The system is consuming 400 Watts when not mining (in idle condition), and 800 Watts when mining. Meaning that GPU's are taking only 400 Watts and the remaining 400 Watts is being used by my Dell Precision T7500. I'd not never suggest anyone to buy branded system, instead, build one. That should save lot's of power.
Please keep in mind that efficiency of PSU is also important. If your PSU is not efficient it might pull more from the wall than others. I do not think that HP/Dell put highly efficient PSUs into their Workstations.