Power Distribution in Three PCs (2012)

Power | The Silent Front
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HEAVY LOAD POWER TEST RESULTS

The amount of power consumed on heavy load is a determining factor in how much capacity is required for a power supply and how much cooling is required to keep things running smoothly. Like all electrical components, CPUs, GPUs, and motherboard VRMs aren't 100% efficient, resulting in waste heat during operation. The more energy the system draws, the more heat needs to be dissipated, so bigger heatsinks and faster, louder fans might be required.

Power Distribution: HandBrake Video Encoding
System
i7-965 XE + HD 5870
i5-2500K + HD 6870
X4 630 + HD 5550
+12V
(8-pin CPU)
7.5A
4.2A
5.2A
+12V
(6-pin GPU)
0.4A
0.3A
N/A
+12V Total
11.2A
6.5A
6.2A
+5V
1.5A
1.7A
2.9A
+3.3V
9.7A
2.4A
2.4A
Total DC Output
175W
96W
97W
Total AC Input
204W
123W
119W
Approx. Efficiency
86%
78%
82%

Our first heavy load test is encoding a video using HandBrake, a tool commonly used to re-encode video formats so that they're compatible with various devices. It's purely a software encoder, using CPU power to do all the work. As a result, almost all the extra load came on the +12V lines with some marginal increases on the +5V rail for the i7-965 and X4 630.

For many users, this may be the most stressful thing performed on their system and yet the total system power consumption was quite low, about 120W AC for both the Sandy Bridge and Athlon II based machines. The i7-965 is a bit antiquated and inefficient and even it used only ~200W.

Power Distribution: Prime95
System
i7-965 XE + HD 5870
i5-2500K + HD 6870
X4 630 + HD 5550
Pent. D 820 + GMA950
Athlon 64 3500+ + 6800GT
+12V
(8-pin CPU)
8.9A
4.8A
6.2A
11.0A
4.0A
+12V
(6-pin GPU)
0.4A
0.3A
N/A
N/A
N/A
+12V Total
12.7A
7.3A
7.2A
11.4A
7.8A
+5V
2.1A
2.6A
3.5A
3.5A
3.8A
+3.3V
10.2A
2.2A
2.6A
0.6A
3.8A
Total DC Output
198W
109W
113W
155W
125W
Total AC Input
229W
134W
134W
 
-
Approx. Efficiency
86%
81%
85%
Note: The older Pentium D and Athlon 64 systems were tested with CPUBurn rather than Prime95.

Using Prime95, a synthetic CPU stress tool, again most of the extra power is derived from the +12V rail with some moderate increases on the +5V rail. The total power consumption was not that far off from our video encoding test, about 10~25W more.

Power Distribution: Prime95 + FurMark
System
i7-965 XE + HD 4870 CrossFireX
i7-965 XE + HD 5870
i5-2500K + HD 6870
X4 630 + HD 5550
+12V
(8-pin CPU)
9.0A
9.2A
4.9A
6.0A
+12V
(6-pin GPU)
15.2A
12.1A
7.4A
N/A
+12V Total
36.0A
28.3A
17.8A
9.3A
+5V
2.3A
2.3A
2.5A
3.5A
+3.3V
12.2A
10.6A
2.6A
2.7A
Total DC Output
484W
389W
237W
139W
Total AC Input
547W
438W
271W
162W
Approx. Efficiency
88%
89%
87%
86%

The real party didn't begin until we threw FurMark into the mix, a tool used to stress graphics cards more than actual gaming. Once again, the +12V rail ruled supreme, but this time the 6-pin PCI-E power cables provided much of the extra load. The HD 5870 and 6870 are fairly power hungry, doubling the total power consumption of their respective systems, while the HD 5550 didn't make much of a dent.

Our video card selection is somewhat limited (the HD 5870 is our highest draw card, using about 220W on load) so to show you what you can expect with a more high-end configuration we added results with a pair of HD 4870s in CrossFireX on the i7-965 machine. The older 4870s are notoriously inefficient, taking the AC input of the Nehalem PC up to 547W. Even combining components with intentionally power sucking hardware, our 650W power supply was only pushed to about 75% capacity.

This is the type of activity that allows power supplies to reach peak efficiency levels, with our test unit exceeding 85%. It should be noted that in our X-650 review, efficiency reached up to 92%. The discrephancy is likely the result of a combination of three factors: the difference in load distribution in our power supply testing, the different type of load (resistive) we use to stress power supplies, and to a lesser degree, the inaccuracy of our clamp meter. As a result the real world efficiency was up to 10% lower in some cases.



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