That page does not show 0 (zero), but (*), meaning that the info is not available. So, comparing to other 27-inch iMacs, maybe in the range of 90-110w for idle.
I don't think that "idle" means "sleep", which on most Macs is less than 5w, IIRC.
Idle should mean "no load"
After searching around, some Macs use less than 1 w/hour in sleep mode.
Hmmm
You DID ask about sleep mode, so I leaped to the conclusion that you were interested in power consumption during sleep mode. That is clearly not your need. Your personal sleep time is just not the same as the Mac's sleep time.
Easy enough to put only the display to sleep, which should help considerably with your power needs.