I tested Power Nap on my MacBook Pro Retina (Aug 2012) and my Time Machine backup hasn't made any progress. Even the hard drive light is out while the machine sleeps.
Energy saver is at the default setting of sleep after 15 minutes and Enable Power Nap is checked.
It just doesn't seem to work. Are my expectations wrong? Shouldn't it just back up while in "sleep" mode?
This is a newly re-installed OS with no special apps installed and no custom software besides MS Office.
Update: My Mac is plugged into a Thunderbolt monitor (power and TB). The drive is chained off the monitor. Backups don't occur if the drive is directly connected (no monitor) nor through the monitor. The laptop is always plugged in.