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Power, Efficiency, And Heat

Amazing SLI Scaling: Do Two GeForce GTX 460s Beat One GTX 480?
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Power consumption numbers are measured for the entire system by a meter gauging use from the wall outlet. Our OCZ-Z1000 power supply is 90% efficient (±1%) at the “Full GPU Load” power consumption levels charted below. So, anyone who wants to know the actual consumption of system components need only multiply the “Full GPU Load” readings by 0.9.

Efficiency is a comparison of work done to energy used. There is no metric to determine 100% efficiency using bits-per-watt or even FPS-per-watt, since bits and rendered frames are intangible. The best we can hope for is a comparison of how much more or less efficient one system is compared to another. To do that, we must first see how much more energy each system used than the lowest-consuming system, which in this case is the single GeForce GTX 460-based configuration.

Next, we need to know how much more performance the three top systems have over the fourth. We averaged the frames per second of each system and divided the result by that of the lowest system.

Dividing the performance difference (above) by the power consumption difference (two charts above) gives us a “full load” performance-per-watt difference, with the lowest system representing 100%. On the other hand, since the work produced by idle systems is invariable, comparing the efficiency of idle systems requires simply dividing each configuration’s idle power by that of the most miserly configuration. Because there is no such thing as 100%-efficient, subtracting the 100% baseline from each result allows us to show the difference between configurations.

Adding a second GeForce GTX 460 gives us a 90% performance boost, while requiring only 53% more total system power, yielding a performance-per-watt increase of 24% (1.90 / 1.53). The same system, at idle, consumes 13.1% more power than the single GeForce GTX 460 baseline.

And now for heat:

The temperature numbers above are for the first card in the system. In our SLI configuration, with two cards spaced less than one expansion slot apart, the bottom card actually runs cooler than a single card. The is likely due to the system increasing the speed of both fans simultaneously. Our motherboard also supports spreading the cards several spaces apart, and a test with two spaces between cards revealed identical temperatures in SLI and single-card configurations.

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