A NAS for Sonos that goes to sleep

[Norman]

As the title says, I need a NAS to store MP3 music for my Sonos system that is available 24/7 but goes to sleep after a pre-set period of inactivity and wakes up when the Sonos requires it.
I currently have an Intel SS-4000 NAS (4x400GB RAID5) which works great except that it will cost me in the region of £60 a year in electricity if left on 24/7 - an amount I can't justify for one electrical device

This NAS should ideally be RAID1 and it must be usable directly from the Sonos so something like the Netgear SC101 isn't suitable.