The electricity which comes to your home, your AC supply is a pure sinusoid wave. But there are two types of UPS in the market: 1) Pure Sinewave UPS: which produce pure sinusoid AC. 2) Stepped Sinewave UPS or Modified Sinewave UPS: which produce an 'approximate' sinusoid AC. Power Factor correction is a mechanism which you find in good PSUs, that helps in reducing input harmonics if you have a "noisy AC input". PFC can either be implemented by just using a simple filter circuitry (passive PFC) or by using feedback and control (active PFC). The former is cheaper, and the latter is better and expensive for the manufacturer. Now, the reason why I'm creating this thread is, good PSUs have Active PFC, and few of these PSUs don't tend to play nice with Stepped Sinewave UPSs. Not all, but some. I'm creating this thread to tabulate, and would be looking for contributions from you guys. [table="width: 750"] PSU UPS Model and Output Type Comments Corsair CX430v2 (Active PFC) APC Back UPS RS600 (BR600CI-IN) with stepped sinewave output Has worked fine for me > 4 years. System = A8-3870K + HD 6950.