Following is an answer I posted on this thread:
if there are still people struggling with this subject, you can find more information in this Application Note by APC. (You can also do a Google search for "APC active PFC application note" should the link expire.)
Here is a summary:
Because of the way active PFC’s operate, they can sometimes overload the UPS with momentary high inrush current. This can occur when the UPS transfers from online to on-battery operation, creating a momentary loss of power (<8ms). The PFC supply may respond by temporarily drawing an excessive amount of current. Also some PC’s, when awoken from standby (or ‘sleep’) mode, will draw a momentary high inrush current, potentially overloading the UPS if it happens to be running on battery. All APC UPS’s are designed to protect themselves when there is a severe overload while on battery. Some general serverclass UPS’s such as APC’s core Smart-UPS® models will protect itself by actively limiting the overload to a level that it can manage. Other, more economical UPS designs such as Back-UPS® or the Smart-UPS® SC will protect itself by shutting down quickly when it detects a severe overload. This potential for incompatibility should be considered when selecting a UPS – sometimes the most economical choice is not the best one. It is important to note that not all PFC power supplies will cause the UPS overload. However, the incompatibility is most acute in the one of the following situations: • A large server class PFC supply (e.g. rated 500W or more) is used with a Back-UPS or Smart-UPS SC. • The server is equipped with redundant PFC supplies (has two line cords) that are powered by the same UPS. • More than one PFC supply is plugged into the same UPS, bringing the total power rating (nominal) of the power supplies to 500W or more. • A workstation class PC (or high-end gaming PC) is equipped with a PFC power supply rated 500W or more. In any of these situations, APC recommends that a true, pure sine wave, server class UPS be used. Acceptable models include APC’s Smart-UPS®, Smart-UPS® XL and Smart-UPS® RT family of UPSs. However if, a Smart-UPS SC or Back-UPS RS is to be used, the UPS should be sized accordingly.
A critical factor to consider in avoiding an overload trip fault is the ‘nominal’ power rating of the power supplies, not the actual steady state power consumption. For example, a server may have two 600W power supplies in parallel-redundant mode, for a total power rating of 1200W. But the steady state power consumption in this case will be less than 600W. In another example, a high-end workstation with an 850W PFC power supply may only consume 350W under normal operation. So proper sizing of a UPS with active PFC power supplies, to better handle momentary overloads, must take into consideration the maximum power rating of the power supply, not just the actual power consumption of the load. Also keep in mind that if a power supply is rated for 600W output, it’s maximum ‘input’ power will be higher depending on its efficiency. For example, an Energy Star 4.0 compliant power supply has to be more than 80% efficient. That means when it is delivering 600W output power, its input power can be as high as 750W. This ‘input’ power should be the basis for sizing the UPS. Currently not all UPS selectors take these factors into consideration when recommending a proper UPS for servers with active PFC power supplies. Therefore the following guidelines should be followed when recommending a UPS for a PFC load.