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ck64's Guide to Choosing the Right Power Supply

#1 crystalklear64   Ded CAGiversary!   13616 Posts   Joined 13.8 Years Ago  


Posted 01 June 2011 - 05:41 PM

In this thread, you will learn how to research and choose the proper power supply for your PC. Fair warning, it gets a little technical and probably isn't as straight forward as it could be because I like to include information that is interesting though not essential. I'll probably add a "I don't care about the theory I just want to put this to practice" ezmode version of the guide at the end. Note that I, like you, am seeking to expand my knowledge on the subject, so if you see I've gotten something wrong or have something you'd like added please let me know. Also this is a work in progress. I'll add pictures and work on formatting as I can. Hope you enjoy.

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Useful Resources:
Johnny Guru
An excellent site that provides reviews of power supplies.

Silent PC Review

Another great PSU review site with a focus on quiet PSUs.

eXtreme Power Supply Calculator
A useful tool for determining the amount of wattage you will need from a PSU to power your system and the things plugged into it. Also its EXTREME. !!!.
Glossary/More Useful Stuff:
  • Power Supply Unit= PSU
    Gives your PC components the juice baby! Plugs into your wall and raises your electric bill.
  • Modular
    A feature on some PSUs that allows you to remove the ends of cables you aren't using meaning you have a tidier case.
  • Rail
    A path through which power travels. I'll be using rail and path interchangeably in this guide.
  • Watts=w
    A unit of power commonly listed in the PSU's name and almost always the most advertised part of a PSU.
  • Amperes=amp
    Another unit of power and the most important when looking for the right PSU.
  • Volts=v
    A variable you'll need to know for the following formula. Wiki says its a unit of electromotive force. k.
  • Bomb Ass Formula for Finding Volts, Watts, or Amps if You Have the Other Two
    Watts= Amperes*Volts

ck64's Guide to Choosing the Right Power Supply
Let the Learning Begin:
When building a new PC it is always important to research the individual components and ensure they can provide you with what you want while maintaining a good price to performance ratio. In other words, you want the best part for your money put into the context of your requirements. A power supply is no different, but its a part many people neglect to put time into researching often thinking that if the PSU has enough watts it'll work, or overpaying for a particular brand that provides way more power than they need. This is largely due to misinformation and misconceptions that have become popular throughout the internet, so lets take this time to clear a few things up...

    As you will soon find out, amps are far more important than watts, but typically units with a similar wattage have a similar amount of amps if they are in the same price range, so watts are useful in determining the general area for you to look in.
    If you go too cheap with a PSU, especially with a brand or model that has proven to be faulty, there is a very good possibility that it will fail and depending on how it fails, it could surge and fry all the stuff in your computer. It could also just stop working but thats not nearly as dramatic.
    Contrary to internet belief, not everyone needs a 1000 watt Corsair modular PSU. By doing your own research you can find a more than suitable PSU for a reasonable price. Of course, not everyone wants to take the time to do the research and this makes Corsair very happy.
The Role Watts Play:
With that out of the way lets jump into finding the right PSU for you. First, you'll need to have an idea of the other components you'll be using in your computer. This includes all the parts such as the CPU and video card as well as peripherals such as webcams, external HDDs, and USB mug warmers. Once you have all that in mind, head on over to the PSU Calculator.
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Fill in all the information as best as you can. It goes quite in depth, even to the LED's on your case fans. The last field is capacitor aging which accounts for reduced efficiency of your capacitors after awhile, so bump that to 20% if you'll be keeping your computer for awhile (1+ year). Once you fill it all in, click Calculate down at the bottom. This will produce for you a wattage amount. In my test setup it told me I needed a minimum of 572 watts with a recommended of 622. PSUs, as far as I have ever seen, only come in multiples of 50 watts ex. 600, 650, 700, etc. So, with a recommended of 622, I'll will be looking at 650, 700, and possibly 750w PSUs depending on my future plans for the system. Great!

Once You Know...:
Now that you have a wattage to start looking at, you now need to figure out how many amps your parts will suck up. The most important path or "rail" of power in a PSU is the 12v rail. This powers your video card, cpu, and other heavy power requirement things like fans and motors. So, you'll need to look up the amp requirements of the two major things that use the 12v rail:
  • Your video card(s)
  • Your CPU
The video card is simple enough. Just find the specs from the manufacture's website or google. I just looked up what a 6970 takes and its 32 amps. If you can't find the amps but you can find the watts, remember the video card uses the 12v rail meaning you can plug in the watts with 12 volts in the formula and figure it out. (watts=volts*amps)

For the CPU you'll have to do some math. Head back over to the PSU Calc now.

In the CPU section pick your CPU and look below it. You'll find a little checkbox that says, "Overclock my CPU." Check that box and just enter the say numbers from stock vcore and stock speed and click "Overclock." It will give you a wattage.
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Now, since we know that the CPU will use the 12v rail, we can plug the numbers into our formula to get the amps.
In this my CPU needs 95 watts. So, (w=a*v) 95=amps*12. 95/12=amps. 7.91=amps.

So, 32 (video card amps)+7.91 (cpu amps) gives us about a 40 amp draw. Factor in a minimal draw from your other parts and whether or not you'll be overclocking and give yourself some stretching room. I'll be looking for 650, 700, 750 watt PSUs with at least 45 amps on the 12v rail. Its time to head over to your computer parts store of choice...

Your Requirements:
Now that we have both a wattage guideline and an amp goal to shoot for, finding the right PSU comes down to a matter of price and reliability. Load up your favorite computer parts website and lets start searching. Filter your results based on the wattage, sort the list from low to high, and pick a PSU. Now, check out the specs. On newegg you can find them here:
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Now there are a few major things you're looking for here:
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1. The Connectors

These are the things that will plug into your components. Obviously you will need the right kind of connections and you'll need enough of them. I can't cover this in depth because I don't know what stuff you'll be using in your case, but just make sure if something needs power, you have a connection that will work for it. As an example, a 6970 requires a 6 pin and an 8 pin PCI-Express power connector. If we look at the specs we see the PSU has 2 PCI-E connections. A 6-pin and a 6+2 pin (8). So, we're good. If your PSU does not have an 8 pin connection, most video cards that require it come with an adapter you can stick on x2 4 pin connections to make an 8 pin. These are listed in more detail if you scroll down in the detail's list.

2. The Amps
This is where you see the amount of amps the PSU delivers on its various rails. I talked about the 12v rail, but there is also a 5v rail and a 3.3v rail and generally you don't have to worry about them. The 12v rail comes in 2 flavors, split and single rail. A single rail 12v has all the amps coming through one path. A split rail divides those amps into multiple 12v paths. To get the total amps of a split 12v rail PSU YOU HAVE TO CAST A MAGIC SPELL. The split rail was originally made for safety since having too many amps in a single rail can cause heat, but Intel discontinued the requirement of splitting rails at 20amps years ago. Just be aware if you see 12v1 12v2 12v3, etc. like in the PSU pictured, its a split rail and you just CAST THE SPELL. So make sure, whether split or single, the PSU is providing enough amps. In this case we have 22+22+25=69amps total which is well over our projected 45amp requirement. Maybe I should look at a cheaper and smaller PSU...

3. The Form Factor

The PSU has to fit in your case, so make sure your case to hold whatever size your PSU is.

Other things to consider:
  • Is it Crossfire/SLI Certified?
    If the PSU is not certified/approved/ready for one of these technologies and you plan on using one of them, find a different PSU. PSUs that haven't been certified might still work but if its been certified you know its been tested and approved for it.
  • Is it Modular?
    A nice bonus if it is. This will allow you to remove unused power cables from the PSU resulting in a tidier case which helps with airflow and is more visually appealing to some.

Once you find a PSU that meets all your requirements, look up a review of it. If a review doesn't exist, check out a different PSU in the same price range. I heavily advise against trying a new or unknown PSU before someone has gotten their hands on one and taken it to task. This will help you to avoid "bad" PSUs.

Bad PSUs:
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What do we mean by "bad"? When someone says a PSU is "bad", they're usually talking about the PSUs ability to push out its promised power and its stability. For example, a 12v rail that delivers 60 amps should be a total of 720 watts. However, this is simply what the PSU has been rated to do. In practice those 12vs might fluctuate. A bad PSU might be rated to deliver 720w, but when you take a multimeter to it, the 12v might be all over the place changing from 12v to 10v meaning you're getting a variable amount of power which is not a good thing for parts expecting a certain amount of power having it surge up and down. A good PSU will be very close to 12v and have very little if any fluctuations in that voltage even when under a heavy load. Certain brands are known for having bad PSUs such as Rosewill or Sunbeam but it really comes down to individual models so you'll have to read reviews.

Extra Notes:
If you plan on doing this make sure your PSU is SLI/Crossfire certified. Also be sure it has enough/the right connections. When calculating the amount of amps 2 video cards need you DO NOT DOUBLE THE AMPS OF ONE CARD. It will be a different amount of amps that you'll have to lookup.

Peak/Continous Wattage
Some PSUs use their peak wattage as their advertised output. This is stupid because if it can even achieve peak in the first place, it will deteriorate over time. Look for a continuous wattage rating when sorting through your results.

Edited by crystalklear64, 12 June 2011 - 03:16 AM.


#2 crystalklear64   Ded CAGiversary!   13616 Posts   Joined 13.8 Years Ago  


Posted 01 June 2011 - 05:43 PM

TL;DR Lite Version
1. Go to PSU Calc and put in all your shit. Get a wattage.
2. Look up your video card's amps.
3. Calculate your CPU's amps.
4. Go to newegg/whatever site you use and sort PSUs by wattage.
5. Check amps, connections, size, and whether its SLI/Crossfire certified and if its modular or not.
6. Try to find a review of the PSU @ Johnny Guru.
7. Find a review somewhere else.

Edited by crystalklear64, 02 June 2011 - 12:20 AM.


#3 Hydro2Oxide   Shazbot! CAGiversary!   4773 Posts   Joined 9.2 Years Ago  


Posted 01 June 2011 - 05:53 PM

Good reading. The place I've screwed up the most in my builds are with the PSU. I've undershot, and also relied on some shitty ones that burnt out. I'm just glad that so far, I've come out without any exploding motherboards.

#4 j-cart   PAPER TRAILS on a mountain CAGiversary!   5539 Posts   Joined 8.3 Years Ago  

Posted 01 June 2011 - 05:54 PM

GOOD STUFF. Can't wait to see this 2.0 version of this.

#5 crystalklear64   Ded CAGiversary!   13616 Posts   Joined 13.8 Years Ago  


Posted 02 June 2011 - 12:21 AM

changed some numbers and stuff around to make it more consistent with the images. made it look more pretty.


#6 Firvagor   CAGiversary! CAGiversary!   305 Posts   Joined 7.4 Years Ago  


Posted 03 June 2011 - 12:33 PM

Tom's Hardware has an article that explains brand vs manufacturer, which is helpful to determine what PSUs within a brand are more solid than others. The comments section and on that and the previous version of the article are also helpful.
For the more advanced, there's a brief guide on how to determine quality by the internal components (which I guess doesn't really help when buying, but it's still nice to know).

Newegg also has a PSU Calculator, although some hardware is missing from the listings and the wattage displayed is a bit different. It's normally found by looking under the Power Supplies category and clicking on the "What Power Supply Do I Need?" picture.

#7 piscian18   CAGiversary! CAGiversary!   238 Posts   Joined 12.5 Years Ago  

Posted 03 June 2011 - 02:02 PM

I stopped valuing johnny gurus opinions when he recommended a PSU that died on a friend of mine and me 4 times.

I don't think it's fair to recommend PSU's that start at $100 for average users.

I5 760 3.75Ghz Corsair H50 W7 U 64bit, MSI P55-GD80, 2x4GB OCZ Platinum 1333Mhz, GTX460 SLI Surround, 3x1 HP LP2465, 2x750GB Raid-0 2x1TB, Cooler Master Storm Sniper, OCZ 1010w GameXtream

This has been my machine for 6 months plus. That PSU is one Johnny Guru has stated is crap from a crap brand. I bought it for $60 refurb from SVC. If you're building a monster with 10 hard drives and a big watt video card like a 590 or a 6990 sure buy some monstrously expensive seasonic psu, but if you're a typical gamer some of these reviews can be a bit elitist.
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"There are times when I'm all by myself and I concentrate as hard as I can to see if I can't make myself catch on fire like the Human Torch." Dr.Cox - Scrubs

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