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).
Even though HP had the burliest videocard of the bunch, it nearly tied Dell for consuming the least amount of energy under load. We tested this by running a combination of Prime95 (CPU and RAM) and Furmark (GPU), a potential lethal combination, but one that gives us a glimpse of a worst case scenario.
Dell pulled slightly less power from the wall than HP under load, and while both feature a 460W power supply and the same CPU, other parts of these two systems are different, though apparently not different enough to warrant choosing one over the other based on power consumption.
A bigger power supply doesn't mean you'll use more power, but in this case, iBuyPower's system consumed nearly 100W more under load than the other two systems, which we attribute to the Nvidia GeForce GTX 550 Ti graphics card and liquid cooling setup more than anything else.