How much power do video cards use after Windows turns your display off?

Many modern cards usage very low power when idle for example my sapphire hd5770 consume 18w when idle and 108w when at its max. i don't comment on any specific card but commonly new cards consume less power during idle state. And even if window turn off the display it can not turn off the gpu and video ram. there will be very low or nothing difference between idle and display off state.

there is one exception. when you have hybrid sli(onboard and discrete graphics running in sli mode) then windows can completely turn off the discrete graphics card like 9800 gx2. you find this kind of setup more in notebooks where you can turn off the gpu.

That very much depends on the graphics card in your system.

I can only find this for certain relating to ATI Radeon graphics cards.

Some graphics cards do have a 'low power' mode where they reduce their power consumption considerably when they activate DPMS (Display Power Management Signaling).

From the Linux Radeon manual page:

Option "DynamicPM" "boolean"
      Enable dynamic power mode switching.  This can help reduce  heat
      and  increase  battery  life  by  reducing  power usage when the
      system is idle (DPMS active). The default is off.

I don't believe anyone has done a negative power consumption test - all the tests I have found have been while running the cards at maximum. It would be an interesting project to test various cards in running, idle and DPMS modes.