Component(s) Power Requirements

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Component(s) Power Requirements

Postposted on Sun Nov 22, 2009 1:40 pm

I am not sure how the relationship between nominal PSU output vs. components(s) power requirements works that is why I decided to ask.

Does the PSU total nominal wattage output have to be equal to or greater than each component's power requirement separately? Or does the PSU total nominal wattage output have to be equal to or greater than all components' power requirement combined?

The reason that I am asking this is because what if individually all components' power requirements below PSU total nominal wattage but if they are all combined they will exceed the PSU total nominal wattage. So how does that work?
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Re: Component(s) Power Requirements

Postposted on Sun Nov 22, 2009 1:54 pm

The PSU needs to be able to handle the combined power usage of all of your components added together.

You should also pay attention to how much wattage the PSU can supply on each rail, and whether this is adequate for the components which will be connected to that rail. This is less important than it used to be, since pretty much all high-power components draw from the +12V rail now, and PSU specs have shifted accordingly; however may still be an issue for PSUs which split the +12V across multiple outputs. (In the past, some motherboards/CPUs drew the bulk of their power from the +5V or +3.3V rails, but this practice was phased out several years ago.)

I also suggest factoring in at least 30% headroom (e.g. if your components draw 300W, make sure you get a 400W PSU), since running a PSU at 100% capacity will make it run hot and shorten its life.

Also be aware that many off-brand PSUs tend to over-state the wattage the unit can reliably handle. If you really can't "just say no" to a cheap off-brand PSU, increase the headroom to 100%, and don't whine when it still decides to self-destruct and toast your motherboard in the process.
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Re: Component(s) Power Requirements

Postposted on Sun Nov 22, 2009 2:04 pm

And in practice a reasonably powerful modern system doesn't use as much power as a lot of people think; certainly not as much as the popularity of 600W+ PSUs would lead you to believe. If you're running multiple graphics card (especially the last generation) you can certainly see much higher usage, and a system with many hard drives can demand a lot of juice at start up (if you don't have a provision for staggered spin up), but for more mainstream rigs a quality 500W (or even less) gives plenty of headroom and keeps the PSU in its high-efficiency band (PSUs tend to be less efficient at both the high and low ends of their power output).
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Re: Component(s) Power Requirements

Postposted on Sun Nov 22, 2009 2:27 pm

I ordered Alienware Aurora with the highest PSU rated at 875W. My specs are as follows:
-Core i7 920 2.66GHz.
-Radeon Dual 5870 CrossFire
-12GB of RAM 6x2GB at 1066MHz.
-1.5TB HD.

If Dell offers these specs with a 875W PSU, it should be sufficient.
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Re: Component(s) Power Requirements

Postposted on Sun Nov 22, 2009 3:45 pm

In TR's review, the 5870 Crossfire setup pulled just 460W at the wall in a system with an i7-965 Extreme CPU. Accounting for typical decent PSU efficiency (let's say 85%), that's only 390W being output from the PSU. Even if you assume all of this is on the 12V rail, you're looking to cover just 33A. A decent 650W PSU beats this by nearly 20A, so would have had more than enough headroom for your system.
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Re: Component(s) Power Requirements

Postposted on Sun Nov 22, 2009 3:50 pm

I went overkill on my power supply when I built my system. I went with a 700 watt power supply from OCZ. Figured too much wattage was better than too little. The power supply only outputs what is needed anyway, and that leaves me room for expansion. I'm probably only using 300 to 350 watts max. I would recommend getting a good brand like OCZ, Corsair, etc. etc. This is only my opinion, but I figure the better quality parts you use, the higher the longevity of your computer, leaving you with fewer failing parts and fewer headaches.
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Re: Component(s) Power Requirements

Postposted on Sun Nov 22, 2009 7:36 pm

MrJP wrote:In TR's review, the 5870 Crossfire setup pulled just 460W at the wall in a system with an i7-965 Extreme CPU. Accounting for typical decent PSU efficiency (let's say 85%), that's only 390W being output from the PSU. Even if you assume all of this is on the 12V rail, you're looking to cover just 33A. A decent 650W PSU beats this by nearly 20A, so would have had more than enough headroom for your system.


In these reviews http://www.xbitlabs.com/articles/video/ ... html#sect0 and http://www.anandtech.com/video/showdoc.aspx?i=3643&p=26 they used some "stress" software which pulled way over 600W but I guess in real world applications in would never happen

dustyjamessutton wrote:I went overkill on my power supply when I built my system. I went with a 700 watt power supply from OCZ. Figured too much wattage was better than too little. The power supply only outputs what is needed anyway, and that leaves me room for expansion. I'm probably only using 300 to 350 watts max. I would recommend getting a good brand like OCZ, Corsair, etc. etc. This is only my opinion, but I figure the better quality parts you use, the higher the longevity of your computer, leaving you with fewer failing parts and fewer headaches.


I have a 7 year old computer which uses some cheap generic power supply and it works fine. On the other hand in my other system I had a "high end" ThermalTake which started making noises that sounded similar to shorting/sparking wire. I decided to take no chances and replace it when it was over a year old, when I called ThermalTake they did not even return my call.
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Re: Component(s) Power Requirements

Postposted on Sun Nov 22, 2009 11:12 pm

michael_d wrote:On the other hand in my other system I had a "high end" ThermalTake which started making noises that sounded similar to shorting/sparking wire. I decided to take no chances and replace it when it was over a year old, when I called ThermalTake they did not even return my call.

Yeah, my opinion of Thermaltake has soured a fair bit in the past ~3 years. Their PSUs seem to be second-rate, and their fans (both their case fans and the ones they use on their CPU coolers) seem to have a rather short lifetime as well. They've moved from my short list to my avoid list.
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Re: Component(s) Power Requirements

Postposted on Sun Nov 22, 2009 11:15 pm

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Re: Component(s) Power Requirements

Postposted on Sun Nov 22, 2009 11:59 pm

I think I've said it before: I don't trust any power supply manufacturer. The power supply is, for me, the most often failed component since I started building my own computers ~10 years ago.
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Re: Component(s) Power Requirements

Postposted on Mon Nov 23, 2009 8:55 am

I will say this again too. Please refer to the required readings on the subject.

viewtopic.php?t=38038
http://www.silentpcreview.com/article28-page1.html
http://www.anandtech.com/casecoolingpsu ... spx?i=3413

michael_d wrote:In these reviews http://www.xbitlabs.com/articles/video/ ... html#sect0 and http://www.anandtech.com/video/showdoc.aspx?i=3643&p=26 they used some "stress" software which pulled way over 600W but I guess in real world applications in would never happen
The Xbitlabs test are running the systems overclocked. The power draw will be up quite a bit when that happens.

But back to the config. CrossFire 5870? What resolution are we talking about? Or just doing your part in stimulating the economy? :o
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Re: Component(s) Power Requirements

Postposted on Mon Nov 23, 2009 9:08 am

michael_d wrote:In these reviews http://www.xbitlabs.com/articles/video/ ... html#sect0 and http://www.anandtech.com/video/showdoc.aspx?i=3643&p=26 they used some "stress" software which pulled way over 600W but I guess in real world applications in would never happen

Looks like you need to budget for about 200W extra on the PSU if you want to run stress testing software rather than games!
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Re: Component(s) Power Requirements

Postposted on Mon Nov 23, 2009 7:18 pm

Flying Fox wrote:I will say this again too. Please refer to the required readings on the subject.

viewtopic.php?t=38038
http://www.silentpcreview.com/article28-page1.html
http://www.anandtech.com/casecoolingpsu ... spx?i=3413

michael_d wrote:In these reviews http://www.xbitlabs.com/articles/video/ ... html#sect0 and http://www.anandtech.com/video/showdoc.aspx?i=3643&p=26 they used some "stress" software which pulled way over 600W but I guess in real world applications in would never happen
The Xbitlabs test are running the systems overclocked. The power draw will be up quite a bit when that happens.

But back to the config. CrossFire 5870? What resolution are we talking about? Or just doing your part in stimulating the economy? :o


I ordered 30" 2560x1600. :D
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