Replace night_surplus_wh with pv_start_battery_wh and forecast_min_battery_wh

- Rename the MQTT metric night_surplus_wh to pv_start_battery_wh
  (battery level in Wh above MIN_SOC at the next net-charging crossover)
- Add forecast_min_battery_wh (minimum battery level over the full
  forecast horizon, slot-by-slot simulation with floor/ceiling clamping)
- Extract ForecastMetrics class in src/batcontrol/forecast_metrics.py
- Rename test_night_surplus.py to test_pv_battery_metrics.py
- Remove legacy MQTT discovery entry for night_surplus_wh
- Add TODO(0.9.1) tombstone for backward-compat removal
- Add docs/integrations/forecast-metrics.md with metric descriptions,
  decision matrix, and use cases (heat pump, EVCC automation)
- Update docs/integrations/mqtt-api.md Solar Surplus section accordingly

Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

Author: MaStr

docs/integrations/forecast-metrics.md

@@ -0,0 +1,243 @@
+# Forecast Metrics
+
+`ForecastMetrics` is a stateless module that derives three battery indicators
+from the production/consumption forecast arrays and the current battery state.
+These indicators are published via MQTT and are intended to drive downstream
+automation decisions such as "should I run the heat pump now or save the battery
+for tomorrow's solar charge?"
+
+All three values are updated once per evaluation cycle (every 3 minutes by
+default). They are based on the same forecast window that the main optimizer
+uses, so their horizon is `min(max available price hours, max available solar
+hours)`.
+
+## The Three Metrics
+
+### `solar_surplus_wh` — Current-Window Solar Overflow
+
+**MQTT topic:** `{base}/solar_surplus_wh`
+
+Expected energy in Wh that the solar production in the **current production
+window** will generate above what the battery can absorb.
+
+- **solar_active = true (slot 0 is producing):** surplus is the net solar
+  production of the ongoing window minus the remaining free battery capacity.
+  `surplus > 0` means that PV power will be exported to the grid even if the
+  battery is managed optimally.
+- **solar_active = false (nighttime or break before next window):** surplus
+  accounts for the overnight discharge first — the battery will self-discharge
+  from consumption before solar starts, which creates room. `surplus > 0` means
+  even after that extra room is created the next solar window will still
+  overflow.
+
+A value of `0` means the battery can absorb everything the upcoming solar
+window produces. A value `> 0` means some PV will inevitably be exported; it
+is safe to run flexible loads (heat pump, EV charging) from the grid right now
+because the solar surplus will offset them.
+
+**Use case:** If `solar_surplus_wh >= estimated_load_wh`, running a flexible
+load (heat pump, EV charging via evcc) has no net grid cost over the forecast
+horizon. See [Use Cases](#use-cases) for concrete automation examples.
+
+---
+
+### `pv_start_battery_wh` — Battery Level at Next Charging Point
+
+**MQTT topic:** `{base}/pv_start_battery_wh`
+
+Battery level in Wh (above MIN_SOC) at the moment when solar production first
+exceeds household consumption (`net_consumption < 0`). This is the point where
+the battery transitions from discharging to charging.
+
+- Simulated slot-by-slot from the current moment forward.
+- If the battery hits `0` (MIN_SOC) before solar starts, the value is `0`.
+- If the battery has no net-charging slot in the forecast at all, the value
+  is `0`.
+- If slot 0 is already a net-charging slot (solar already exceeds
+  consumption), the value equals the current stored usable energy.
+
+**Use case:** "How much charge will the battery have left when solar starts
+tomorrow?" A low value (e.g. < 500 Wh) means flexible loads tonight should
+be reduced; a high value means overnight loads can run freely. See
+[Use Cases](#use-cases) for heat pump and evcc automation examples.
+
+!!! note
+    `pv_start_battery_wh` depends on `net_consumption < 0`, not just on
+    `production > 0`. The battery does not switch to charging until solar output
+    exceeds household consumption — on a partly-cloudy morning the cross-over can
+    happen later than sunrise.
+
+---
+
+### `forecast_min_battery_wh` — Forecast Minimum Battery Level
+
+**MQTT topic:** `{base}/forecast_min_battery_wh`
+
+The lowest battery level in Wh (above MIN_SOC) reached at any point during the
+entire forecast horizon, based on slot-by-slot simulation with proper
+floor/ceiling clamping.
+
+- A value of `0` means the battery is expected to hit MIN_SOC at some point in
+  the forecast — a signal that the system will be energy-constrained.
+- The simulation respects both the floor (MIN_SOC = 0 usable Wh) and the
+  ceiling (MAX_SOC = stored_usable + free_capacity), so multi-day
+  charge/discharge cycles are tracked correctly.
+- The horizon covers the full forecast window (same as the optimizer), not
+  just the next 24 hours.
+
+**Use case:** "Will the battery run out at any point in the planning horizon?"
+`forecast_min_battery_wh == 0` means batcontrol may need to grid-charge later —
+block or reduce flexible loads. A value comfortably above zero means the battery
+has buffer and loads can run. See [Use Cases](#use-cases).
+
+---
+
+## Decision Matrix
+
+The three metrics form a natural 2-D decision space for flexible load control:
+
+| `solar_surplus_wh` | `forecast_min_battery_wh` | Recommended action |
+|--------------------|--------------------------|-------------------|
+| > 0 | > 0 | Run flexible loads freely — PV will cover them and battery stays healthy |
+| > 0 | = 0 | PV surplus exists but battery will be short later — run light loads only |
+| = 0 | > 0 | No surplus but battery OK — use `pv_start_battery_wh` to judge night loads |
+| = 0 | = 0 | Constrained — block flexible loads, preserve battery |
+
+`pv_start_battery_wh` refines the third row: if it is high, the overnight
+discharge is gentle and a moderate flexible load (e.g. heat pump one cycle)
+is fine. If it is near zero, defer to the next solar window.
+
+---
+
+## MQTT Topics
+
+| Topic | Unit | Retained | Description |
+|-------|------|----------|-------------|
+| `{base}/solar_surplus_wh` | Wh | No | PV overflow that cannot be stored in the battery |
+| `{base}/solar_active` | bool | No | `true` if solar is producing in slot 0 |
+| `{base}/pv_start_battery_wh` | Wh | No | Battery level at next net-charging crossover |
+| `{base}/forecast_min_battery_wh` | Wh | No | Minimum battery level over entire forecast horizon |
+
+All values are published after each evaluation cycle (together with the
+inverter control decision). See [MQTT API](mqtt-api.md) for the full topic
+reference and configuration options.
+
+### Home Assistant Auto-Discovery
+
+The following HA entities are created automatically when
+`auto_discover_enable: true` is configured:
+
+- **Solar Surplus** — sensor (energy, Wh)
+- **Solar Active** — binary sensor (on/off diagnostic)
+- **PV Start Battery** — sensor (energy, Wh)
+- **Forecast Min Battery** — sensor (energy, Wh)
+
+---
+
+## Use Cases
+
+### Heat Pump Control
+
+A heat pump is a flexible load: it can pre-heat a buffer tank or run an extra
+heating cycle when energy is cheap or free — but running it at the wrong time
+can deplete the battery before the next solar window.
+
+The three metrics together answer the key questions for heat pump automation:
+
+**"Can I run a heating cycle right now without net grid cost?"**
+
+Check `solar_surplus_wh >= estimated_cycle_wh`. If yes, the PV will produce
+more than the battery can store anyway — running the heat pump consumes what
+would otherwise be exported. No additional grid draw over the forecast horizon.
+
+**"Is the battery safe enough for a cycle tonight?"**
+
+Check `pv_start_battery_wh`. A high value (e.g. > 2000 Wh) means the battery
+will still have a comfortable charge when solar starts tomorrow; the heat pump
+can run. A low value (< 500 Wh) means overnight consumption will nearly deplete
+the battery — defer to the next solar window.
+
+**"Should I block flexible loads entirely?"**
+
+Check `forecast_min_battery_wh == 0`. If the battery is expected to hit MIN_SOC
+at some point in the forecast, batcontrol may need to grid-charge later;
+flexible loads should wait.
+
+A simple Home Assistant automation combining all three:
+
+```yaml
+# Allow heat pump if PV will overflow OR battery is healthy and not forecast-constrained
+condition:
+  - condition: or
+    conditions:
+      - condition: numeric_state
+        entity_id: sensor.batcontrol_solar_surplus_wh
+        above: 1500          # surplus covers one heat pump cycle
+      - condition: and
+        conditions:
+          - condition: numeric_state
+            entity_id: sensor.batcontrol_pv_start_battery_wh
+            above: 1000      # enough battery charge at dawn
+          - condition: numeric_state
+            entity_id: sensor.batcontrol_forecast_min_battery_wh
+            above: 0         # battery not forecast to run empty
+```
+
+---
+
+### EV Charging via evcc
+
+[evcc](https://evcc.io) supports a `min_soc`/`target_soc` model as well as
+charging from PV surplus. The batcontrol metrics integrate naturally with evcc's
+MQTT API to let you charge the car only when it does not compete with the
+battery.
+
+**Scenario: charge only from true PV overflow**
+
+`solar_surplus_wh` tells you exactly how much energy the PV will produce above
+what the battery can absorb. Pass this value to evcc's `pv_action` or use it
+in an automation to set the evcc charging mode:
+
+- `solar_surplus_wh > 0` → switch evcc to **PV** mode (charge from surplus)
+- `solar_surplus_wh == 0` and `forecast_min_battery_wh > 0` → switch to **Min+PV**
+  (keep a minimum charge rate, fill up with PV where possible)
+- `forecast_min_battery_wh == 0` → switch to **Off** or **Min** only (battery
+  needs the energy)
+
+**Scenario: opportunistic overnight charge**
+
+Use `pv_start_battery_wh` to decide whether to allow evcc to draw from the
+battery overnight. If the battery is forecast to still be above a threshold at
+dawn, a slow overnight charge (e.g. 6 A / 1.4 kW) will not noticeably affect
+the next day's solar cycle.
+
+---
+
+### General Pattern for Any Flexible Load
+
+| Question | Metric to check | Threshold example |
+|---|---|---|
+| Is PV overflowing right now? | `solar_surplus_wh` | `> estimated_load_wh` |
+| Will battery survive the night? | `pv_start_battery_wh` | `> 500 Wh` |
+| Is the battery forecast-constrained? | `forecast_min_battery_wh` | `> 0` |
+
+All three are dimensioned in Wh, so you can directly compare them against the
+energy consumption of the load you want to schedule.
+
+---
+
+## Implementation Notes
+
+- All values are computed in `src/batcontrol/forecast_metrics.py` by the
+  `ForecastMetrics` class.
+- Slot 0 is time-adjusted: the elapsed fraction of the current interval is
+  subtracted so that a slot already 80% elapsed only contributes 20% of its
+  forecast energy.
+- `net_consumption = consumption - production`; negative = battery charging,
+  positive = battery discharging / grid draw.
+- `stored_usable_energy` is the energy above MIN_SOC. `free_capacity` is the
+  space between the current level and MAX_SOC.
+- The slot-by-slot simulation clamps at both ends:
+  `battery = max(0, min(stored_usable + free_capacity, battery - net))`.
+  A simple net-sum over slots would overestimate available energy because it
+  ignores that the battery cannot go below 0 or above MAX_SOC.

docs/integrations/mqtt-api.md

@@ -95,9 +95,13 @@ Batcontrol publishes data to the following topic structure (assuming base topic
 - `house/batcontrol/reserved_energy_capacity` - Energy reserved for discharge in Wh
 
 ### Solar Surplus Information
-- `house/batcontrol/solar_surplus_wh` - Expected solar surplus energy in Wh (>0 means usable surplus available)
+
+See [Forecast Metrics](forecast-metrics.md) for a detailed explanation of these values and how to use them for flexible load control.
+
+- `house/batcontrol/solar_surplus_wh` - Expected PV overflow in Wh that cannot be stored in the battery
 - `house/batcontrol/solar_active` - Whether solar is currently producing (`true`/`false`)
-- `house/batcontrol/night_surplus_wh` - Expected battery surplus in Wh at start of next production window
+- `house/batcontrol/pv_start_battery_wh` - Battery level in Wh at the next net-charging crossover
+- `house/batcontrol/forecast_min_battery_wh` - Minimum battery level in Wh over the entire forecast horizon
 
 ### Configuration Limits
 - `house/batcontrol/always_allow_discharge_limit` - Always discharge limit (0.0-1.0)

mkdocs.yml

@@ -64,6 +64,7 @@ nav:
       - Price Difference Calculation: features/price-difference-calculation.md
   - Integrations:
       - MQTT API: integrations/mqtt-api.md
+      - Forecast Metrics: integrations/forecast-metrics.md
       - MQTT Inverter: integrations/mqtt-inverter.md
       - EVCC Connection: integrations/evcc-connection.md
   - Development: