In the above screenshot you can see the five GPUs we have installed and activated in this PC. For our test we used several pieces of software including the Cryostasis PhysX Demo which allows us to go hard on the GTX 295 (Quad SLI) AND ALSO use Physx on the additional GeForce 280. During the test runs we simply monitor voltage fluctuations.

The reality is harsh though, with all that equipment and tweaks we just could not pass ~725 Watts load. A year or two ago with this very system we'd have passed 800 Watts no doubt. But not only do PC components get more efficient, PSUs themselves are becoming more and more efficient and green as well, in the end saving on overall power draw from the wall socket.

Some notes here. Powered down the PSU consumes very little current, we measured 2~3 Watts only. And again, though we had so much gear plugged into his system, the power consumption remained fairly low, making me believe that 85% power efficiency is very likely.

Great stuff really. 

For those interested, you can download the Cryostatis PhysX Demo and give it a try yourself.