UPS seems to be overloaded?

[Anonymous user]

Can anyone tell me if this looks correct (see attached screen shot)...I have a Back-UPS RS900 on a workstation, WinXP Pro and PowerChute PE is telling me I am overloaded but only using 178 watts of power. the only thing pluggedin to the ups is the workstation. I even took of the LCD monitor! During a power blip the ups will not hold, not even for a second.
Any ideas...do i not understand something here?
Thanks for any insight!

[TheNotoriousKMP_apc]

Bob,

Do you have a power factor corrected (PFC) power supply on the desktop?

I've been told that the capacitors in the PFC power supplies would drain upon UPS transfer to battery to accomodate the step approximated sine wave. Upon discharge of those capacitors, as soon as the UPS is on battery, the capacitors decide they need to recharge fully at that time, creating an inrush power draw that would exceed the rated capacity of the UPS. Then as soon as the capacitors are full, the power draw will go back to what you're seeing it in PowerChute. The only problem is that temporary overload while on battery is forcing the UPS to drop the load.

There is some other information posted here:

Referenced content removed

[Anonymous user]

I did a little investigating after reading your response. Assuming I have a 750 watt PFC power supply, and i only need say 5 to 10 min. of battery time during an outtage, what size UPS would be appropriate? The APC selector doesn't have any mention of PFC power supplies.

[TheNotoriousKMP_apc · Accepted solution]

Bob,

The tricky thing here is trying to determine how much runtime the capacitors are going to eat up by recharging. I would say if you had a UPS that was sized above 750W, after that initial recharge state of the capacitors, the UPS's capacity would drop back down to a stable state, and the runtime would increase.

That's probably why there's little to no information on our site about it. It's an extreme wild card that makes it tough to size.