OS Neutral How much GPU power does an external monitor use?

I game on mac and windows via bootcamp on my 15" rMBP. For some games id like to get a bigger screen such as the Apple thunderbolt display or another third party like samsung (cheaper) and hook it up to my mac.

MY question is how much more GPU power does this draw? will i have to turn down the graphics on my games that i play with the monitor more than i would regularly need to without the monitor?

thanks