It depends on the display type. Some displays emit energy based on the intensity of the image (CRT, Plasma). Others always generate light but then mask out the light with a matrix display which typically has a pretty constant energy usage (LCD, Projection). Still others only require energy only when they change (eInk, future color pigmentation eInk variations).

And of course power variations in each technology, FL LCD v LED LCD. Older Plasma displays vs newer Plasma displays. The size of the display, (your 52" TV v 3" cellphone)

For CRT/Plasma there is also refresh rate to consider...

etc etc etc.