SunSpec Model 712 — DER Watt-Var
SunSpec Model 712 is a standardized DER Watt-Var information model that maps reactive power output as a function of active power for grid-connected inverters and distributed energy resources. This template communicates over Modbus TCP and gives your portal visibility into the model header registers (id, l), the function enable flag (ena), multi-curve management registers (adptcrvreq, adptcrvrslt, npt, ncrv), the reversion timer (rvrttms, rvrtrem, rvrtcrv), and both scale factors (w_sf, deptref_sf). Any SunSpec-certified DER that declares model 712 in its register map can be monitored and configured with this template, making it directly reusable across inverter brands without per-device customization.
- Protocol
- Modbus TCP
- Connection
- Ethernet, Modbus TCP
- Registers
- 12, read and write
- Brand
- SunSpec
- Last updated
Registers (12)
12 read, 4 write
| Name | Unit | Access | Address (Hidden until you enter your email) | Function (hidden) | Type (hidden) |
|---|---|---|---|---|---|
| id | Read | ||||
| l | Read | ||||
| ena | Read and write | ||||
| adptcrvreq | Read and write | ||||
| adptcrvrslt | Read | ||||
| npt | Read | ||||
| ncrv | Read | ||||
| rvrttms | Secs | Read and write | |||
| rvrtrem | Secs | Read | |||
| rvrtcrv | Read and write | ||||
| w_sf | Read | ||||
| deptref_sf | 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
- Verify that the Watt-Var function is enabled on a DER after commissioning by reading the ena register.
- Monitor the reversion countdown via rvrtrem to confirm a timed curve change expires as configured.
- Track curve adoption results through adptcrvrslt to detect failed curve-switch requests on any site.
- Alert on unexpected ena state changes that could indicate unauthorized remote reconfiguration of reactive power behavior.
- Compare ncrv and npt values across multiple DER units to verify uniform curve capacity before bulk configuration.
Installation
Enable Modbus TCP on the target DER through its web interface or commissioning tool, typically under the communication or Modbus section, and note the configured TCP port (default 502).
Connect the DER to the same LAN segment as the ModbusCloud Gateway, ensuring no firewall rules block port 502 between the Gateway and the device.
In the ModbusCloud portal, add the Gateway, create a new device, import the SunSpec Model 712 template, and enter the DER's IP address and Modbus unit ID.
Read back the id register and confirm it returns 712, then check the l register value to verify the model instance is fully accessible at the configured address offset.
Things to watch for
The id register does not return 712. What is wrong?
SunSpec models are placed at dynamic offsets in the register map, not fixed absolute addresses. The template uses relative offsets from 0. You must first discover the model's base address by scanning for the SunSpec identifier 0x5375 at address 40001, then walking the model chain until model ID 712 is found, and configure that base address in the portal.
The ena register reads 0 and reactive power output is flat. Why?
The Watt-Var function is disabled by default on most DER implementations. Write value 1 to the ena register (RW, address offset 2) using function code 16 to activate the function. Confirm the device accepts the write by reading back the register. Some devices require a specific Grid Guard code or firmware version before the write is accepted.
After writing adptcrvreq, adptcrvrslt shows an error code. What now?
An error result in adptcrvrslt typically means the requested curve index exceeds ncrv (the number of stored curves), or the curve has not yet been written to the device. Verify the requested index is within the range 1 to ncrv and that curve data registers have been populated before issuing the adopt request.
Is it safe to write rvrttms and rvrtcrv while the DER is generating?
Writing reversion registers while the DER is live will take effect immediately. If rvrttms is set too short, the active curve may revert to the default (rvrtcrv) before the intended control window ends, altering reactive power output unexpectedly. Always confirm the reversion curve index points to an acceptable profile, and test during low-generation periods to avoid tripping grid protection limits.
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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