Use Dell's Enterprise Infrastructure Planning Tool (EIPT) That will tell you all the power/heat/air flow information you need to size.
Dear Experts,   We plan to buy 3 sets of R750 with "Dual, Hot-Plug, Power Supply (PS) Fault Tolerant Redundant (1+1), 1400W, Mixed Mode". I wonder what is the power consumption of that one.  If 1 PS runs and 1 PS is in sleep mode, I think it is around 1.4 kW per set, is that right? We need to calculate their power consumption in order to buy proper UPS.   Thanks!      
Power Consumption for PowerEdge R750