St-ems wattsonic (grid)
The St-ems wattsonic is a hybrid inverter platform sold under multiple brand names, including Wattsonic Gen3, and used in residential and light-commercial solar-plus-storage installations. This Modbus TCP template reads the grid connection point of the wattsonic inverter: total grid exchange power, per-phase active power across all three phases, and per-phase AC current. All values are signed, so import and export are distinguished without additional configuration. Connecting the ModbusCloud Gateway to the wattsonic over Modbus TCP gives installers live three-phase grid visibility in a single dashboard without polling the manufacturer cloud.
- Protocol
- Modbus TCP
- Connection
- Ethernet, Modbus TCP
- Registers
- 7, read only
- Brand
- St-ems
- Compatible models
- wattsonic
- Last updated
Registers (7)
7 read, 0 write
| Name | Unit | Access | Address (Hidden until you enter your email) | Function (hidden) | Type (hidden) |
|---|---|---|---|---|---|
| power | W | Read | |||
| currents_a | A | Read | |||
| currents_b | A | Read | |||
| currents_c | A | Read | |||
| powers_a | W | Read | |||
| powers_b | W | Read | |||
| powers_c | W | Read |
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If a device is not in the library yet, we build the template for free as soon as we have the register data.
Use cases
- Monitor total grid exchange power in real time to verify self-consumption performance across a site.
- Track per-phase active power on phases A, B, and C to detect load imbalance between phases.
- Alert on sustained grid import above a threshold to identify unexpected consumption during solar hours.
- Compare per-phase currents to spot a phase overloaded relative to the others before a breaker trips.
- Report historical grid power trends to building owners as evidence for energy contract renegotiations.
Installation
Confirm that the wattsonic inverter has its LAN dongle connected and has obtained a local IP address; note the IP, as it will be used as the Modbus TCP target on port 502.
Connect the ModbusCloud Gateway to the same local network as the wattsonic inverter, ensuring no firewall rules block TCP port 502 between the two devices.
In the ModbusCloud portal, add a new device, import the St-ems wattsonic (grid) template, set the host to the inverter IP, port to 502, and unit ID to 255.
Trigger a manual poll and verify that the power register at address 11000 returns a plausible signed watt value matching the current grid import or export shown on the inverter display.
Things to watch for
All registers return zero or no data appears in the portal.
Confirm the wattsonic LAN dongle is active and reachable by pinging its IP from another device on the same network. The Modbus TCP listener runs on port 502 with unit ID 255. If the device is unreachable, check DHCP assignment or set a static IP via the inverter display menu under network settings.
Phase current values look ten times too high or too low.
The current registers at addresses 11010, 11012, and 11014 store values as uint16 in tenths of an ampere, so a raw reading of 152 represents 15.2 A. Verify that your polling client or dashboard formula applies a scale factor of 0.1 to convert raw register values to amperes.
The power register reads the wrong sign: import shows as positive when export is expected.
The wattsonic sign convention for grid power can differ by firmware version and configuration. Some community integrations apply a scale of -1 to register 11000 to match the expected sign. Check the inverter display for the current import or export direction and invert the sign in your channel configuration if needed.
Reads occasionally fail with an illegal data address error on some registers.
A known Wattsonic quirk is that some register address tables are documented with a zero-based offset, while the inverter expects a one-based address over Modbus. If a register consistently fails, try adding 1 to the address. This applies particularly when mixing register tables from different firmware documentation versions.
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