Picking The Right Power Supply: What You Should Know

How To Determine Your Power Requirements

A Matter Of Simple Addition

While there are many online calculators meant to help you figure out how much power your build will use, they all estimate its maximum requirement. They then take into account the PSU’s peak efficiency at a load of 50 to 55 percent and make a (generous) projection based on those numbers. The drawback is that this method does not consider consumption when the computer is idle, which we already determined is where efficiency often appears worst.

The following table is meant as a guideline, showing you how much power a certain component is likely to use under a specific load. If you know the exact figures for your components, plug them in instead and do the math.

 Component Type    Description Idle (W)Load (W)Number Installed
CPUCurrent dual-core CPU
Current dual-core CPU, overclocked
Current quad-core CPU (Mid-range)
Current quad-core CPU (High-end)
Current quad-core CPU, overclocked (High-end)
Older dual-core CPU (AMD)
Older dual-core CPU (Intel)
Older single-core CPU
20
25
35
40
45
35
55
35
65
90
95
125
140
90-125
125-140
60-90
1
MotherboardCurrent microATX board without graphics
Current microATX board with graphics
Normal mid-range board without graphics
Normal mid-range board with graphics
High-end board
High-end board, overclocked
15
30
20
25
35
40
25
40
35
50
45
55
1
RAMCurrent DDR2 or DDR3 RAM, per 2GB module
Current DDR2 or DDR3 RAM, per 4GB module
Overclocked RAM per module (estimated)
2
3
4
4
5
6
1-4
HDDConventional hard drive28-10Varies
SSDCurrent solid-state drives14Varies
ODDDVD-ROM only
DVD burner
Blu-ray reader/burner
1
1
2
6
10
12
1
FanNormal CPU fan, mandatory
Silent case fan
Performance case fan
1
2
3
1-3
2
3
1
Varies
Varies
Add-In CardExternal sound card
TV card
Controller card
2
1
1
8
2-5
2
0-1
0-1
Varies
Graphics CardsCurrent office-oriented card
Current mid-range gaming card
Current high-end gaming card
10-16
16-30
25-35
35-75
75-180
180-375
1
1-4
1-4
OtherCold cathode lighting, modding parts, etc.See manufacturer infoVaries

Now that we have an idea of how much power each component draws, calculating the overall consumption at idle and under load is easier. Based on this information, we will define a power window in the next chapter and choose the best PSU for the job.

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30 comments
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  • A few more words about active power factor correction. APFC won't save you money on your electric bill although the electric companies will love you for it as it minimizes loss over the power lines saving them money, it does however enable you to use a much lower rated battery backup system. A hypothetical example a computer that uses say 200W without APFC would require a backup system of 700W or much more to cope with the large peaks in current where as a power supply with APFC would require a backup system of 250W or so.
    Therefore APFC is only worthwhile if you were to use it with a battery backup system.
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  • Toms knows everything... yeah right.
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  • Very well written article! Just one thing. You say:
    Quote:
    Regardless of whether the PC is idling or under full load, voltages may not deviate from their spec by more than five percent according to the ATX spec.

    But the ATX specification seems to disagree. According to the spec, full load or "peak loading" allows 10% deviation from the nominal voltage for the 12V rail.

    http://www.formfactors.org/developer/specs/Power_Supply_Design_Guide_Desktop_Platform_Rev_1_2.pdf

    Also, Q about the power factor correction. It's probably the most difficult topic to understand. In this case, you say the load would be anything that used power. Are you talking about hardware like a GPU or the internals of the PSU like capacitors and such? Also, say the computer is putting load on the PSU. How is there idle current then?
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  • Voltage Stability ?

    Ripple ?
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  • Unaligned tables :|
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  • There have been quite a few instances in the past where you could get an XFX PSU 550w or so for 40$ or less with rebates. I know compared to more modern PSUs they may not stack up, but they used to be pretty decent. Ya, more often than not, the cheaper the PSU the worse the quality, but you really do need to do your homework.
    Budget PSU
    https://www.youtube.com/watch?v=Ezk9OA7aKOE
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  • The newest ATX spec defines 5% at peak load as well. The 10% is only for the -12V rail which is now optional. The newest ATX spec is confidential (dont know why)
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  • While I can understand that having a beefy power supply on idle state wont be too efficient, its on loads where you want it as efficient as possible.
    Somehow, having a low efficiency under a 65W load is less expensive than low efficiency at 500W load, go figure :D.
    1
  • In all fairness, a PC is not a self-maintenance Robot.
    If you want a PC to last a good 10-15 years you need to take care of it:
    Clean dust, replace fans when they fail, replace thermalpaste, check your temperatures from time to time, not turn it on-off-on too fast, keep your Hard drives with some spare space and defraged if they are HDDs....

    There is quite some work for a PC to keep their form, but its not like a human can lay down in bed eating cheese and drinking cola looking like a model either.

    PSUs however have this strange aura of magic around them since some people vastly overestimate what power supply they need (I got a 700W TT one for a load of 320, go figure) and others buy things that are simply bad products, no matter how high the W are.

    I did once burn a PC due to a bad PSU (and I even OCed the damn PC, went down in smoke.. I gotta say it was quite fun, but expensive), so I stay on the safe side (I just simply add an extra 20% for 12v rail amps as long as the price of a quality supply is not doubling).
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  • Is this write up inteded for who needs to replace a psu on a old system? Test systems are very ineficient really... So we are talking about 80 plus bronze gold on sandy bridge gtx 580 ... Enthusiast system?! My casual gaming needs ask me to upgrade 2-3 generations... I know is just a test for power of psu but not elocvent for some people who would want to use systems as a guide... Otherwise good writeup as now rather to explaining to people why you need a good psu and a optimal size can resume at passing tbis link.
    -2
  • This could be a stupid question but the power coming from the outlet is ac does it get changed to dc somewhere to be reconverted in to ac again or what?
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  • there are no stupid questions, no worries.

    • AC socket: 100-240 VAC
    • After bridge rectifier: full-wave DC (square root (2) x AC input)
    • After APFC's bulk capacitor(s): 380 VDC
    • After primary switchers: chopped DC signal (feeds the main transformer's primary winding)
    • Transformer's secondary winding output: high frequency AC signal
    • Secondary side output: DC rails (12V, 5V, 3.3V, 5VSB, -12V)
    -2
  • You should've added a picture of the power supply that actually caught on fire in your low end power-supply comparison!
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  • my pc is powered by an unknown chinese brand labeled as zero 1000 ,it is 1000 watts ,which cost me 50 egyptian pounds about less than 5 us dollars , and yet it is still running my core 2 duo e8400, one stick 8 gb teamelite ram ,nvidia gt 740, gigabyte g41 motherboard and 3 harddisks 4 terbytes in capacity . i am still worried about its safety measures
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  • I'm curious to learn, at what point is a new PSU a must for new CPU's, mobos and GPUs? I have a Seasonic X-750w I bought new in 2012. What do I look for to know when I need to get a new PSU to go with a new system?

    http://www.newegg.com/Product/Product.aspx?Item=N82E16817151087
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  • Thanks, I need to replace an old PSU that is starting to go. I was going to get Gold or Platinum but now I will get Titanium for the extra efficiency at 10% loads.
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  • Quote:
    my pc is powered by an unknown chinese brand labeled as zero 1000 ,it is 1000 watts ,which cost me 50 egyptian pounds about less than 5 us dollars , and yet it is still running my core 2 duo e8400, one stick 8 gb teamelite ram ,nvidia gt 740, gigabyte g41 motherboard and 3 harddisks 4 terbytes in capacity . i am still worried about its safety measures


    That build most likely doesn't consume more than 200-300 W (rough guesstimate) at peak load. Try to load your PSU to actual 1000 W and watch the fireworks. Cheap PSUs *never* deliver the wattage they're rated for. At least in the EU and probably in the US regulations forbid selling PSUs that can't actually deliver the rated wattage or don't hold up to safety standards...
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  • Thank you very much for the chart. The one bad thing about most calculators is they don't usually break things down very well. I think it's part of the reason why people end up with more PSU than they need: They want to leave room for upgrades, but aren't sure how much or little each extra thing'll cost 'em.
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  • Nice article, thank you.

    I recently built a work station / game machine for rendering and the occasional demanding games. I chose a massive power supply that can output a bit more than I need to future proof an upgrade to whatever the 2nd gen Titan X Nvidia is coming out with later on this year.

    The funny thing is, I have a OC 6 core CPU and I can hear the water cool CPU fans spool up and increase in speed when I am rendering - however the PSU fans never turn on at all because it is so efficient and runs so cool. Very impressed technology has advanced so much from over 20yrs ago when I last built a machine. I switched to laptops and a NUC for a few years but got back into building my own full desktop PC.
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