Powerware 9120 BUS Unbalanced error display

The user's Guide for the PW9120 handles 700-3000va - this is the complete range for the PW9120. It does not list a possible alarm of "'BUS Unbalanced''. Unless your unit is a European version, it will be 120 input and output and will have only one inverter internally.
Get the manual or download the full manual from the Powerware site. Go through the UPS settings to ensure they match the factory settings. (By the way, the factory setting for the Input Voltage Frequency should be +/- 5%).

If all the settings are correct, you have may have dead batteries. What does the display show you for battery charge % and battery backup time? If your individual battery voltages are less than 11 volts, your batteries are dead. Test at the battery output connector at the back of the UPS. 9120-700 2 batteries, 9120-1000 3 batteries, 9120-1500 4 batteries - all in series, so the 12 volt battery bus voltage is additive. Use PowerSonic PS-1290 F2, rated at 9.0 Amp. Hr. for the best battery replacement.

I've had this same issue what it boils down to is that the Power IN is not being distributed properly across the Power Outlets.

Step 1. unplug everything from the unit and power the ups down. If the alarm clears itself the UPS is mostly likely fine. if it doesn't not clear send it in for repair

Step 2. If step 1 cleared the alarm try distributing the decives across the outlets differently.

There are two inverters so two output if everything is plugged into one side (ie a power bar with 10 things plugged into it) and nothing on the other side this could cause the alarm.

hope this helps