For testing power consumption, we took our test system and plugged
it into a Seasonic Power Angel. For idle numbers, we allowed the system
to idle on the desktop for 15 minutes and took the reading. For load
numbers we measured the peak wattage used by the system while running
the OpenGL benchmark FurMark 1.7.0 at 1920×1200 resolution.
Power Consumption Results: The Radeon HD 5870 and 5850 running in mixed CrossFire overall performed a little slower than a pair of Radeon HD 5850’s in CrossFire. Running mixed CrossFire also uses more power as you can see from the load test results shown above. At 518 Watts this configuration uses ~40W more than the faster Radeon HD 5850 CrossFire setup. This is right on target with AMD’s maximum board power ratings as the 5870 is 188W and the 5850 is 151W when it comes to max board power. This is a 37W difference and our testing shows a 40W increase in power draw over a pair of Radeon HD 5850’s!
Keep in mind that the less GPU bottlenecked that the system is,
the harder the CPU will work, thus increasing power consumption by the
CPU. In simple terms, not all of the additional consumed power is going
to the graphics card.