Feature: negative-price PV export curtailment via Growatt export-limit register

[johanzander]

Background

Raised by @Frank-Leysen in #126 (comment 2026-07-11T13:36:32Z): on his Luminus Dynamic contract, sell/injection price goes negative on sunny midday hours. His HA scripts block grid export when the injection price is <= 0, but BESS's optimizer has no equivalent — when solar surplus exceeds house load plus available battery charge room, the excess is unconditionally exported at whatever the sell price is, including negative.

Root cause (confirmed in code)

core/bess/dp_battery_algorithm.py — _state_transition / _compute_reward (STORE and IDLE dispositions, ~lines 192-223 and 294-314): surplus_exported = max(0.0, surplus - solar_to_battery) is computed unconditionally once solar exceeds min(max_charge_power_kw, remaining battery room). The reward term already penalizes this correctly when sell_price < 0 (-grid_exported * current_sell_price becomes a cost), so the DP "wants" to avoid it — but there is no action available to actually avoid it. There is no PV curtailment action anywhere in the DP action space, and no export-limit integration in growatt_sph_controller.py / growatt_min_controller.py.

Hardware capability (confirmed, not yet integrated)

Growatt hybrid inverters (SPH / MOD-XH series, includes Frank's MOD 5000TL3-XH GEN4) support a genuine export-limit / curtailment function via Modbus:

• Register 201: export limit value (or percentage on some models)
• Register 202: enable export-limit control

This uses a CT/smart-meter at the grid connection point (e.g. SPM-CT-E) to measure actual export and throttle PV/MPPT production down — it curtails real panel output, not just a grid-meter cap that gets bypassed once the battery is full. One forum report noted a small residual overshoot (~20-50W) even at a 0 W setting, so it isn't perfectly airtight, but it does what's needed here.

Sources:

• https://growatt.pl/wp-content/uploads/2020/01/Growatt-Export-Limitation-Guide.pdf
• https://eco-asset.eu/en/blog/post/2-growatt-s-sph-4-10ktl3-bh-up-totalsingle-export-limit-setting
• https://diysolarforum.com/threads/export-limit-with-growatt-inverters.31391/

Open question for Frank: does he have the CT/smart-meter hardware connected/available? This feature is only viable if that's present.

Proposed feature

When forecast sell price for a period is negative (or below a user-configurable floor), and the DP has no better use for the surplus (battery full / no headroom), write the Growatt export-limit register (0 or a configured cap) for that period instead of allowing full-rate export; clear/restore the limit once price is non-negative again. No change to the DP objective is needed — the reward function already penalizes negative-price export correctly; this is purely a new actuation path.

Scope

• New: export-limit register read/write in the Growatt GEN4 Modbus controller
• New: per-period decision of when to apply the limit (likely a straightforward "sell_price < 0" check against the existing schedule, not a DP action-space change)
• Confirm hardware prerequisite (CT/smart-meter) with affected users before committing to implementation

Not a regression — no current version of BESS ever had PV curtailment.

[Frank-Leysen]

Hi Johan,

Answering the open question on #269 — yes, we have external grid metering hardware installed.

---

Hardware

Chint energy meter at the grid connection point, RS485 to our Growatt MOD 5000TL3-XH (GEN4 Modbus). The inverter reads it as Meter 1 for grid import/export measurement and export-limit control.

Relevant HA entities (solax_modbus):

• select.pv_growatt_limit_grid_export — Disabled / Meter 1 / Meter 2 / CT Clamp
• number.pv_growatt_grid_export_limit — export cap (%)
• sensor.pv_growatt_total_grid_import / total_grid_export — lifetime grid energy

So the hardware prerequisite for export-limit / PV curtailment via Growatt registers should be met on our system.

---

How we block export today (HA scripts, not BESS)

We already solve the negative-injection problem in Home Assistant. BESS has no equivalent yet — that is why we reported it on #126.

Injection price source: sensor.prijs_elektriciteit_injectie_per_uur — hourly Luminus Dynamic export compensation from Belpex H:

injectie = Belpex_kWh × 1.018 − 0.012685   (EUR/kWh, 0% VAT)

Orchestrator automation: growatt_solar_export_belpex_hourly
Triggers: top of hour, injection price changes, battery SOC ≥ 98%, or net export detected while injection price ≤ 0.

Rule: if sensor.prijs_elektriciteit_injectie_per_uur <= 0 → run script.growatt_solar_export_block
If injection price > 0 (and not in evening battery-export window) → script.growatt_solar_export_unblock

growatt_solar_export_block (no paid negative export):

Battery SOC	Action
< 98%	growatt_herstel_normaal — Load First, PV charges battery, Limit Grid Export = Disabled (Meter off)
≥ 98% (full)	Meter 1 + grid_export_limit = 0% — Chint-measured zero export cap; surplus PV is curtailed at the inverter instead of being exported at negative injection prices

growatt_solar_export_unblock (positive injection hours): restore normal Load First (growatt_herstel_normaal, Meter off, PV 100%).

Evening battery→grid export is a separate path (growatt_export_start / Grid First TOU slots) and only runs when injection price > 0, SOC above our export-start threshold (~65%), and in the evening peak window — not during negative midday injection.

---

Relation to #269

Your proposed feature (export-limit register when sell < 0, battery full / no headroom) is essentially what our growatt_solar_export_block already does manually via Meter 1 + 0% export limit when SOC ≥ 98% and injection ≤ 0.

We would be happy to beta-test native BESS support for this on Growatt GEN4 Modbus once available — it would replace a fair amount of HA scripting.

Frank

[johanzander]

Live-test evidence from @Frank-Leysen on #126 (19 Jul 2026 17:06 CEST, v9.9.0b20): battery reached 100% SOC around 17:00, and from then on the plan is SOLAR_EXPORT with SOE flat at 15→15 kWh — i.e. once full, PV overflow is dumped to the grid at negative sell price (-0.012 EUR/kWh) with no way to curtail. Distinct from the (fixed) #336 case, which was export with headroom available — this is the unavoidable-once-full case this issue already tracks. Debug bundle: bess-debug-2026-07-19-170622.md, gist: https://gist.github.com/Frank-Leysen/499ff1b50749ef6f73cfe1a2272d4dd1

[Frank-Leysen]

Live evidence — SOLAR_EXPORT at negative sell price with battery nearly empty (25 Jul 2026 ~09:02 CEST)

@Frank-Leysen / Growatt MOD 5000TL3-XH, 15 kWh, Luminus Dynamic / Belpex H, 9.9.0b23, live TOU.

This is stronger than "full battery overflow": BESS is exporting PV at a negative injection price while SOC is only ~11% and charge rate is forced to 0%.

Observed (~09:01–09:02 CEST)

Strategic Intent	SOLAR_EXPORT (block planned ~07:00–11:29)
Battery Mode	Load First
SOC / SOE	11% / 1.6 of 15 kWh (Min SOC / Discharge Stop = 12%)
Charge Power Rate (BESS → inverter)	0%
Discharge Power Rate	100%
Live	PV ~1.4 kW, home ~0.5 kW, export ~0.7 kW, battery charge only ~0.2 kW
Injection / sell price (Luminus all-in)	−0.0014 EUR/kWh (09:00–10:00); more negative 10:00–17:00

Inverter schedule overview: current period SOLAR_EXPORT, Target SOC stuck ~11–15%, Charge % blank/0, Storage only planned from 11:30.

Energy Flow chart (sell price ON): blue export area overlaps the negative dashed sell-price trough through midday.

Desired

When sell/injection price is ≤ 0:

1. Do not choose SOLAR_EXPORT as the economic intent.
2. Prefer SOLAR_STORAGE / charge at available rate while SOC ≪ 100% (here ~11% — almost empty).
3. If surplus remains with battery full: curtail via Growatt export-limit / Meter 1 (Feature: negative-price PV export curtailment via Growatt export-limit register #269) — do not pay to inject.

Our previous HA scripts blocked export when injection <= 0. Paying to export while the battery still has ~13 kWh of free capacity is clearly wrong.

Evidence

• Debug: bess-debug-2026-07-25-090230.md
• Screenshots: Dashboard 09:01, Energy Flow (negative sell + export), Inverter schedule (SOLAR_EXPORT + Charge 0%)

Gist: https://gist.github.com/Frank-Leysen/499ff1b50749ef6f73cfe1a2272d4dd1

Happy to leave live for the rest of the negative window (until ~17:00) if you want more samples, or temporarily force Storage / charge 100% on our side for the day.

[Frank-Leysen]

Live retest — negative-price PV export still not curtailed (2 Aug 2026, 9.9.0b29)

@johanzander — following up on this closed issue. On our Growatt MOD 5000TL3-XH + Chint Meter 1 setup, native BESS still does not stop paid export when sell/injection ≤ 0. We had to fall back to our HA scripts again.

1) Morning — SOLAR_EXPORT with headroom (same class as 25 Jul evidence)

~10:30 CEST today:

Intent	SOLAR_EXPORT (planned until ~11:00)
Injection / sell	−0.010 €/kWh (negative through ~17:00)
SOC	~19% (far from full)
EMS charging rate	0% (BESS)
Live	PV ~2.9 kW, export ~0.9 kW, battery charge only ~0.1 kW

So again: paying to export PV while the battery still has ~12 kWh free capacity. Our temp HA guard (#269 workaround: force charge 100% when injection ≤ 0 and SOC < 95%) had to override this.

2) Afternoon — battery nearly full, still no Meter 1 / 0% curtailment

~15:08 CEST:

Injection / sell	−0.013 €/kWh
SOC	96%
EMS charge rate	100% (already charging)
Live	PV ~3.5 kW, charge ~0.7 kW, export ~1.5 kW

Limit Grid Export stayed Disabled / export limit not applied by BESS. We manually set Meter 1 + grid_export_limit = 0% → export dropped to 0 W within seconds. That is exactly the hardware path described earlier on this issue (and what our growatt_solar_export_block does when SOC ≥ 98%).

Our Belpex hourly orchestrator that normally runs that block is off during the BESS live test — so today is a clean observation of BESS-only behaviour.

Ask

Could this be reopened (or tracked as a follow-up)? From the live site it still looks like:

1. Do not choose SOLAR_EXPORT / charge 0% when sell ≤ 0 and SOC ≪ 100%.
2. When sell ≤ 0 and battery is full / no headroom: apply Growatt export-limit via Meter 1 (we confirmed the register path works on GEN4).

Happy to leave Meter 1 + 0% on until ~17:00 today and share another debug export if useful.

[Frank-Leysen]

Reopening based on live retest 2 Aug 2026 (9.9.0b29): SOLAR_EXPORT + charge 0% at SOC ~19% under negative sell, and no Meter 1/0% curtailment at SOC 96% — see comment above.

[Frank-Leysen]

Debug export for the comment above (2 Aug 2026, 15:16 CEST, 9.9.0b29):

https://gist.github.com/Frank-Leysen/09e099473f1ec31f77cc6d211e328c28

Captured shortly after we observed ~1.5 kW export at negative sell with SOC ~96–99%, and after we manually applied Meter 1 + export limit 0% on the inverter (HA side). Health snapshot in the bundle still shows overall OK; schedule history includes morning SOLAR_EXPORT slots through the negative-price window.