Overclock.net › Forums › General Hardware › General Processor Discussions › CPU Power Efficiency
New Posts  All Forums:Forum Nav:

CPU Power Efficiency

post #1 of 2
Thread Starter 
(I started this thread a few days ago in order to learn more about appropriate CPU/GPU power for a very basic PC. After not having recieved much feedback and with the purchase date rapidly approaching, I began to thik of this question in different terms: power efficiency over lifetime. This seemed interesting enough to do a new thread about, and it also makes sure I'm not doing it wrong!! )


I'm putting together a budget build for a family member and I was having a hard time deciding on an appropriate CPU. Right now I'm torn between the Intel Core i3-2100 and the AMD Phenom II x4 840, and I got to thinking about long-term power efficiency and it's effects on the Total Cost of Ownership of a given CPU. The best I could do to find numbers that were comperable was this article which has both the i3 and the 840 on it's power consumption page and uses the same methods and hardware for both. The numbers:

i3-2100, ASRock Pro3-M Z68 - $220.48, 56w idle, 86w load
x4 840, GIGABYTE GA-880GM-D2H - $142.40, 91w idle, 166w load

As we can see, the i3 is more expensive but more efficient. I want to know how this plays out over a expected usage lifetime of four years. My knowledge of 'electricity math' is limited, so check my numbers! I saw somewhere that the 2010 national average is 9.88¢/kWh, so that's what I'll use. Here's what a four years of idling gives us:

i3 - $194.00 = 1,963.58 kWh @ 9.88¢ each
840 - $315.25 = 3,190.82 kWh @ 9.88¢ each

Wow, we can really see the effects of power efficiency magnified over time! How about a more extreme example: four years at full load.

i3 - $297.93 = 3,015.50 kWh @ 9.88¢ each
840 - $575.08 = 5,820.62 kWh @ 9.88¢ each

Granted, full load for four years is...unlikely...but it makes for a neat example. Now what would the TCO of these chips be for a typical family machine? It's only on for, let's say, an average of 7 hours a day. Average CPU load is going to look like 15% at most on either of these chips. By my (hopefully correct!) approximation, that takes the power consumption of the i3 to 60.5w and the 840 to 102.25w. What would our electricity cost look like over four years?

i3 - $61.13 = 618.73 kWh @ 9.88¢ each
840 - $103.32 = 1,045.71 kWh @ 9.88¢ each

So now we come down to it - Total Cost of Ownership, or the cost of purchase added to the cost of use. Will the power efficiency of the i3 win out out, or will the significantly cheaper 840 retain the lead for this light workload?

i3 - $281.61
840 - $245.72

I'll admit that I'm quite a bit surprised; I thought that the i3 would catch up to the 840 much more quickly. Obviously heavier lifting and longer uptime favor the i3, but how long would it take the i3 to surpass the 840 in terms of cost-over-time?

Idle - 2 years, 92 days, 17 hours, 5 minutes
Load - 27 days, 1 hour, 6 minutes


So, did I do my math right?

Note: This is obviously quite simplified and was mostly done for my own amusement. I'm aware that the i3 will complete more work 'per load' and thus increase it's efficiency advantage further, but I judged it an unnecessary complication in this instance. Additionally, AMD's integrated GPU for the 840 is weaker than the the Intel HD 2000 which may necessitate a discreet card for certain tasks and thus tilt the balance in the i3's favor even more.

Which of these would you get for a basic home office/web machine (if either)? Is the HD 4250 graphics sufficient for these tasks?
My System
(13 items)
 
  
CPUMotherboardGraphicsRAM
Core i5-2500k ASUS P8P67 B3 Sapphire Radeon HD 6850 1GB GDDR5 G.Skill 4GB (2x2GB) DDR3 1600MHz 7-8-7-24 
Hard DriveOptical DriveOSMonitor
Samsung Spinpoint F3 1TB 7200RPM Lite-On 24x DVD Burner Windows 7 Home Premium 64-bit HP 2310m 
PowerCase
NZXT Hale 90 650W NZXT Phantom White 
  hide details  
Reply
My System
(13 items)
 
  
CPUMotherboardGraphicsRAM
Core i5-2500k ASUS P8P67 B3 Sapphire Radeon HD 6850 1GB GDDR5 G.Skill 4GB (2x2GB) DDR3 1600MHz 7-8-7-24 
Hard DriveOptical DriveOSMonitor
Samsung Spinpoint F3 1TB 7200RPM Lite-On 24x DVD Burner Windows 7 Home Premium 64-bit HP 2310m 
PowerCase
NZXT Hale 90 650W NZXT Phantom White 
  hide details  
Reply
post #2 of 2
Nice post, rep+ This is a kind of reasoning I've done when buying server parts, so I can relay to what you're saying. I'm going to add some complexity to the analysis

One other thing to consider when computing the cost of ownership is the uptime. In the case of an average desktop PC one would keep it running for a few hours per day, most of them idle and a few at load. If the PC isn't gamer-oriented chances are it's going to be more idle than any other usage pattern. Also one can assume the PC is used more during weekends when @home than during workdays. As such the idle/load power consumption of the CPU is even less relevant - supposing an average of 4h/day ilde usage and 1h/day full load usage (I used excel btw) the time it'd take the i3 to surpass the 840 in terms of cost-over-time could be easily be 5 times the idle value, thus making any power efficiency comparison irrelevant.

Noise factor - lower power CPUs can run more silently with a cheaper HSF, so often a 125W PhII albeit cheaper would need some 30$ CM 212+ to run as quiet as a 65W i3 w/ stock cooling - thus negating much of the pricing advantage. Since silent computers are important to me I quantified the cost of cooling @low noise in the initial price.

Undervolting - this is the same as OCing in my book from a standpoint. So you have a stock speed or a max OC and you want to achieve it with as low voltage as possible - or that's the theory. Here's an example: a Phenom II 940 w/ 5850 with a fresh Win7 install and BIOS defaults runs at some 135W Idle and 220W CPU load. Using K10Stats the CPU is undervolted from the stock load voltage of 1.4 to 1.25V and idle voltage is decreased from 1.1 to 0.85V. New idle power: 115W, load 180W. The benefits are there, but it's difficult to quantify them before putting one's paw on the CPU and seeing what it can do. Still, most who tried agree that AMD CPUs are woefully overvolted at stock speed, be that Phenom, Llano or Turion in laptops, at the same time Intel actually properly tunes idle/load voltage per CPU and not per batch of CPUs - so at stock they're better, but there's little room for tuning.

Bottom line is that in my opinion for certain use cases some CPUs have clearly better cost of ownership than others. If you talk about a typical file server with 24/7 uptime and very low CPU usage then the CPU idle power is most important. The crossover point depends a lot on the uptime and average load, as well in a lesser extent to other collaterals like acceptable noise level and tweakability.

P.S. The 4250 is easily god enough for all the 2D and eye candy in Win7, HD2000 isn't any better. The HD2000 has an advantage in the few games where Intel's drivers actually work, otherwise the 4250 is faster, or at least better. You shouldn't base your purchase decision on the speed of the IGP, they're both bare minimum. Also you'll need Dx11 to accelerate Win8's desktop, so they won't work.
Edited by dragosmp - 11/3/11 at 4:55am
Centurion
(14 items)
 
  
CPUMotherboardGraphicsRAM
Phenom II 940 @3.3GHz 1.25V Gigabyte 790X-DS4 XFX 5850 8GB OCZ DDR2-800 
Hard DriveCoolingOSMonitor
Corsair Force 3 TRUE Win 7 Pro x64 ACER 23" TN  
PowerCase
400W Silverstone Strider Antec P182 
  hide details  
Reply
Centurion
(14 items)
 
  
CPUMotherboardGraphicsRAM
Phenom II 940 @3.3GHz 1.25V Gigabyte 790X-DS4 XFX 5850 8GB OCZ DDR2-800 
Hard DriveCoolingOSMonitor
Corsair Force 3 TRUE Win 7 Pro x64 ACER 23" TN  
PowerCase
400W Silverstone Strider Antec P182 
  hide details  
Reply
New Posts  All Forums:Forum Nav:
  Return Home
  Back to Forum: General Processor Discussions
Overclock.net › Forums › General Hardware › General Processor Discussions › CPU Power Efficiency