Attached is anandtech powersupply system measurement for 2x SLI, 3x SLI for your reference.
http://www.anandtech.com/video/showdoc.aspx?i=3518&p=11
If you want to make sure you can do your own measurement and be independent of this published measurements. DO IT YOURSELF...
use a CLAMP AMMETER to measure the actual current on each rail & DIGITAL VOLMETER to measure the actual voltages.
That will give you the actual current and voltages of whatever PS you are using.
Power Watts = I(Amperes) x V(Volts).
You need to factor in the different conditions when making your measurements:
- Current Surge at Power-Up. This is the current surge ~ 150% of the load during power up. This is normal on all CMOS circuits.
- Idle ... No activity after the computer complete the boot up process
- Loaded... For GPU common practice is measurement during 3D-Application or game is running, hard drive activity is high, memory I/O is high...etc.
If you have a logic analyzer you can trigger your measurement at specific conditions of the computer. Condition base measurement becomes more accurate. This can be initiated through computer interupt or software TRAP conditions.
Nvidia and ATI power supply recommendation is to some degree sandbagged. It does not provide details on the actual specific load of the computer. Load condition, what hardware is on the system is very specific to every computer so it varies.
When in doubt do your own measurement.