Some time ago I built NAS from Raspberry Pi. I also played with my wattmeter, and wrote down results.
NAS was build from Raspberry Pi 1 Model B (see Wikipedia, active USB hub Unitek Y-206P and HDD Toshiba 2,5" 250GB (can't recall exact model), connected via USB.
When whole set was powered off (after poweroff command), this set used 2.9W. Powering it on, increased energy draw to 5.2W, so I believe there's no point in powering off Raspberry Pi, at least not this way.
After putting some load on RPi with Perl script and a simple loop, energy use increased to 5.9W, and highest value was when HDD got busy (cp on NTFS partition). This NAS used then 7.3W.
IMHO ~5W is affordable for having online Linux machine at home, which can act as personal hosting etc.