SunSpec Model 306 — Reference Point Model
SunSpec Model 306 is the Reference Point Model, a standardized SunSpec information block used in solar PV monitoring to capture site irradiance conditions. Deployed on any SunSpec-compliant weather station or irradiance sensor with Modbus TCP, this template reads the model header registers (id, l) and three irradiance channels: global horizontal irradiance (ghi), plane-of-array irradiance (a), and a second irradiance channel (v), plus an environmental temperature point (tmp). Importing this template into your ModbusCloud portal gives installers consistent, vendor-neutral irradiance data alongside inverter performance registers for yield verification.
- Protocol
- Modbus TCP
- Connection
- Ethernet, Modbus TCP
- Registers
- 6, read only
- Brand
- SunSpec
- Last updated
Registers (6)
6 read, 0 write
| Name | Unit | Access | Address (Hidden until you enter your email) | Function (hidden) | Type (hidden) |
|---|---|---|---|---|---|
| id | Read | ||||
| l | Read | ||||
| ghi | W/m2 | Read | |||
| a | W/m2 | Read | |||
| v | W/m2 | Read | |||
| tmp | W/m2 | 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
- Monitor global horizontal irradiance continuously to verify modelled versus actual solar resource at your site.
- Compare plane-of-array irradiance against inverter output to detect soiling, shading, or module degradation over time.
- Track the second irradiance channel alongside GHI to identify discrepancies between sensor positions on large rooftops.
- Alert on irradiance dropping below a threshold while inverter output remains high, flagging potential sensor faults.
- Report irradiance and temperature readings alongside energy yield data for performance ratio calculations in the ModbusCloud portal.
Installation
Enable Modbus TCP on the SunSpec-compliant weather station or irradiance sensor through the device web interface or configuration tool, typically under the communications or network section.
Connect the device to the same LAN segment as the ModbusCloud Gateway, confirm the device has a static IP address, and verify TCP port 502 is reachable from the Gateway.
In the ModbusCloud portal, add the Gateway, create a new device, import the SunSpec Model 306 template, and assign the correct unit ID matching the device configuration.
Spot-check register ghi (address 2) in the live register view during daylight hours to confirm a non-zero W/m² reading, verifying the full poll chain is working.
Things to watch for
Why do all irradiance registers return zero at midday?
Confirm the sensor is powered and oriented correctly. Also verify the ModbusCloud Gateway is polling the correct unit ID and IP address. Some SunSpec devices only expose model 306 data after the SunSpec magic marker and Common Model block are successfully read; ensure auto-discovery has completed before reading model-specific registers.
The gateway connects but the id register shows an unexpected value. What now?
Register id (address 0) must return 306 for this model block. If it returns a different value, the Gateway is reading a different model at that offset. SunSpec register addresses are relative to each model's starting offset, not absolute. Confirm the base address of model 306 on your device matches what is configured in the template.
Why does the tmp register show implausible values given its unit is listed as W/m²?
The SunSpec Model 306 specification defines tmp as a temperature point. The W/m² unit in this template reflects a data entry issue in the source definition. Treat the raw register value as a temperature reading and apply any scale factor your device documents. Cross-check against the device's native interface to confirm the correct engineering unit.
Values appear intermittently, then the poll times out. What is causing this?
SunSpec sensors on TCP can drop connections if multiple clients poll simultaneously. Ensure no other Modbus TCP client, such as a local datalogger or SCADA system, is holding an open connection to port 502 at the same time as the ModbusCloud Gateway. Reduce poll frequency if the sensor's connection queue is saturated.
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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