Picking The Right Power Supply: What You Should Know

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How does a power supply work? Why is it important to choose a sufficiently powerful and efficient model? We guide you through discussions of efficiency and tips for getting the best deal before we go on to explain why less can be more in the PSU market.

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.

Swipe to scroll horizontally
 Component Type    Description Idle (W)Load (W)Number Installed
CPUCurrent dual-core CPUCurrent dual-core CPU, overclockedCurrent 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 CPU202535404535553565909512514090-125125-14060-901
MotherboardCurrent microATX board without graphicsCurrent microATX board with graphicsNormal mid-range board without graphicsNormal mid-range board with graphicsHigh-end boardHigh-end board, overclocked1530202535402540355045551
RAMCurrent DDR2 or DDR3 RAM, per 2GB moduleCurrent DDR2 or DDR3 RAM, per 4GB moduleOverclocked RAM per module (estimated)2344561-4
HDDConventional hard drive28-10Varies
SSDCurrent solid-state drives14Varies
ODDDVD-ROM onlyDVD burnerBlu-ray reader/burner112610121
FanNormal CPU fan, mandatorySilent case fanPerformance case fan1231-3231VariesVaries
Add-In CardExternal sound cardTV cardController card21182-520-10-1Varies
Graphics CardsCurrent office-oriented cardCurrent mid-range gaming cardCurrent high-end gaming card10-1616-3025-3535-7575-180180-37511-41-4
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.

Aris Mpitziopoulos
Aris Mpitziopoulos
Contributing Editor

Aris Mpitziopoulos is a contributing editor at Tom's Hardware, covering PSUs.

30 Comments Comment from the forums
  • abryant
    Archived comments are found here: http://www.tomshardware.com/forum/2916-56-picking-power-supply-know
    Reply
  • pjmelect
    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.
    Reply
  • redgarl
    Toms knows everything... yeah right.
    Reply
  • turkey3_scratch
    Very well written article! Just one thing. You say:
    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?
    Reply
  • JackNaylorPE
    Voltage Stability ?

    Ripple ?
    Reply
  • Aks_X
    Unaligned tables :|
    Reply
  • jossrik
    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
    Reply
  • Aris_Mp
    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)
    Reply
  • cats_Paw
    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.

    Reply
  • cats_Paw
    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).
    Reply