During the stress tests we measured power consumption based on the power draw from the entire PC.
The methodology is simple: We have a device constantly monitoring the power draw from the PC. After we have run all our tests and benchmarks, we look at the recorded maximum peak; and that's the bulls-eye you need to observe as the power peak is extremely important. Bear in mind that you are not looking at the power consumption of the graphics card, but the consumption of the entire PC.
Our test system is a power hungry Core i7 965 / X58 based and overclocked to 3.75 GHz. Next to that we have energy saving functions disabled for this motherboard and processor (to ensure consistent benchmark results).
I'd say on average we are using roughly 50 to 100 Watts more than a standard PC due to higher CPU clock settings, water-cooling, additional cold cathode lights etc. Keep that in mind. Our normal system power consumption is a little higher than your average system. But with multi-GPU setups .. well that fact fades away real fast. Check it out.
So yeah with 2-way SLI we noticed our power consumption for the GTX 460 768MB peaked to roughly 435 Watts, that's not bad really as that is slightly higher than one GTX 480 yet we certainly are more powerful.
BTW we stress the GPUs here,. not the processor, so you need to add another 100~150W for overall power consumption.
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