Ok, a lot of talk is now going on about PSU and UPS compatibility. That is mostly because, many of us who prefer buying better to best components for our beloved rigs find that arguably most of the best PSU-s (Corsair, Tagan etc.) are not compatible with the undoubtedly best UPS-s i.e the house of APC. The Problem: Most new and sophesticated PSU-s have active PFC-s. Active PFC is incompatible with simulated sine wave output, it would only work with pure sine wave. The problem is more financial in nature. It is not that APC does not produce UPS wich output pure sine wave but they are the best server class pieces which come at a premium ~>10k. Now many of us have never thought about that while buying, once bought then only we find this out. So we are left with either changing our UPS or PSU!!! What will happen if I run my active PFC PSU along with simulated Sine wave UPS? In an ideal case, with power cut in this scenario the UPS starts providing simulated sine wave, the PSU expecting a pure sine wave would power down killing your system, the UPS sensing no load will shutdown as well. In short your system will behave like it has no UPS backup at all. A lot of deviations have been seen though. For example the Corsair VX450 is reported to have run without any hitch, other good psu-s would run with a heavy humming noise due to overload. We would all agree it would be a bad idea to take a chance with our precious monnies err.. hardware. A little about PFC: A lot of articles already here on this very forum about this. So will keep it short. A complex electrical system has mainly two kinds of loads resistive(simple resistance) and reactive(capacitance and inductance). In an AC circuit the lag or angle between the voltage and current depends on the reactive part of the circuit. The actual power consumed by the circuit is proportional to the product of voltage, current and cosine of the angle between voltage and current. So the more reactive circuit means more lag means lesser value of the cosine factor means the ciruit will be able to draw much less power even when it feeds on a main line which is actually able to give a maximum power equal to voltage multiplied by current that is when cosine of lag is 1 that is when lag is zero => only resistive load. The power factor of a circuit = true power / apparent power in this case it is (voltage X current X cos [lag]) / (voltage X current). So obviously to increase the power factor would mean to make the circuit more efficient. That is where PFC (Power Factor Correction) comes in. Not delving into complex descriptions let us just say that with PFC we add something to the actual circuit so that from outside the circuit behaves more like a pure resistive circuit thus increasing it's efficiency. PFC can active where a circuit sucks power from the mains in a resistive way, and feeds it to the low power factor circuit on the other side, isolating the mains from whatever the circuit is doing. Passive PFC uses a capcitive or inductive filter to enhance power factor. Active PFC is more efficient than the passive method. So what to buy? All APC Smart UPS and high end models have pure sine wave, but the cost a lot. So you can opt for that if you want to( and are rich :hap2: ). For people who are already stuck with lower models of APC UPS(which also do not come cheap :no: ) there is the option of none PFC and passive PFC models. All el cheapo unreliable PSU-s are none PFC. Passive PFC is becoming rarer by the day but still avaiable the Cooler master Extreme Power series and the Gigabyte Superb power series fall into this category. Please add more if you know of. Anyways that is all I wanted to tell. It might be something that you already know, but for noobs like me I thought it might be helpful. Post your suggestions.