We used SeaSonic's Power Angel Power Meter to measure the amount of power our test system pulled from the wall. You'll find three figures below: power supply's maximum rated wattage, peak power consumption under a full CPU/GPU load, and how much the system pulled from the wall when idle, following a fresh system boot.
We only have two metrics listed above because Dell didn't disclose the power supply rating, which is an internal unit as opposed to an external power brick. At idle, the XPS One 27 pulled 71W from the wall. When we upped the ante with a potentially lethal combination of Prime95 and FurMark, peak power usage bobbed back and forth between 199W and 200W before creeping up to 209W.
NoiseOnly when we fully stressed Dell's system did noise become an issue. With both the CPU and GPU at full tilt, the internal fan(s) slowly but surely ramp up to where they might be a distraction if you require a quiet work environment. Even then, it was never worse than a strong whir.
At idle and during routine tasks, the XPS One 27 is nearly whisper quiet.