Our overclocked hexa-core CPU pushes baseline power numbers fairly high, but we pushed them even higher by setting these graphics cards to run at their highest fan speeds. That extra cooling turned out to be necessary in order to get three highly overclocked GeForce GTX 660 Tis running in three-way SLI, particularly since the middle card's intake fans were stifled by the back of the bottom card. The cost of a 100% duty cycle ranged from 5 W on the stock GeForce GTX 680 2 GB to 20 W for three GeForce GTX 660 Ti cards in SLI. Under heavier GPU loads, the difference wasn't as significant.
The third GeForce GTX 660 Ti appears to idle down to around 18 W, while the second GeForce GTX 680 4 GB appears to idle down to around 25 W. Default fan speeds drop those numbers to around 12 W and 19 W, respectively.
The average performance chart could be a spoiler for the conclusion, but it's required for our efficiency calculation. Fortunately, we can still discuss some of the issues not easily identifiable from this chart, though.
Gigabyte’s overclocked 4 GB GeForce GTX 680 outperforms the stock 2 GB version by around 4% in our tests. Yet, higher power consumption nudges its efficiency gain down to 2%. A pair of Gigabyte’s 680s provides the best performance-per-watt, while high energy use keeps three GeForce GTX 660 Tis from scoring an efficiency lead.