St-ems wattsonic (battery)
The St-ems wattsonic is a hybrid solar inverter with integrated battery storage, sold under the Wattsonic brand and rebadged by St-ems. The ModbusCloud template connects over Modbus TCP and reads two key battery registers: instantaneous charge or discharge power (address 40258) and state of charge (address 43000). It also exposes a writable minimum-SOC setpoint (address 52503) that lets an energy management system reserve battery capacity. Importing this template into your ModbusCloud portal gives you live battery visibility and basic remote control without manual register configuration.
- Protocol
- Modbus TCP
- Connection
- Ethernet, Modbus TCP
- Registers
- 3, read and write
- Brand
- St-ems
- Compatible models
- wattsonic
- Last updated
Registers (3)
2 read, 1 write
| Name | Unit | Access | Address (Hidden until you enter your email) | Function (hidden) | Type (hidden) |
|---|---|---|---|---|---|
| power | W | Read | |||
| soc | % | Read | |||
| limitsoc | % | Write |
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Use cases
- Monitor real-time battery charge and discharge power to verify the system follows the expected self-consumption pattern.
- Track state of charge over time to detect capacity degradation across heating seasons.
- Alert on SOC dropping below a threshold during grid outages to trigger manual load shedding.
- Report daily battery cycling data to building owners as part of a monthly energy performance summary.
- Verify that the limitsoc setpoint takes effect after writing, by comparing the register readback against the intended value.
Installation
Enable Modbus TCP on the Wattsonic inverter: connect to the inverter's local IP address via a web browser or the Wattsonic app and activate the Modbus TCP server under the communication or EMS settings section.
Connect the ModbusCloud Gateway to the same LAN as the Wattsonic inverter; confirm the inverter is reachable on port 502 using a Modbus test tool such as QModMaster before proceeding.
In the ModbusCloud portal, add a new device, import the St-ems wattsonic (battery) template, and set the unit ID to 255, which is the default Modbus slave ID used by the Wattsonic LAN dongle.
Verify the connection by checking that the power and soc registers return live values in the portal dashboard; a value of zero for power during the day with no load is normal when the battery is idle.
Things to watch for
Why does register 43000 always return zero or an error?
The Wattsonic LAN dongle uses Modbus unit ID 255 by default. If you set a different unit ID in the portal, the inverter will reject all read requests with an illegal data address error. Confirm the unit ID is 255 and that the inverter's Modbus TCP server is active.
The gateway connects but the power register shows garbage values.
Address 40258 returns a signed 32-bit integer spanning two consecutive 16-bit registers. If your polling reads only one register instead of two, the value will be incorrect. Confirm the data type is set to int32 (two registers, big-endian) in the portal template configuration.
Writing to limitsoc does not seem to change inverter behaviour.
Writes made directly via Modbus may not reflect immediately in the Wattsonic cloud app, because register changes over Modbus are not always pushed back to the cloud. Check the inverter's local display or read back register 52503 to confirm the value was accepted.
Is it safe to write aggressive values to the limitsoc setpoint?
Use caution when writing to the limitsoc register (address 52503). Setting the discharge floor too low may stress the battery cells and could affect warranty. Always stay within the limits documented in the inverter's operating manual and test with small changes before deploying automation.
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