All three registers return zero or no response. What is wrong?

Modbus TCP is disabled by default on Victron GX devices. Go to Settings, then Services, then Modbus-TCP on the GX device and confirm the service is enabled. Also verify the GX device is reachable on TCP port 502 from the ModbusCloud Gateway before retrying.

I get a gateway path unavailable error on all registers. Why?

This error typically means the unit ID is incorrect. Grid system registers 820 to 822 belong to com.victronenergy.system, which must be queried with unit ID 100. Unit ID 0 also maps to the same service but many Modbus clients reject it; always use 100.

Phase L1 reads a large negative value unexpectedly. Is the scaling wrong?

No scaling factor applies: these registers are raw int16 watts with a scale of 1. A large negative value on power_a simply means the installation is exporting power to the grid on L1. Confirm with the GX device display that the sign matches the expected grid direction.

The GX device shows three-phase data but only one phase returns valid readings.

Each phase maps to a separate register: 820 for L1, 821 for L2, and 822 for L3. Confirm the Modbus poll requests all three addresses individually. Querying a contiguous block starting at 820 with a count of 3 is the most reliable approach and avoids gaps caused by non-existent registers between them.

Free Modbus template: Victron Energy (grid)

Victron Energy (grid) is a system-level Modbus TCP data source exposed by Victron GX devices, such as the Cerbo GX and Venus GX, in residential and commercial energy installations. The template reads three registers from the com.victronenergy.system service: grid power on phases L1 (820), L2 (821), and L3 (822), each as a signed watt value. Positive values indicate power drawn from the grid; negative values indicate export. Connecting the ModbusCloud Gateway to your GX device over LAN gives you live, per-phase grid power visibility without any additional hardware.

Name	Address	Function	Type	Unit	Access
power_a Grid power on phase L1, watts, positive import, negative export.	820	FC04	int16	W	R
power_b Grid power on phase L2, watts, positive import, negative export.	821	FC04	int16	W	R
power_c Grid power on phase L3, watts, positive import, negative export.	822	FC04	int16	W	R