Solar PV systems in commercial and high-end residential buildings generate significant rooftop power that should preferentially feed controllable loads — heat pumps, EV chargers, immersion heaters — rather than export to the grid at low feed-in tariff rates. KNX building automation provides the real-time load control capability to implement self-consumption optimisation, and Fronius Symo/Gen24 inverters provide Modbus TCP access to PV yield data and grid import/export. This guide covers the Fronius Modbus register map, KNX/Modbus gateway setup, self-consumption logic, SG Ready heat pump integration, and energy dashboard implementation.

Fronius inverter Modbus TCP interface

Fronius Symo, Primo, and Gen24 series inverters support Modbus TCP (SunSpec protocol) on port 502. Enable in inverter webserver: Browse to inverter IP (DHCP or static, found via Fronius Solar.web app) > Settings > Modbus > Enable = Yes, Port = 502, SunSpec = Enabled. The Fronius Solar API v1 (HTTP REST) is also available but Modbus TCP is preferred for real-time integration (faster, no HTTP overhead). Fronius Symo 15.0-3-M: SunSpec Common Model (register 40001–40069) + Fronius-specific extensions. Key registers (Modbus address offset, holding registers, read via function code 03): 40070 = AC Power output (int16, Watts), 40071 = AC Power factor, 40080 = AC Energy today (uint32, Wh), 40100 = DC Power input total (int16, Watts from all MPPT). For Gen24 with smart meter: 40225 = Grid power (int16, positive = import, negative = export, Watts). 40226 = Grid energy import (uint32, Wh), 40227 = Grid energy export (uint32, Wh).

Modbus to KNX gateway setup

Intesis IN701KNX (BACnet/Modbus to KNX gateway) configured in Modbus TCP master mode: MAPS software > New signal > Source: Modbus TCP, IP: [inverter IP], Port 502, Register: 40070, Type: int16, Scale: 1. Target: KNX, DPT 13.010 (active energy Watt, 4-byte signed), GA 9/0/0 (PV Power W). Signal 2: Register 40225 → DPT 13.010 → GA 9/0/1 (Grid Power W, negative = export). Signal 3: Register 40080 (today energy) → DPT 12.001 (4-byte unsigned Wh counter) → GA 9/0/2 (PV Energy Today Wh). Upload signal table to IN701KNX. Verify: ETS6 Group Monitor → observe GA 9/0/0 — should show inverter AC output in Watts (varies with solar irradiance). Poll interval: set to 10 seconds in MAPS for real-time control. For visualisation only: 60-second poll is sufficient.

Self-consumption logic in ETS6 or Loxone

KNX self-consumption logic using MDT Logic Module SCN-LOGM.01 or Loxone Miniserver: Input 1: GA 9/0/1 (Grid Power W, signed) — negative means export surplus. Threshold trigger: if Grid Power < −1000W (exporting >1kW surplus): send telegram to EV charger or heat pump to increase consumption. EV charger integration (Easee Home with KNX-Modbus adapter or OCPP to home automation): GA 11/0/0 (EV Charger Current limit, DPT 9.021, Amperes). Logic: surplus >2000W → increase EV charger from 6A to 10A → surplus >4000W → increase to 16A. Heat pump SG Ready: GA 11/1/0 (SG Ready input, DPT 5.001, 0–3): send SG Ready signal 3 (excess PV available, run at maximum) when surplus > 3000W. Immersion heater (if equipped): KNX binary output relay GA 11/2/0 → send 1 when surplus > 500W. Priority sequence: (1) heat pump SG Ready, (2) EV charger increase, (3) immersion heater. Total controllable load: 3kW heat pump + 3.6kW EV (1-phase 16A) + 2kW immersion = 8.6kW controllable.

Fronius Gen24 battery integration

Fronius Gen24 Plus with BYD Battery Box Premium HV: adds battery SOC data via Modbus. Register 40493 = Battery SOC (0–100%). Register 40495 = Battery charging/discharging power (int16, W). KNX mapping: GA 9/1/0 = Battery SOC (DPT 5.001, 0–100%). GA 9/1/1 = Battery Power (DPT 13.010, positive = charging, negative = discharging). Self-consumption logic with battery: if Battery SOC > 80% AND Grid export > 500W → activate controllable loads. If Battery SOC < 20% → pause controllable loads (prioritise charging). If Frost risk (outdoor temp GA < 0°C, from weather station) → maintain battery SOC > 30% (lithium cell protection). ETS6 OR/AND logic: MDT Logic Module conditions for multi-input decisions.

Eastron SDM630 Modbus sub-metering alongside Fronius

For BREEAM Energy credit or EN ISO 50001 energy reporting: add Eastron SDM630 Modbus RTU sub-meters on each load circuit (HVAC, lighting, EV charging, server room). Integration: Eastron SDM630 Modbus RTU (RS-485) → WAGO 750-362 Modbus RTU slave/master coupler OR MDT KNX/Modbus gateway → KNX GAs 9/2/0 through 9/2/7 (per circuit energy Wh). Combine with Fronius PV data (GA 9/0/2) in Gira X1 or IP-Symcon energy dashboard: (1) Total consumption = grid import + PV self-consumed. (2) Self-consumption rate = PV self-consumed / PV total yield × 100%. (3) Self-sufficiency rate = PV self-consumed / total consumption × 100%. Report monthly: export CSV from visualisation → BREEAM assessor or energy auditor. Target: self-consumption rate > 70% for typical office with PV + EV charging + heat pump.

SG Ready EN 14825 heat pump integration

Vaillant arotherm plus (Vaillant's heat pump range with KNX/SG Ready): SG Ready is a 2-wire dry-contact signal from the electricity smart meter or building automation controller to the heat pump. 4 states: State 1 = blocked (grid overload, heat pump off), State 2 = normal (standard operation, default), State 3 = encouraged (cheap tariff or PV surplus — heat pump runs at elevated power, raises buffer tank to 55°C), State 4 = run-at-max (maximum PV utilisation — heat pump runs at full capacity, heats buffer tank to 60°C). KNX SG Ready: MDT binary output module → 2 relay outputs → SG Ready terminal S1 and S2 on heat pump. KNX logic: S1 relay GA 11/1/0, S2 relay GA 11/1/1. State 3 = S1=0, S2=1. State 4 = S1=1, S2=1. ETS6 logic: PV surplus > 3kW → send S2=1 (state 3). PV surplus > 6kW → send S1=1, S2=1 (state 4). Grid import high price flag (from smart meter P1 telegram) → send S1=1, S2=0 (state 1, blocked). Vaillant arotherm plus also supports KNX directly via the vSMART controller with KNX module — provides setpoint control DPT 9.001 alongside SG Ready relay signals.