An uninterruptible power supply, also uninterruptible power source, UPS or battery backup, is an electrical apparatus that provides emergency power to a load when the input power source fails.
From Wikipedia, the free encyclopedia
We found out that STUDIO XPS 435T/9000 has challenges when connecting to majority of UPS because of certain sine wave compatibilities. Luckily we also found some answers online
FROM DELL COMMUNITY FORUM
Some XPS desktop systems connected to a UPS (Uninterruptible Power Supply), may turn off or drop power when the UPS is running on its internal battery. This issue is noticed with line interactive UPS units that output a squared, stepped approximated, or quasi sine AC waveform while on battery power. Dell recommends to use UPS that output a true sine waveform while on battery power. Some models are suggested below:
*APC Smart-UPS 750VA USB & Serial 120V, Part number SUA750
*APC Smart-UPS 1000VA USB & Serial 120V, Part number SUA1000US
*Dell 2200VA - 120 Volt UPS, Part number GU085
*Dell 1500VA - 120 Volt UPS, Part number 2K481
*Dell 750VA - 120 Volt UPS, Part numbers U6829
*TrippLite SmartPro 750SLT - UPS - 500 Watt - 750 VA 60Hz AC 120V, Part number SMART750SLT
also a similar answer from APC Forum.
FROM APC FORUM
"...Yes, the BackUPS line of products output a psuedo-sine wave, which is somewhere between a sine wave and a square wave. Lately, power supplies have been incorporating PFC technology to meet European regulatory requirements and they don't always work with the psuedo-sine wave. In particular, the XPS 9000 does not seem to like it. You would be better served with a SmartUPS."
So there you have it. If you plan to buy a UPS for your system purchase a SmartUPS just to be sure.