mock_mgr.publish.assert_not_called()
+
+
+def test_publish_states_noop_when_not_connected() -> None:
+    """publish_states must be a no-op when MQTT is configured but not connected."""
+    settings = _make_settings(mqtt_enabled=True, ha_discovery_enabled=True)
+    mock_mgr = _make_mock_manager(is_connected=False)
+
+    with (
+        patch("app.services.ha_discovery.build_runtime_settings", return_value=settings),
+        patch("app.services.ha_discovery.mqtt_manager", mock_mgr),
+    ):
+        from app.services.ha_discovery import publish_states
+        from sqlalchemy import create_engine as _ce
+
+        eng = _ce("sqlite:///:memory:")
+        with Session(eng) as session:
+            publish_states(session)  # must not raise
+
+    mock_mgr.publish.assert_not_called()
+
+
+# ---------------------------------------------------------------------------
+# 8. Integration: build_discovery_payload does not IndexError for energy entities
+# ---------------------------------------------------------------------------
+
+
+def test_build_discovery_payload_no_index_error_for_energy_entities(energy_db) -> None:
+    """build_discovery_payload must NOT raise IndexError for energy_cost entities.
+
+    Validates that the 2-element identifiers=('energy-cost', 'energy-cost') tuple
+    satisfies the ha_discovery.py requirement to access identifiers[1] as node_id.
+    This is a regression guard: if _energy_cost_provider used a 1-element tuple,
+    this call would raise IndexError.
+    """
+    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, "Expected 4 energy_cost entities in catalog"
+
+    for entry in energy_entries:
+        # Must not raise — specifically no IndexError from identifiers[1]
+        topic, config = build_discovery_payload(entry.entity, "homeassistant")
+
+        # Basic sanity checks on the result.
+        # Note: ha_discovery._node_id() replaces hyphens with underscores, so
+        # identifiers[1]="energy-cost" → node_id="energy_cost" in the topic.
+        assert "energy_cost" in topic, (
+            f"Expected 'energy_cost' (hyphen→underscore) in discovery topic, got {topic!r}"
+        )
+        assert topic.endswith("/config"), (
+            f"Discovery topic must end with /config, got {topic!r}"
+        )
+        assert "unique_id" in config
+        # unique_id = "<identifiers[1]>_<key.replace('.','_')>"
+        # identifiers[1]="energy-cost" (hyphens NOT replaced in unique_id, only in node_id)
+        assert "energy" in config["unique_id"], (
+            f"unique_id must contain 'energy', got {config['unique_id']!r}"
+        )
+        assert "device" in config
+        assert "energy-cost" in config["device"]["identifiers"]
+
+
+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
+    from app.services.ha_discovery import build_discovery_payload
+
+    device = DeviceInfo(identifiers=("energy-cost", "energy-cost"), name="Energy Cost")
+    entity = ExposableEntity(
+        key="energy.import_cost_total",
+        component="sensor",
+        device=device,
+        device_class="monetary",
+        unit="EUR",
+        name="Energy Import Cost Total",
+        state_class="total_increasing",
+    )
+
+    topic, config = build_discovery_payload(entity, prefix="homeassistant")
+
+    # node_id: hyphens replaced with underscores → "energy_cost"
+    expected_node = "energy_cost"
+    assert f"/{expected_node}/" in topic, (
+        f"Expected node_id 'energy_cost' in topic {topic!r}"
+    )
+    assert topic.startswith("homeassistant/sensor/"), (
+        f"Topic must start with homeassistant/sensor/, got {topic!r}"
+    )
+    assert "state_class" in config
+    assert config["state_class"] == "total_increasing"
+    assert config.get("device_class") == "monetary"
+
+
+# ---------------------------------------------------------------------------
+# 9. Key stability: fixed strings, not derived from mutable data
+# ---------------------------------------------------------------------------
+
+
+def test_energy_entity_keys_are_stable_fixed_strings(energy_db) -> None:
+    """Entity keys must be fixed strings, not derived from DB ids or session state."""
+    from app.integrations.expose import build_catalog
+    from datetime import timedelta
+
+    t0 = datetime(2025, 6, 1, 0, 0, tzinfo=timezone.utc)
+    t1 = t0 + timedelta(minutes=15)
+
+    # Insert two periods with different currencies to ensure key does not drift
+    with Session(energy_db) as session:
+        _make_period(
+            session,
+            period_start=t0,
+            currency="EUR",
+            pricing={"kind": "tibber", "buy": "0.25", "sell": "0.10"},
+            degraded=False,
+        )
+        session.commit()
+
+    with Session(energy_db) as session:
+        catalog1 = build_catalog(session)
+        keys1 = {e.entity.key for e in catalog1 if e.entity.key.startswith("energy.")}
+
+    with Session(energy_db) as session:
+        _make_period(
+            session,
+            period_start=t1,
+            currency="EUR",
+            pricing={"kind": "tibber", "buy": "0.30", "sell": "0.12"},
+            degraded=False,
+        )
+        session.commit()
+
+    with Session(energy_db) as session:
+        catalog2 = build_catalog(session)
+        keys2 = {e.entity.key for e in catalog2 if e.entity.key.startswith("energy.")}
+
+    # Keys must be identical across both builds (price changed, key must not)
+    assert keys1 == keys2, (
+        f"Entity keys must be stable across different data states. "
+        f"First: {keys1!r}, Second: {keys2!r}"
+    )
+
+    expected_keys = {
+        "energy.buy_price_now",
+        "energy.sell_price_now",
+        "energy.import_cost_total",
+        "energy.export_revenue_total",
+    }
+    assert keys1 == expected_keys, (
+        f"Energy keys must be exactly {expected_keys!r}, got {keys1!r}"
+    )
+
+
+# ---------------------------------------------------------------------------
+# 10. Provider registered: energy_cost entities co-exist with modbus entities
+# ---------------------------------------------------------------------------
+
+
+def test_energy_provider_registered_alongside_modbus(energy_db) -> None:
+    """Both modbus and energy_cost providers must be registered and produce entities."""
+    from app.models.modbus import ModbusDevice
+    from app.integrations.expose import build_catalog
+
+    now = datetime.now(tz=timezone.utc)
+    with Session(energy_db) as session:
+        device = ModbusDevice(
+            uuid="cccccccc-0000-0000-0000-000000000099",
+            friendly_name="Co-exist Meter",
+            host="10.0.0.1",
+            port=502,
+            unit_id=1,
+            profile="sdm120",
+            poll_interval_s=5,
+            enabled=True,
+            created_at=now,
+            updated_at=now,
+        )
+        session.add(device)
+        session.commit()
+
+    with Session(energy_db) as session:
+        catalog = build_catalog(session)
+
+    all_keys = {e.entity.key for e in catalog}
+
+    # Modbus entities present
+    assert any(k.startswith("modbus.") for k in all_keys), (
+        "Expected modbus entities in catalog"
+    )
+    # Energy entities present
+    assert "energy.import_cost_total" in all_keys, (
+        "Expected energy.import_cost_total in catalog"
+    )
+    assert "energy.export_revenue_total" in all_keys, (
+        "Expected energy.export_revenue_total in catalog"
+    )
diff --git a/tests/test_expose_catalog.py b/tests/test_expose_catalog.py
index 0b886c0..29bafa8 100644
--- a/tests/test_expose_catalog.py
+++ b/tests/test_expose_catalog.py
@@ -203,16 +203,29 @@ def test_register_provider_direct_call():
 
 
 def test_build_catalog_empty_with_no_devices(expose_db):
-    """build_catalog with no enabled devices must return an empty list."""
+    """build_catalog with no enabled modbus devices must contain no modbus entities.
+
+    The energy_cost provider is always registered and always produces its 4 entities
+    regardless of device state, so the catalog will not be empty.  This test checks
+    that no *modbus* entities are present when there are no enabled modbus devices.
+    """
     from app.integrations.expose import build_catalog
 
     with Session(expose_db) as session:
         catalog = build_catalog(session)
 
-    # The modbus provider will find no enabled devices; other providers may
-    # add entries only if registered.  Since we only register the modbus
-    # provider at module load, the result should be empty.
-    assert catalog == []
+    # The modbus provider finds no enabled devices → no modbus entities.
+    # The energy_cost provider always produces 4 entities, so the catalog is non-empty.
+    modbus_entities = [e for e in catalog if e.entity.key.startswith("modbus.")]
+    assert modbus_entities == [], (
+        "Expected no modbus entities when no modbus devices are enabled"
+    )
+
+    # The 4 energy_cost entities should always be present.
+    energy_keys = {e.entity.key for e in catalog if e.entity.key.startswith("energy.")}
+    assert len(energy_keys) == 4, (
+        f"Expected exactly 4 energy_cost entities, got {energy_keys!r}"
+    )
 
 
 def test_build_catalog_produces_entities_for_enabled_device(expose_db):