It seems like that would be a simple thing to do, but its unfortunately really hard to do. You need to monitor the current in each of the hot wires going to the motherboard, as well as the voltage on each line to get an accurate reading, there unfortunately isnt a way to just sum up the 12V current going to the board once it gets to the board as it splits off and goes multiple directions then, so you would need to monitor it on the power supply cables, so its not possible for a software application to do this for you, it can tell you the current power your CPU is consuming, but not the motherboard.
If you want the full system power consumption get a kill-a-watt meter, that will give your AC draw, multiply it by about .85 to figure out your DC power.
okay guys, i got a built in psu that gives a max output of 120W. Then i've burned it and pluged in a 350W power supply that i had on a AMD machine. SO i beleive the current system doesnt pull more that it needs, so my guess it the DC max pull is 120ws
^ True, but if you leave selections blank and only fill them in one at a time (and blank the previous one), you can get numerical estimates for each and every single component.
It's still not power consumption ( jsc would call it SWAG [ scientific wild assed guess ] )
Slight changes will give different results
My cpu ( I7-930, 130 watt TDP )
They start off with a minimum of 34w so thats the base we'll use
Using thier V and subtracting the min. wattage 109w
Using my OC voltage (1.2v ) drops that to 79w ( Wow , -.17v equates to -30 watts )
Now add my overclock ( 3.8 )and it jumps to 112w
Now use thier V and my OC, we get 144w
Change the MB field to high end ( this raises the min. to 55w )
Stock CPU and thier V , 106w ( 3 watts less due to a high end board )
Use my OCV, 97w ( 18 watts more due to a highend board, and only -9 watts due to the -.17v ? )
Add my OC , 129w ( 17 watts more due to a highend board )
Now use thier V and my OC, we get 144w, ( What ? No difference due to the high end board ? )
The psu calc. works good as a guide for choosing a psu with enough wattage to run the components you input, not for single component power consumption.