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.