Picking The Right Power Supply: What You Should Know

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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.
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.


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?


Contributing Editor
Feb 5, 2015
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)
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.

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).


Dec 5, 2015
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.


Jan 24, 2015
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?


Contributing Editor
Feb 5, 2015
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)


Jun 9, 2016
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


Jan 12, 2015
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.


Mar 10, 2011
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...


May 14, 2013
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.


Mar 31, 2013
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.


Oct 2, 2008
The first and second pages just introduce more confusion than clarifying anything.
It could have shown AC waveforms and converted DC waveforms to show how it works. By the way, introduction on AC and DC currents is a must.


Jun 11, 2016
Hoping this is the right forum or that you folks are willing to entertain this one:

I have a system purchased from an engineer whose wife ordered it out of her living room. About $600 for:

Gigabyte GA-Z87X-HD3
Intel i7-4770k
EVGA GeForce GT-740 FTW 1GB

Some Cooler Master cooler that barely breaks a sweat.
At 100% load the system runs 60-65 W and 65 degrees C according to the Intel Power Gadget.

Samsung EVO 240 GB SSD
2 TB WD Green (most storage is on a NAS)
Corsair CX500M PSU (four years old?)

Soprano case

Now, I'm running three monitors: 2 off the GT740 and one off the integrated. That third monitor usually only has documentation or mail up for viewing. Most of my work is office level junk with the occasional heavy Excel Solver LP/Evol. work, random forests in R, video editing in Camtasia or visualization in Tableau.

Now, about a month ago, I had an OCZ SSD go bad but also ended up with a system that would only boot to the BIOS and freeze. Draining the CMOS worked and its been solid ever since. I've tried adding a few other older IDE items (DVD, and 250 Zip drive) using converters but it seemed to be unreliable for booting it would sometimes come up and sometimes would refuse to even power up. The system has also complains occasionally about the power use on the USB (iBeam whiteboard system?).

When I'm not using it the CPU is pegged doing work for the World Community Grid using the BOINC grid client. So the CPU runs max most of the time.

Someone suggested the PSU was at its limit.

Now, I've thought about swapping out the PSU for a Corsair HX850i (refurb) which is currently on sale. I assume it will be a simple swap.

I also just received a Sapphire Radeon HD7850 2GB card (free)

Now, I'm hoping a new PSU will deal with the power issues and perhaps be a bit more efficient given the current 100% constant load. It would be nice to have room for upgrades for when I win the lottery and spring for a pair of 1080s. In fact, a GPU upgrade might be possible in the near future.

So the questions are:

1. Is the Corsair HX850i going to solve the too much load problem (assuming that is the problem).

2. How about the graphics cards, should I use both the Nvidia and AMD or simply use one OR use one or more with the integrated Intel graphics. Will any of these cause bottlenecks?

3. While I'm at it, is adding another 8GB of memory worthwhile? I've hit a memory wall a few times (using R I've gotten a 64GB image running) but I'm wondering if more will actually help on the day to day stuff.

Would love to hear your thoughts and thank you.

- David
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