Solis UN-BUS: DC Bus Voltage Too Low
UN-BUS (DC bus undervoltage, code 1023) means the Solis internal DC bus, the voltage rail between the input and the AC output, is too low to operate. The usual reason is simply not enough input: weak sun, or on a hybrid system a deeply discharged battery. It often recovers on its own as the input comes back.
What usually causes UN-BUS
- Low PV input, such as early morning, dusk or a heavily clouded sky.
- A deeply discharged battery on a hybrid system.
- A transient that a restart clears.
How to handle it
- Check the input. In good sun (or once the battery has charged), the DC voltage rises and UN-BUS normally clears.
- Power-cycle once. AC breaker off, then the DC isolator, wait 5 minutes, restart, to clear a transient.
- If it persists in good sun with a charged battery, contact your installer. A persistent UN-BUS can point to a measurement or hardware issue.
Related Solis codes
FAQ
UN-BUS shows at dawn and dusk then clears.
That is normal: when the sun is weak the input voltage is low, so the inverter waits. Once there is enough light, UN-BUS clears and it starts producing.
Helpful guides
Sources
- Solis inverter alarm-code list (UN-BUS = DC bus voltage too low; check input voltage, restart).
- Solis installer documentation.
⚠️ Safety disclaimer. Solar inverters carry lethal DC and AC voltage even when "off". This guide covers a safe power cycle and checking input only. Internal faults must be handled by a licensed installer.