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6970 Crossfire PSU

post #1 of 12
Thread Starter 
Hello everyone,

I was just curious as to how much watts I should aim for when I put another 6970 in my rig.

The main specs are:

3770k @ 4 GHz
Corsair H60
8 gb 1600 MHz RAM
6970 xfire
2 HDD's
1 SSD
Antec Truepower New 650 Watts

I was thinking that I might need 750W, but if I can get away with my current PSU, it would save me a lot of money.

Any insight would be greatly appreciated, thanks!
post #2 of 12

You're in luck:  the 650W TruePower New is still overkill:

 

http://www.guru3d.com/articles_pages/radeon_hd_6850_6870_crossfirex_review,4.html

 

With two 6870s under full load in their system, their PSU pulled 421W from the wall outlet.  Their CPU was idling, so 150W needs to be added for an overclocked 3770K under full load.  This makes the PSU pulling 571W from the wall outlet if the two 6870s and the 3770K are under full load at the same time.  If the PSU is 85% efficient while pulling 571W from the wall, then that means the system is pulling about 485W from the PSU.

 

Even if I add 200W instead of 150W, that still comes out to being only about 530W being pulled from the PSU (that is, 421 + 200 x .85 = 527.85).

 

The 650W TruePower New is able to easily deliver 650W continuously if it's ever needed, so you're set for life as far as I'm concerned.

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post #3 of 12
Thread Starter 
Thanks a lot for the post. I meant to ask about two 6970s though, that is my fault.

Would that pull more than 600 watts with my setup?

Thanks again
post #4 of 12
Would likely only use something like 550W under full load, and more like 400-450 under real world usage.
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post #5 of 12

Whoa, I don't know how I messed that one up.  redface.gif  I apologize.

 

I still have good news.  :)  Based on what I found here, it looks like the 650W TruePower New is still plenty.  For two 6970s, the end result of the calculation would come out to be about 610W being pulled from the PSU if the 6970s are left at stock and the 3770K is overclocked.

 

http://www.guru3d.com/articles_pages/radeon_6950_6970_review,11.html

 

They only did one 6970 and their calculation shows that the one 6970 can pull 207W by itself.  Their PSU pulled 361W from the wall outlet with one 6970 under full load while the CPU was idling, so I will add another 207W for a 2nd 6970 which makes 568W being pulled from the wall outlet.  So, if I add 150W for an overclocked 3770K to that, it becomes 718W being pulled from the wall.  So, if the PSU is 85% efficient while pulling 718W from the wall, then the system is pulling 610W from the PSU.

 

I think that overclocking the 6970s would be out of the question, but even still: I'm calculating based on a simultaneous maximum load between the two 6970s and an overclocked 3770K (like maybe 4.5 GHz).


Edited by TwoCables - 12/6/12 at 10:27pm
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post #6 of 12
Thread Starter 
I am just curious as to how psu efficiency works.

you stated that there is a 718w pull in total, but with the psu being 85% efficient, 610w is coming from the psu.

Never really researched on psu efficiency.
post #7 of 12
You lose a certain amount of power converting from AC power that comes out of the wall to DC power which is utilized by digital devices.

PSUs are rated for their output figures on the DC side. Therefore, you would have to factor in the efficiency to determine values that are read on the AC side. So, if you put in a new card and your Kill-A-Watt meter read 150w higher, the card would actually be pulling ~127.5w DC if the PSU were 85% efficient.
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post #8 of 12
yeah as long as you don't overclock the 6970s its fine, they become very power hungry once you start raising the power cap and the voltage
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post #9 of 12
I used a TX750 for over a year with my 6970's and i got well served. Id say anything higher then 650W, grab a 750-850 if you want more head room etc etc. As long as you get a quality PSU you are fine.
post #10 of 12
Quote:
Originally Posted by tr1xst3r View Post

I am just curious as to how psu efficiency works.
you stated that there is a 718w pull in total, but with the psu being 85% efficient, 610w is coming from the psu.
Never really researched on psu efficiency.

 

Some of the power is lost as heat.  It is a little bit like a light bulb.  However, it has nothing to do with the PSU's capacity.  The capacity has to with how much power the system can pull from the PSU, and the efficiency has to do with how much power it has to pull from the wall outlet in order to deliver that power to the system.

 

So, if we had a PSU that is 100% efficient, then the amount of power that it has to pull from the wall outlet would be precisely the same as what the system is pulling from the PSU.  A 100% efficient PSU would mean that absolutely none of the power is being lost as heat.  So the less efficient the PSU is, the more power it has to pull from the wall outlet in order to give the system what it wants.  Again though, this has no relation to the PSU's capacity (its rated wattage).  A PSU's job is to take the raw power from the outlet and convert it into power that can be used by the entire computer system.  So, I think I would say that a PSU's capacity is referring to how much power it has the potential to create from the raw power.

 

So, that's why the efficiency is not as important as the market wants us to think.

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It's a computer!
(19 items)
 
  
CPUMotherboardGraphicsRAM
i5-2500K @ 4.5GHz (1.368-1.384V fixed voltage) ASUS P8P67 EVO B3 (UEFI ver. 1850) GTX 780 ASUS DirectCU II (1228 / 6300, 1.180V) G.SKILL Ripjaws X 8GB (2 x 4GB) 1866MHz, CL9 
Hard DriveHard DriveHard DriveOptical Drive
250 GB Samsung 840 EVO (C:\) 250 GB Samsung 840 EVO (D:\) 150 GB WD VelociRaptor Samsung SH-S243N 24x DVD Burner 
Optical DriveCoolingOSMonitor
Samsung SH-S203N 20X DVD Burner Thermaltake Frio Win 7 Home Premium x64 SP1 Retail AOC G2460PG (24" 1920 x 1080 144Hz G-SYNC) 
KeyboardPowerCaseMouse
Filco Majestouch 104-key Cherry MX Blues w/NKRO Corsair HX650 (Bronze, ordered on 12-12-2009) CM 690 Intellimouse Optical (1.1A) 1000Hz polling rate 
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Basic, but premium round X-Fi Titanium HD Klipsch ProMedia 2.1 (with 16 AWG Monster Cable... 
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