Desktop CPU Power Survey, April 2006

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D. ENERGY CONSUMPTION

Here, we need to look at system power consumption at the AC socket. Realistically speaking, neither the full load nor the idle system power truly reflect energy consumption. In order to get a truer picture of energy consumption, we need to consider how much the system is in idle or at full load during the course of a typical day. Research by the US Environment Protection Agency for the Energy Star program shows that computers are in idle 95% of the time that they are powered on.

But that raises our hackles. Most of us are enthusiasts here; surely we must stress our PCs a little harder than that (we all say with a little machismo and swagger). So let's say 90% idle and 10% full load for PC enthusiasts like us.

Then we can calculate the average electricity consumption of the systems with this simple formula: (0.9 x Idle power) + (0.1 x Load Power) = Average Power. The EPA's draft computer specification scheduled for implementation next year states that a PC will be an Energy Star when it consumes no more than 49W in idle. So for argument, let's use 49W as the reference and calculate how much more (or less) energy each system consumes, expressed as a percentage. Having gone this far, we might as well calculate the annual energy consumption in kWh assuming that each computer is turned on an average of eight hours a day.

Processor
Idle Power (W)
Load Power (W)
Avg. Power (W)
re: Energy Star
kWh/yr
Intel P-M 770 Dothan
40
53
41.3
-16.2%
120.6
AMD Turion 64 ML-40 Lancaster
40
54
41.4
-16.2%
120.9
AMD Sempron 3400+ Venice
44
66
46.2
-5.7%
134.9
AMD A64 3000+ Venice
44
61
49.3
+0.6%
144.0
AMD A64 4000+ San Diego
51
76
53.5
+9.2%
156.2
AMD A64 X2 3800+ Toledo
51
90
54.9
+12.0%
160.3
Intel Core Duo T2600 Yonah
53
75
55.2
+12.6%
161.2
AMD A64 3500+ Winchester
54
80
56.6
+15.5%
165.3
AMD A64 3500+ Venice
55
83
57.8
+18.0%
168.8
AMD A64 X2 4800+ Toledo
53
115
59.2
+20.8%
172.9
Intel P4 630 Prescott
64
128
70.4
+43.7%
205.6
Intel PD 820 Smithfield
71
142
78.1
+59.4%
228.1
Intel PD 930 Presler
75
146
82.1
+67.6%
239.7
Intel PD 950 Presler
74
160
82.6
+68.6%
241.2
Intel P4 670 Prescott
84
195
95.1
+94.1%
277.7

It's easy to see that the idle power is much more important than the peak. The idle power is a much better indicator of typical energy consumption than load power. This would not hold true if you are a gamer with a high performance rig that is turned on only when you play. Then, the load power consumption becomes much more indicative of total energy consumption. Of course, all your power numbers would go up as much as >200W depending on what graphic cards you're using.

E. THE EFFECT OF PSU EFFICIENCY

Looking at all the data so far, it's clear that small differences in CPU power, either at load or idle, don't really change the total energy consumed. Especially when the higher cost of low power processors like the Turion 64, Pentium M or Core Duo are considered, this doesn't seem a very cost effective way to improved power consumption. What about a more efficient power supply?

The FSP Green 400 PSU used for all the testing is no slouch for efficiency. But we also happen to have a Seasonic SS-400HT that is 80 PLUS certified. It has even better efficiency. We've tested both units in our labs.

FSP Green 400W PSU
SEASONIC SS-400HT, 80 PLUS
DC Output Load (W)
AC Input (W)
Calculated Efficiency
AC Input (W)
DC Output Load (W)
40.4
58
69.7%
76.6%
55
42.1
63.7
84
75.8%
81.5%
78
63.6
89.7
114
78.7%
82.8%
109
90.2
150.6
185
81.4%
85.3%
180
153.5
199.3
245
81.3%
85.3%
231
197.2

So what happens when we swap the FSP Green out for the Seasonic SS-400HT 80 PLUS? (Both PSUs can be found online for ~$70.) We chose just a few systems to try this, as time was running short. Only the data on system power is cited, as there is no change in the power delivered to the CPU / 2x12V connector.

Processor
Idle Power (W)
Load Power (W)
Avg. Power (W)
FSP Green
Seasonic 80 Plus
FSP Green
Seasonic 80 Plus
FSP Green
Seasonic 80 Plus
Intel P-M 770
40
37
53
49
41.3
38.2
AMD Turion 64 ML-40
40
37
54
50
41.4
38.2
AMD A64 4000+
51
47
76
72
53.5
49.5
AMD A64 X2 4800+
53
50
115
108
59.2
55.8
Intel PD 930
75
71
146
138
82.1
77.7
Intel P4 670
84
78
195
187
95.1
88.9

It's clear that power supply efficiency is a key determinant in system efficiency. With the higher power processors, the gains are most dramatic, but they are measurable even with the lowest power processors. Especially when there is no price difference between the two power supplies, the choice is obvious.

It should be noted that we are comparing two relatively high efficiency power supplies. If we were to compare the Seasonic to a typical or generic PSU which has 70~72% efficiency at best and <65% efficiency at low loads, the real differences in energy consumption would exceed 15W average power in many systems. Over the course of a year or for an enterprise running hundreds of PCs in a building, this difference is very significant.

NOTE: SLEEP MODE VS. HIGH EFFICIENCY PSU - The jury is still out on whether high efficiency PSUs or full implementation of effective sleep and advanced idle modes is a more effective means of reducing energy consumption by computers. For various points of view on this matter, please see the relevant PDF presentations at the Energy Star Computer Specification web page. HP, for one, contends that effective, universal implementation of Sleep Mode is not only much cheaper than a more efficiency power supply, it actually leads to much greater energy savings all around.



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