Power By Numbers
As we have noted in our various processor reviews in the past several months, power consumption has quickly become an item of importance when looking at various PC components. The CPU and the GPU are by far the biggest single users of power from the PSU and your choices about which of these components you use will have an effect on what requirements you put on your power supply.
NVIDIA recommends a 350 watt power supply for a system with a single 7800 GTX card in it and a 500 watt PSU for an SLI setup.
Just as we were told by NVIDIA, the 7800 GTX does in fact use less power than the 6800 Ultra, at least at idle. When we give the system a full load though, the single 7800 GTX does surpass the 6800 Ultra by a slight margin of 4 watts. SLI is a different story though as at both idle and load the pair of 7800s use 20 watts less power at idle and 13 watts less power when fully loaded. The ATI X850 XT PE still uses the least power of all the configurations however.
Performance Per Watt
Another interesting aspect of the power consumption numbers we saw above is to look at what kind of performance per watt each card is getting for the entire system. We can find this information by simply taking the average frame rate that we got in our benchmarks and dividing it by the wattage we measured during full load with each card. For example, for our Far Cry numbers the 7800 GTX had a frame rate average of 51.4 and a total wattage draw of 273, for a result performance/watt rating of 0.1883. The 3DMark05 results that are at default settings were divided by 100 for better scale.
The results show as we would expect that the 7800 GTX is much more efficient than the NV40; we see a 47% improvement on HL2 and a 30% improvement on Far Cry. The ATI X850 XT PE is a contender with the 7800 GTX in this respect already, so I am curious to see what the new R520 architecture is able to pull off.