Picking The Right Power Supply: What You Should Know

Of Power Factors, Apparent Power, And Effective Power

PFC (Power Factor Correction): It Does What It Says

Don’t worry, you won’t need your high school physics text book for this part. We just want to point out another characteristic that can be used to tell a high-quality PSU from a bad one. Once you know the basics, you’re a lot less likely to make a bad purchasing decision. So, let’s dive right in.

Idle Current And Idle Power

One problem that plays an especially big role in the case of switching power supplies is the so-called idle current which results from inductivity. Note that idle current has nothing to do with the idle state of your computer. This idle current only transports energy between a generator and a load, but doesn’t end up benefiting the load in any way. Again, load in this context has nothing to do with a PC at full throttle, but refers to a component that uses power. Think of it as a power shuttle that isn’t involved beyond its role as a transporter. This idle current has to be reduced as much as possible and as early as possible, since it causes power loss in conjunction with ohmic resistors, manifesting itself as heat. This idle power consumption is a waste, and should be kept as low as possible through appropriate circuitry.

Effective Power And Apparent Power

Unlike idle power consumption, effective power measures the power that is actually used, while apparent power refers the sum total of the effective power and the cumulative idle power.

Power Factor

This factor is the result of the ratio between effective power and apparent power, falling somewhere between 0 (worst) and 1 (ideal). Thus, you should always ensure you are buying a power supply with a high power factor, since the factor is an extremely good indicator of the quality of the PSU circuitry.

Practical Application

Active PFC

As the name implies, active power factor correction (PFC) employs a circuit that actively corrects the power factor. As we just explained, the power factor is an important parameter in the world of PSUs, since it describes the ratio between effective power and apparent power:

Pros:
• An almost ideal effective power of nearly 99%
• Very stable current output
• Very low power loss
• Very low heat dissipation
• Does without iron, leading to reduced weight

Cons:
• Higher initial cost
• More susceptible to faults
Passive PFC

Passive PFC attempts to reduce idle currents by using large choke coils. While this method is simpler and cheaper, it is also much less effective.

Pros:
• Cheaper
• Practically no electro-magnetic interference

Cons:
• Requires better cooling
• Not suitable for higher power levels
• Increased power consumption due to power loss
• Heavier due to the choke coil
• Lower effective power ranging from 70% to 80%

Power supplies with passive PFC should be considered outdated and can be considered inferior.

Note: Do not confuse efficiency and PFC!
Summary
• 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.
• redgarl
Toms knows everything... yeah right.
• turkey3_scratch
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?
• JackNaylorPE
Voltage Stability ?

Ripple ?
• Aks_X
Unaligned tables :|
• 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
• 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)
• 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 .
• 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).
• SpAwNtoHell
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.
• mrkillall
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?
• Aris_Mp
there are no stupid questions, no worries.

• Travis Hershberger
You should've added a picture of the power supply that actually caught on fire in your low end power-supply comparison!
• Islam_5
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
• josejones
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
• Soaptrail
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.
• amk-aka-Phantom
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...
• rayden54
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.
• RobAC
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.
• zodiacfml
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.
• ssf_coffee
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
24 GB RAM
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.

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
• youcanDUit
the one thing that's really important about psu's is jonnyguru.com is the place you an trust.
• turkey3_scratch
593370 said:
the one thing that's really important about psu's is jonnyguru.com is the place you an trust.

Jonnyguru is not the only reputable site. I don't see why people seem to think this.