Well.....<deep breath>
I have a plug-in power meter and my rough measurements so far as follows:
1. MC PC (Intense PC Pro (expensive!) with an i7 Ivy Bridge mobile processor, silent operation (no fan), 128GB SSD, Win7 Pro, 16GB RAM, attached DAC that receives power via USB) - 5W in standby, 20W idling, doesn't change much from that while playing a HD movie. Quoted max power for this machine under full load is 26W.
2. My eventual NAS (DS411slim or similar) is 20W under access and around 10W while disks are hibernating. I believe this 2011 model doesn't allow the CPU to sleep, hence the higher-than-expected idle power, but I'm hoping the long-awaited 2014 model will improve this.
The PC handles everything MC can throw at it, apart from ROHQ. It can only manage Standard. Therefore the PC and storage are around 40W max (fully-populated drive bays assumed) while playing a single HD movie and not much lower while idling. The combination is around 10W-15W in standby, which is why I would like to keep them in standby and have instant start-up times (hence SSDs). I've even found that I can shut down the PC completely instead of putting it to sleep and JRemote can wake it up from shutdown, and because of the SSD it's available within about 15 seconds. That's not too bad I think. The major power consumption though comes from....
3. TV (modern "energy-efficient" 42" LG) - 50W.
4. AV Amp (Denon AVR-2805) - 80W.
I suppose you have to consider infrastructure too, e.g. gigabit switch, router, HDMI switch - but these do not use much power, I would guess around 10W altogether, I haven't done measurements on those yet.
With regard to hassle-free backups, I don't know as yet. MC does automatic library backups but you need to manually copy them elsewhere I guess, together with the actual media files. If you want automatic backups to another NAS I guess you have to factor in that being continuously running together with the actual PCs (or have a timer so that they wake up occasionally to do backups) or else do it manually every so often.