The UPS is too small.

A ballast typically operates at 5-10% over whats listed. The cheap ones operate at 5-10% under whats listed. My guess is your 425 watt UPS is probably truly overloaded. If you have a dimable ballast, try turning it down a notch or two, this should solve that. If its not a dimable then your kinda SOL on that.

Definitely put the fan on a different plug though. I would unplug the fan, and cycle the UPS and ballast to see if it makes a difference, if not, get a bigger UPS... Or to solve your power flickers, replace your breaker in the breaker box... Should be alot easier than buying a UPS.

- Edit addition:
Wanted to add, that just like ballasts, cheap UPS's can operate over or under specifications. You might only be getting 385 watts out of it. Another option... Open the UPS and remove the stupid little speaker.
low_rdr Reviewed by low_rdr on . Analog Timer with 400 MH Ballast Overloads Surge Protector Approaching Lights Off I just bought a new Battery Backup/UPS for my 400 Watt MH ballast (HarvestPro Switchable), as brief power flickers would cause it to turn off with a 4+ minute delay before starting up (my HPS ballasts don't seem to have this issue). As I was crawling into bed, the beeping began... Observations -the UPS (un-interruptible power supply) manual says that continuous beeping indicates "working under overload condition" -the ballast and lamp were on, apparently uninterrupted -main circuit Rating: 5