I was told by cisco engineer that in the idle mode RPS with 2x750W power consumption depends on temp in the rack and other enviromental conditions.
After all he suggested me to use estimate 100W (AC, 200V) per power supply in RPS, so in your case with 2x750W it would be 200W in idle mode.
Seems like too much for the idle mode for me, but I have no other authoritatative answer...