SunSpec Model 131 — Watt-PF
SunSpec Model 131 is a cross-vendor grid-support control model for solar PV inverters that implements a Watt-PF (active power versus power factor) curve function. Via Modbus TCP, this template reads the model identifier, block length, supported curve count, and points-per-curve capacity, and provides write access to the active curve index, the module enable flag, and the timing registers: window time, reversion timeout, and ramp time. Three scale factor registers align watt, power factor, and ramp values to physical resolution. Use this template to monitor and control cos-phi-of-P behaviour on any SunSpec-certified inverter from your ModbusCloud portal.
- Protocol
- Modbus TCP
- Connection
- Ethernet, Modbus TCP
- Registers
- 12, read and write
- Brand
- SunSpec
- Last updated
Registers (12)
12 read, 5 write
| Name | Unit | Access | Address (Hidden until you enter your email) | Function (hidden) | Type (hidden) |
|---|---|---|---|---|---|
| id | Read | ||||
| l | Read | ||||
| actcrv | Read and write | ||||
| modena | Read and write | ||||
| wintms | Secs | Read and write | |||
| rvrttms | Secs | Read and write | |||
| rmptms | Secs | Read and write | |||
| ncrv | Read | ||||
| npt | Read | ||||
| w_sf | Read | ||||
| pf_sf | Read | ||||
| rmpincdec_sf | Read |
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Use cases
- Verify that the Watt-PF function is enabled on a newly commissioned inverter by reading the modena register.
- Monitor the active curve index across a fleet of inverters to confirm consistent grid-support profiles are applied.
- Alert when the reversion timeout register is zero, indicating a curve change will persist indefinitely without reverting.
- Track the ramp time setting to ensure power factor transitions stay within grid operator ramp-rate requirements.
- Compare the number of supported curves and points per curve across inverter models during multi-vendor site audits.
Installation
Enable Modbus TCP on the inverter under its communication or network settings, and note the IP address and TCP port (default 502 for SunSpec devices).
Connect the inverter to the same LAN segment as the ModbusCloud Gateway, or ensure IP routing between them is in place.
In the ModbusCloud portal, add the Gateway, import the SunSpec Model 131 template, and set the unit ID to match the inverter (commonly 1 or 126 depending on manufacturer).
Read back the id register and confirm it returns 131; then read ncrv and npt to verify the device reports a non-zero curve capacity before writing any curve parameters.
Things to watch for
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.
Knowledge base articles for this device
SunSpec Modbus: one register map for many inverters
SunSpec Modbus is the open register map SMA, Fronius, SolarEdge and Kostal share. The layout, int+SF vs float, the TCP port per brand and reading a mixed fleet.
Fronius Modbus: read a Symo GEN24 over Modbus TCP
Fronius Modbus explained: enable Modbus TCP on the Symo GEN24 on port 502, choose between int+SF and float, read the right SunSpec registers and scale them correctly. Practical guide for solar installers.
SMA Modbus: reading a Sunny Boy over Modbus TCP (installers)
SMA Modbus explained: enable the Modbus TCP server on port 502, choose between the SMA profile (unit ID 3) and SunSpec (unit ID 126), read the right registers, and handle the NaN values at night. Practical guide for installers.
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