Idle power is 31w. Maximum is 168w.
Idle is basically surfing the web and excludes watching any videos.
Maximum means running FurMark which stresses the graphics card more than an actual games does. Probably why it's not recommended to run that benchmark for a prolonged period of time.
Click below for power consumption at various states:http://www.techpowerup.com/reviews/Zotac/GeForce_GTS_450_Amp_Edition/26.html