Technology Explained

Power Supplies Explained: How To Pick The Perfect PSU For Your Computer

Matt Smith 31-07-2012

pick psu computerMost geeks interested in buying new hardware or building a new system think first of the processor, graphics card and perhaps the hard drive. These components have the most impact on performance, so they are given first thought. Somewhere down the list, sitting near the enclosure and optical drive, is the power supply.


This is unfortunate, because power supplies are important. They can have an impact on system stability, they determine the devices you’ll be able to power and they are a long-term purchase. If you buy a good power supply you probably won’t need another for five to ten years. But how do you buy a good one?

It’s Not Just About The Watts

pick psu computer

One of the problems with power supplies is the way they are advertised. Manufacturers rate their units with a given output in watts. There are two problems with this.

First, no one is responsible for fact-checking power supply manufacturers besides journalists, and producers of no-name power supplies usually don’t make their products available for review. A disreputable manufacturer doesn’t have to label a power supply with what it can actually supply. They can just label it with whatever they think they can get away with.

Second, the way a power supply delivers power is more complex than one big number. Power supplies have multiple rails, each of which carry a certain amount of current that is rated in amps. Different devices draw from different rails.  If you draw more power from a rail than it can deliver you will experience system instability. The power supply may even fail.


There is one rail you want to pay particular attention to – the 12V rail. This will be responsible for delivering power your video card, if you have one. A video card is by far the most power hungry component in a system, so you need to give it proper juice. You’ll want about 30 amps on the 12V rail for a computer with a single video card.

You can get by with much less if you only run a mid-range video card (like a Radeon 7750 or GTX 550), but you don’t have to pay a heap of extra cash to gain some wiggle room for future upgrades. The Corsair Builder Series 430W unit, for example, is rated at 28W on the 12V rail. It is priced at $42. Why save $10 or $20 on a power supply that might be just barely adequate?

What Those Certifications Mean

how to pick psu

As you browse for a power supply you will run in to certifications such as “80 Plus.” So what do those really mean?


It has to do with efficiency. A power supply’s job is to take current from your power outlet and convert it into usable power for your computer. Some power is wasted as heat in this process. An 80 PLUS certified power supply is at least 80% efficient, which means only 20% of power is lost as heat.

There are even better certifications including 80PLUS Bronze, Silver and so on. Check out the graph below for a summary of the requirements which must be met to achieve certification.

how to pick psu

A more efficient power supply will use less power at any given load, but don’t expect to shave more than a few quarters off your monthly power bills.


The main reason you should be interested in this certification is build quality. High levels of efficiency can only be achieved with high quality components and good engineering. If you are looking at two similar power supplies with similar prices the certification will likely be the tie-breaker.

Make The Connection

how to pick psu

Power supplies connect to the various components in a system using specific connectors, each of which is designed for a different type of component. PCIe connectors go to video cards, SATA connectors go to SATA drive, and so on.

Once again, video cards require some special attention. Current video cards capable of decent gaming performance usually require at least one 6-pin PCIe connector. Many more powerful cards require two 6-pin connectors. And the biggest, nastiest cards of all time (like the GTX 590) require one 8-pin and one 6-pin or two 8-pin connectors.


Obviously, you’ll want to research your build and make sure you buy a power supply with the connections you need. But it’s also a good idea to leave room for future upgrades.

The Corsair 430W power supply I suggested is great for any budget computer and can handle a decent video card, but you might want to instead buy the more expensive Corsair Gaming Series 600W, which includes an extra 6-pin PCIe and several more SATA connections. This will give you room to move up to a bigger video card and more hard drives without switching the power supply as well.

You also receive more SATA and peripheral connectors, is a boon if you have a tendency to use a lot of hard drives or internal cards.

Sometimes, Basic Works

pick psu computer

With all of this talk of power supplies and connectors and amps, you might be ready to go buy a Seagate Snotstomper 1250W with 20x SATA connections. This is what some people (particularly hardcore gamers) do no matter the system they’re building.

I recommend a more conservative approach. I ran a gaming computer for several years with a similar Corsair 450W power supply. This included a heavily overclocked Radeon 3850 and an overclocked AMD processor. It worked well, as it should. It had enough amps on the 12v to easily handle my video card and plenty of connectors.

You only need to think about more extreme solutions when you start to think about huge arrays of hard drives, multiple video cards or heavily overclocked processors. In these situations you really do need an extreme power supply – but most people reading this aren’t running 12 hard drives, twin GTX 680s and a heavily overclocked Core i7-3770K. You’re probably running a couple hard drives, a mid-range processor and a single video card. For that, a quality 400 to 500 watt power supply will do nicely.

Image Credit: Oliver Aaltonen

Affiliate Disclosure: By buying the products we recommend, you help keep the site alive. Read more.

Whatsapp Pinterest

Leave a Reply

Your email address will not be published. Required fields are marked *

  1. Matthew Miller
    September 12, 2016 at 5:39 pm

    Got a question. I did a small upgrade to my Dell Inspiron 560 by changing its Pentium processor out for a Intel Duo Quad Core extreme. I currently have a 200W power supply. Ill need a bigger fan to keep it cool so whats a recommended brand for a compact power supply thats around 400W that might fit that.

  2. h ref
    February 3, 2015 at 7:50 pm

    something is wrong with my psu !!
    i have psu with
    12 v ~ 8 A
    5v ~ 12 A
    3.3v ~ 8 A
    !!! its too low
    and i can run cpu i3 540 ~ 70 w
    vga ati 6570 ~60 w
    one hdd
    6 gb ram
    one dvd WR
    and big mother board
    i had this pc for 4 years and paly game all the time
    some times pc restart maybe 1 time in month and i know its because of psu
    my qustion is shod i change it or not ?? some one told me it can damage your pc ! is it true??
    because in my country this stuff are pretty expensive

  3. Mark
    September 28, 2012 at 2:08 pm

    didn't know it was this complicated...:(

  4. PatMcGroin
    August 1, 2012 at 11:30 am

    30 AMPS??!?!

  5. Harron Subhani
    August 1, 2012 at 7:02 am

    Amazing and valueable info.

  6. Shakirah Faleh Lai
    August 1, 2012 at 3:02 am

    I don't know what 80 Plus mean before, thank you.

  7. Igor Rizvi?
    July 31, 2012 at 11:20 pm

    Havent seen an article about PSU in ages.Great work!

  8. Rigoberto Garcia
    July 31, 2012 at 9:41 pm

    Excellent article. Recommended reading for anyone embarking on a project that includes the hardware. Many interesting things are not taken into account normally. Thanks Matt...

  9. John Jullies Palma
    July 31, 2012 at 8:22 pm

    Damm. I just bought a PSU yesterday before reading this. With help from a friend. But it will help if i have these knowledge. :D

  10. William Davis
    July 31, 2012 at 8:06 pm

    I have several PSU That I have acquired over the last few years, all top rated units (I do my research before purchase) and most are modular as I hate clutter and love air flow and range from 650 - 1000. My main rig utilizes a Corsair Professional Series Gold AX850 that I got last summer for 50 % off and I love it. Very quiet and handles anything I throw at it. It is one of the most (if not the most)important component of your system and should not be overlooked. Do your homework and you can insure a long and stable life.

  11. Joe Johaneman
    July 31, 2012 at 7:53 pm

    Great article. I'm thinking of building a small cluster (for a college project), so I've been looking at power supplies lately (because I'm looking to use the smallest amount of power possible). This will help a lot.

  12. Elijah Swartz
    July 31, 2012 at 7:32 pm

    I'm really surprised you didn't mention how to figure how many watts you'll need for your computer. An simple way would be to use a power supply calculator to give you an estimate. Here's one.

    Normally you'll want to go over by 20% or 100 watts for some future upgradability, overclocking, and a safety buffer.

    The power supply and the motherboard are the two components that you don't want to cheap out on. If one of those fails, it is possible to take out other components out with it.

    Seasonic manufactures reliable and affordable power supplies. Instead of just buying the Seasonic brand if they are more expensive then you'd like, look for power supplies that they manufacture. As of right now, I think all XFX PSU's are manufactured by Seasonic. XFX occasionally has pretty good sales, rebates, and combo offers making the PSUs quite affordable at times. Seasonic also makes quite a few Antec and Corsair PSUs as well. There are plenty of other good PSUs out there too. You just need to do your research. Look for popular ones that are well reviewed. More input can give you a better feel for the products reliability.

    Getting an 80+ certified PSU is important for more than just having more usable watts. The more heat capacitors are exposed to, the faster they age. If you are in a hot environment and/or have an inefficient PSU creating a lot of heat, you are giving your PSU a shorter lifespan.

    One last thing, not to be nit picky, but a GTX 550 doesn't exist, or not at least yet. There is a GTX 550 Ti though. If the GTX 550 did exist, it would most likely perform less than a GTX 550 Ti.

    One last thing, some computers use "horrible" proprietary power supplies. If they fail on you, normal power supplies won't fit in place and you'll have buy a replacement from your computer's OEM instead. It's something you should double check before just buying a PSU that sounds like it would work.

  13. Pilgrim57
    July 31, 2012 at 6:08 pm

    Interesting but you have a mistake "The Corsair Builder Series 430W unit, for example, is rated at 28W on the 12V rail". This should be 28 amps to give 336watts.
    One other point not mentioned is that if you run a PSU 24/7 then you need a bigger initial output to allow for aging of components that reduce output over several years. This also allows for upgrades. I've gone from a nVidia 260 to a 560Ti & that will be upgraded before I replace my PSU.
    A short 12v 4/8 pin cable can mean getting an extension in a tower case.

    • Trey Mahon
      August 1, 2012 at 2:46 am

      Thumbs up!

  14. phil
    July 31, 2012 at 5:51 pm

    That answered a lot of questions I had about PSU's, thanks