The internal DC bus voltage inside the inverter has dropped below its minimum operating level.
Common causes
PV input voltage too low to maintain bus (early morning or heavy shading)Battery voltage low on hybrid modelsComponent failure in DC-DC converter stage
What to check
STEP 1
Check PV string voltage at inverter DC terminals. Below MPPT minimum (100V for SPH) = inverter will not start, which is normal
STEP 2
On hybrid models, check battery voltage and charge state. A fully discharged battery may cause bus undervoltage at startup
Escalate if
PV and battery voltages within spec but bus undervoltage persists -- escalate for DC-DC converter inspection
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