Active PFC Power Supply and NON true sine wave UPS. - jonnyGURU Forums3

I just spoke with both Silverstone and APC. Silverstone is going to get back to me on this question, but APC was pretty clear -- they strongly felt that PFC power supplies should use true sine wave UPSs. They told me that the reason for this was because PFC power supplies require a much faster changeover time when the battery kicks in, and that due to the design of stepped sine wave UPSs, they were not always fast enough to satisfy the power supply. This could lead to the PSU shutting down when the battery tried to kick in. The guy I spoke with said that the stepped sine wave wouldn't "damage" the PSU, but that it simply might not work, and that if it DID work, it might not "always" work. Apparently it depends on the specific power supply.
Oops! Lesson learned about UPS and active PFC power supplies

Your computer uses a power supply that utilizes Active Power Factor Correction (Active PFC) to improve efficiency. Power supplies with active PFC may experience incompatibility problems with a UPS that does not provide pure sine wave power output when the power supply switches from AC power to UPS battery power. As a result, when a computer system using a power supply with active PFC is attached to a non-sine wave UPS, the system may shut down when it switches to battery power. Also, if the power supply continues to operate, it may produce a humming or high pitch noise while running on battery. This humming indicates the power supply is operating beyond specified tolerances and may damage the power supply