Power Consumption: Drawing Some Conclusions
Who Needs +3.3 V? Not The Radeon R9 295X2
Our first interesting discovery was the complete absence of a load on the +3.3 V rail. Although we did observe a minimum draw of .1 W, that's within a margin of error. We can say fairly confidently that all of the R9 295X2's components run at +12 V now.
PCI Express Slot Measurements
The second and third discoveries concern power consumption from the motherboard's PCI Express slot. Even with the GPUs pegged at 100%, the Radeon R9 295X2 doesn't pull more than 28 W from its host platform. However, the card demonstrates unorthodox behavior compared to the other cards not based on Hawaii at idle. In that state, it does draw most of the power it needs from the PCI Express slot, and only 5 W from the auxiliary connectors.
In the past, we noticed graphics cards getting less power from the motherboard. AMD's Radeon R9 295X2 exemplifies this by maxing out at that very conservative 28 W figure. That also means most maximum power consumption calculations based on an assumed 75 W reading from the motherboard are wrong.
CrossFire And SLI With Single-GPU Cards
Because we don't have six current probes at the lab or the ability to store the immense amount of data generated by two cards at the same time, we recorded separate values for each warmed-up graphics card, one after the other.
Power Consumption At Idle
Although a 28.5 W measurement from the Radeon R9 295X2 is far from ideal, it's less power than you'd need for two Radeon R9 290Xes in CrossFire. And that's before ZeroCore Power kicks in. When ZeroCore is active, you only need 13.5 W to keep AMD's latest dual-GPU flagship running.
Presumably due to its effective thermal solution helping reduce leakage current, AMD's Radeon R9 295X2 beats two 290Xes in CrossFire by 40 W. Then again, our experiments with a single Radeon R9 290X suggest you can shave off 30 W and increase performance by using a more powerful cooler. Multiply that out for an array of cards in CrossFire.
According to our measurements, one Radeon R9 295X2 uses roughly as much power as two GeForce GTX 780 Tis in SLI.
Maximum Load: Compute
The Radeon R9 295X2 finishes well ahead of two GeForce GTX 780 Tis in SLI, though this is attributable to PowerTune intervening to drop the AMD card's peak performance (even though temperatures remained below 66 °C).
If you wanted to really hammer the card hard with a power virus and increase the card's power target, you could certainly push the 295X2 above the 500 W average AMD cites. Compare that to a pair of Radeon R9 290Xes in CrossFire, which approach 600 W before slamming into their speed limiter.