I measured the power consumption of systems with different graphics cards (the monitor excluded) using a specially modified power supply at two loads. The typical gaming load was emulated by running two cycles of the Aliens vs. Predator benchmark at 1920x1080 with 16x AF and 4x MSAA. To create maximum load I launched FurMark 1.8.2 (in stability check mode at 1920x1080 with 16x AF) singly and together with Linpack x64 (LinX 0.6.4, 4750 MB, 5 threads). These two programs load heavily the graphics card and CPU, respectively, so I can determine the peak power draw of the whole system and see what power supply will suffice for it (taking the PSU’s efficiency into account).
You can see the results in the diagram:
The AMD Radeon HD 6790 system needs about 8% more power than the same system with a Radeon HD 5770 and a mere 2% more than the HD 6850 system. This might be expected since the new card is closer to the HD 6850 and HD 6870 than to AMD's junior series in its design and specs. The Radeon HD 6790 is better than its direct rival, GeForce GTX 550 Ti, which consumes 10% more power. Moreover, the GTX 550 Ti configuration turns out to be the most voracious at the default frequencies, consuming more power than the senior GeForce GTX 460. Overclocking changes the picture, though.
Overall, you can see that even if used together with an overclocked six-core CPU (which is rather unlikely considering the pricing of the tested graphics cards) and other components, these cards will not call for a power supply with a wattage rating of more than 550 watts.
According to my tests, the AMD Radeon HD 6790 is exactly midway between the Radeon HD 5770 and Radeon HD 6850 in performance. The new GPU and the 256-bit memory bus make the new card superior to the HD 5770, by about 15% on average, despite the lower frequencies. On the other hand, the HD 6790 has fewer ROPs and shader processors than the Radeon HD 6850, so the latter is 15-16% faster on average. Thus, the new card fits into the previously unoccupied market niche, completing AMD's entry-level product range. Besides, it proves to be overall faster than its direct rival GeForce GTX 550 Ti. It must be noted, however, that these cards from Nvidia feature higher overclocking potential than the Radeon HD 6790 and may use this advantage to catch up with the latter in some games or tests. On the other hand, we are yet to see off-the-shelf HD 6790s and check out their overclockability.
The reference Radeon HD 6790 consumes less power than the GeForce GTX 550 Ti, which is important for mainstream and low-end graphics cards, but produces more noise and has larger dimensions. So, much depends on AMD's partners and their ability to quickly deliver custom-designed Radeon HD 6790s with smaller PCBs, one power connector and factory overclocking. I have no doubt such products will come out very soon. By the way, a good example of such a custom-made graphics card is the HIS Radeon HD 6850 IceQ X Turbo I've tested today.
We, at X-bit labs, will be keeping an eye on this market sector to report to you about such products as soon as they are released.