Can you check whether you have a redundant (Hot Plug) or non-redundant (Cabled) power supply unit. Power monitoring is not supported by iDRAC if server having cabled power supply. 
I just purchased a used Dell r320 and everything is fine except that the front display shows 0 W. Furthermore, I can't read the power consumptions in iDRAC on my r720XD and r630 I have a page show power consumption, but not on this machine. Any idea of why that is? Thanks  
jDell r320 shows 0 W in the front display?