Connect to the inverter's SunSpec Modbus TCP interface (default port 502); scan from holding register 40001 to locate the SunSpec 'SunS' header (0x5375 0x6e53), then walk the model list to find model ID 705 (DER Volt-Var) and 706 (DER Volt-Watt)
For model 705, write the four Volt-VAR curve points to V1–V4 (voltage in 0.01% of nominal) and Q1–Q4 (reactive power in 0.01% of nameplate) per the SunSpec Modbus IEEE 1547-2018 Profile Specification v1.1 (published January 2025)
Set DeptRef register to 1 (% of nameplate kVAR) and enable the curve by writing 1 to the ModEna register in model 705
For model 706 Volt-Watt, write V1–V2 voltage thresholds and P1–P2 active power limits; set ModEna to 1
Write the updated settings to non-volatile storage by toggling the Commit register if supported, or verify persistence by reading back the registers after a simulated power cycle
Validate compliance with IEEE 1547-2018 Table 8 (Category A/B voltage regulation default settings) by comparing written values against the standard's reference curve points
Known gotchas
SunSpec register addresses are relative to the model's base address discovered during the scan; adding 40001 directly to a model-relative offset is a common off-by-one error
Not all inverters implement SunSpec models 705/706 even if they advertise IEEE 1547-2018 compliance; verify the model list at runtime and fall back to manufacturer-specific registers if absent
Volt-VAR Q values use signed integers where positive means inject reactive power (capacitive) and negative means absorb; inverting the sign convention causes the inverter to worsen voltage instead of correcting it
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