The power consumption difference between a 24" LCD monitor with traditional florescent backlight vs a 24" LCD monitor with LED backlight can be significant.
According to the following review of the Dell U2410 (florescent backlight) by CNET, the operational power consumption is about 64w; I am assuming that means right out of the box with no calibration whatsoever. When calibrated power consumption drops to 57w.
http://reviews.cnet.com/lcd-monitors/dell-ultrasharp-u2410/4505-3174_7-33772972-2.html?tag=mncol;rvwBody
The below review of the Dell U2412M (LED backlight) by TFTCentral. With default settings power consumption is about 29w, when calibrated it is 22w.
http://www.tftcentral.co.uk/reviews/dell_u2412m.htm
I will add the caveats that:
#1 - they are not the exact same 24" monitor with different backlights.
#2 - the calibration process is probably a little different between the two review websites.