It turns out that most PC power supply units with active power factor correction (PFC) do not work well with lower-cost uninterrupted power supplies (UPS), e.g. battery back-ups. This I learned the hard way, as my Antec EarthWatts EA 500 PSU, which I adore, will go dead the second the battery on my APC Back-UPS BX1300LCD kicks in.
That’s because APC’s Back-UPS units output power, when on battery, as a modified sine wave, rather than a true sine wave.
In short, if you send a high-end PSU the current created by a lower-end UPS, the PSU’s built-in power factor correction hates it, and cuts power to the PC immediately. As in, the exact thing you bought the UPS to prevent in the first place is exactly what happens.
While I understand the value in having a high-performing PSU — less heat, more power, protection for internal components — a straight-up power off of the PC is never a good thing. So your choices are either to forgo a UPS, purchase a really expensive UPS, or purchase a low-cost PSU.
U.S. household electricity generally comes out of the wall at 60 hertz; that is, 60 “cycles,” or “waves,” of current per second, alternating between 120 volts of positive current, and 120 volts of negative current. This is what a sine wave looks like, an elegant arc of current alternating over time:
For various technical reasons, rather than making a pure sine wave, it’s easiest, when creating a power inverter, to use what is called a “modified sine wave.” Rather than producing elegant parabolic waves, what is produced more closely resembles stair steps, like this:
When a power supply unit that has active power factor correction sees this “square wave,” it doesn’t pass the internal tests the power supply uses to maintain high efficiency and protect your system against power fluctuations. So, in most cases, such as mine, it simply turns off.
Probably the best thing to do is go all in, and spend about $500-$600 for a true sine wave UPS, such as APC’s Smart-UPS products. Or, you could do what I did: purchase a different PSU, one that is generally reliable but doesn’t have PFC built into it. I’m going with a Cooler Master eXtreme Power Plus 500 Watt PSU.