The id register returns 0xFFFF instead of 131. Why?

A value of 0xFFFF (65535) is the SunSpec not-implemented sentinel. The device either does not support Model 131 or it has been excluded from the active profile. Check the inverter firmware version and confirm the Watt-PF function is included in the product's PICS table before polling this model.

I enabled modena but the inverter is not adjusting power factor. What now?

Setting modena to 1 enables the module, but the inverter will only act on the curve pointed to by actcrv. Verify that actcrv references a curve slot that has been pre-loaded with valid watt and power-factor points. A curve slot containing all-NaN or zero values will produce no response.

Scale factor registers w_sf and pf_sf show unexpected negative values. Is that normal?

Yes. SunSpec scale factors are signed base-10 exponents, so a value of -2 means multiply the raw integer by 0.01. Negative values are standard. Apply the scale factor to each curve point value before comparing it to physical units. Never treat scale factor registers as raw measurement values.

Is it safe to write actcrv or wintms during live inverter operation?

Use caution. Writing actcrv switches the active power-factor curve immediately (subject to the ramp defined in rmptms), which can alter reactive power output and may trigger grid protection limits. Always verify the target curve contents before activating it, and set a non-zero rvrttms as a safety reversion fallback. Mis-configuration of curve parameters may affect grid compliance and could impact inverter warranty terms.

Free Modbus template: SunSpec Model 131 — Watt-PF

Name: SunSpec Model 131 — Watt-PF
Brand: SunSpec
SKU: 3fe2ac6e-79e1-4fc9-ad3d-1b0584d27a39

Try the demo Shop

Free Modbus template: SunSpec Model 131 — Watt-PF | ModbusCloud

Registers (12)

12 read / 5 write

Name	Address	Function	Type	Unit	Access
id SunSpec model identifier, fixed value 131 for Watt-PF.	0	FC03	uint16		R
l Length of this model block in Modbus registers.	1	FC03	uint16		R
actcrv Index of the currently active Watt-PF curve, starting at 1.	2	FC03	uint16		R/W
modena Enable flag for the Watt-PF function, 1 to activate.	3	FC03	bool		R/W
wintms Randomised activation delay window, seconds.	4	FC03	uint16	Secs	R/W
rvrttms Reversion timeout before returning to default, seconds.	5	FC03	uint16	Secs	R/W
rmptms Ramp time for transitioning to the new curve setpoint, seconds.	6	FC03	uint16	Secs	R/W
ncrv Number of Watt-PF curves the device supports, read-only.	7	FC03	uint16		R
npt Number of points per curve the device supports, read-only.	8	FC03	uint16		R
w_sf Scale factor for watt axis values, signed base-10 exponent.	9	FC03	int16		R
pf_sf Scale factor for power factor axis values, signed base-10 exponent.	10	FC03	int16		R
rmpincdec_sf Scale factor for ramp increment and decrement values.	11	FC03	int16		R