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Of Power Factors, Apparent Power, And Effective Power

Picking The Right Power Supply: What You Should Know
By

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!
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Top Comments
  • 16 Hide
    iam2thecrowe , September 7, 2011 6:02 AM
    SteelCity1981Like the old saying goes... "You get what you pay for."

    unfortunately that is not always the case, you could buy a resonably expensive Thermaltake TR2 RX series and end up with a piece of crap.
  • 12 Hide
    fakie , September 7, 2011 5:29 AM
    DragonMegaHyperCombatUltra PSU

    i want
  • 10 Hide
    MrHull , September 7, 2011 10:42 AM
    Great article, even an idiot like me could understand it.
    Would like to see more articles written like this.
    Well written, and well done !!!
Other Comments
  • 8 Hide
    SteelCity1981 , September 7, 2011 4:43 AM
    Like the old saying goes... "You get what you pay for."
  • 10 Hide
    aliened , September 7, 2011 4:59 AM
    ^^^^ That's right. I just spent $180 bucks on a Corsair AX850 but I don't care 'cuz I know that I'm getting a great PSU with which I will rarely encounter any problems, my other hardware is safer and I have plenty of room to upgrade my system on a future. IMHO trying to save as much as possible on a cheap PSU is like playing Russian roulette.
  • 1 Hide
    g-unit1111 , September 7, 2011 5:12 AM
    After having two bad experiences with Ultra PSUs, I will never, ever use a no name, low budget PSU.
  • 2 Hide
    bennaye , September 7, 2011 5:18 AM
    The subheading "When Buying Cheap Can Be ReallyExpensive" has a typo, ie. missing a space in between the "Really" and "Expensive". Its on the 6th page , titled "Don't get burned; Safely before Stinginess". And imo, the "Safely" should be a "Safety".
  • 12 Hide
    fakie , September 7, 2011 5:29 AM
    DragonMegaHyperCombatUltra PSU

    i want
  • 7 Hide
    Zero_ , September 7, 2011 5:46 AM
    Quote:
    # Buying too big just to be on the safe side is counter-productive and only makes sense if you actually need the reserves.

    No one really listens to this.
  • 16 Hide
    iam2thecrowe , September 7, 2011 6:02 AM
    SteelCity1981Like the old saying goes... "You get what you pay for."

    unfortunately that is not always the case, you could buy a resonably expensive Thermaltake TR2 RX series and end up with a piece of crap.
  • 3 Hide
    ryandsouza , September 7, 2011 6:20 AM
    Learnt the hard way... Was using the PSU bundled with my case. It was the typical cheap, no-brand PSU. The PC was constantly crashing/ freezing and I never suspected the PSU. Ended up damaging 2 HDDs... Had to RMA both.

    Now I got a CM PSU and no problems since! :D 
  • 3 Hide
    hmp_goose , September 7, 2011 6:22 AM
    Old Skool PCP&C FTW!
  • 10 Hide
    amdfangirl , September 7, 2011 6:33 AM
    I have a fire extinguisher next to my computer.

    Helps keep the Phenom under control.
  • 8 Hide
    alhanelem , September 7, 2011 6:57 AM
    crap just realized my PSU has a passive PFC, gonna replace it ASAP
  • 3 Hide
    radium69 , September 7, 2011 7:00 AM
    No Super Flower Platinum 550W? I have one and they are damn efficient I can tell! ;) 
  • 8 Hide
    alidan , September 7, 2011 8:27 AM
    i thought i should give a bit of context here.

    most good psus are able to out live a pc build under 24/7 use, some able to last 10 years.

    where i live power costs 11 cents per kwatt. it should be 9 like my state average, but whatever, i wont go into that further here.

    now with 11 watts difference, over the coarse of 1 year under 24/7 use, that comes out to about 10$ and out national average is also 11.2 cents, so rounding down to 11 is appropriate.

    now here are some numbers, based on estamates.

    11 watts - 10$ a year
    22 watts - 20$ a year
    33 watts - 30$ a year

    and lets go with a pc thats built right lasts 4 years, and is used for 5 (waiting on parts to be released and such)
    base/4year/5year
    10/40/50
    20/80/100
    30/120/150

    basically you have to look at the long run, and anticipate a mid cycle upgrade, such as new gpu, with 50-100 more watts headroom.

    lets also assume that you turn the pc off at night, some people do, but leave it on all other times. even at half the cost, on the gameing, and mid range, its more cost effective to buy the better psu, the highend though, used a 750 watt Corsair, there should be a 550 or 600~ watt one that would be cheaper and better fit in line with the other psus tested.

    more to the point. we needs a toms chart for psus, right now we have a efficiantcy chart, but what we need is a build chart.

    one that takes the psus, and puts them in a standard pc enviorment, like the ones here, and measures the watts used.

    than gives us a baseline like this one is 100% than how much more the ones above use in %, than in numbers next to it, and than figuring the annual power cost for the extra watts they use, and find out which ones really the best for your builds, assuming a 4 year build cycle with a 5 year use, like i did above.

    i honestly think that could result in an interesting benchmark.
  • -1 Hide
    SteelCity1981 , September 7, 2011 9:35 AM
    iam2thecroweunfortunately that is not always the case, you could buy a resonably expensive Thermaltake TR2 RX series and end up with a piece of crap.


    Yes but in most cases if you buy something cheap the vast majority of the time it's going to be cheaply made so therefor it will break a lot faster then something you buy at a preimuim that's has quality. Your odds of getting a lemon Bently is a lot lower then buying a Dodge that's a lemon. of coure you pay a hell of a lot more for the Bently then your typical dodge but that's because you are paying for the high preimum of quality and craftsmenship that goes into it vs something that is made mostly by a machine.
  • 10 Hide
    MrHull , September 7, 2011 10:42 AM
    Great article, even an idiot like me could understand it.
    Would like to see more articles written like this.
    Well written, and well done !!!
  • 1 Hide
    DSpider , September 7, 2011 11:38 AM
    Rocking a no-name PSU here. I do realise that a slightly more expensive PSU makes more sense if it lasts 5+ years (or at least has the potential to, because you never know...) instead of some cheap crap that won't even last 2 years. But the circuitry design and components today aren't the same as 8 years ago. For instance all motherboards sold today have solid capacitors, even the cheap ones. Meaning they will last much more than their 8 year old equivalents.

    Beware of retailers that don't pass on full specs. Always check the manufacturer website for specs. Some retailers seem to forget to add complete descriptions for PSUs (and products in general too) which they have to order, but seem to add more info to the ones they have on stock. Probably to make them look better so they get rid of it faster. Living in Romania I already have "blacklisted" quite a few retailers from which I'll never EVER buy, based on this solely alone. It was actually very subtle, and it wasn't that they simply "forgot" either. As soon as I noticed this discrepancy I started to question their entire business model. Didn't even bother to tell them (didn't want to) because I'd rather have some other smart person catch on to their scheme instead of guiltying them into correcting it.
  • 8 Hide
    julius 85 , September 7, 2011 12:30 PM
    ryandsouzaLearnt the hard way... Was using the PSU bundled with my case. It was the typical cheap, no-brand PSU. The PC was constantly crashing/ freezing and I never suspected the PSU. Ended up damaging 2 HDDs... Had to RMA both.Now I got a CM PSU and no problems since!

    CM PSUs aren't that amazing either
  • 5 Hide
    cTs Corvette , September 7, 2011 12:39 PM
    Ever wonder who really makes that PSU? Here's a very simple tool that will let you find out:

    http://database.ul.com/cgi-bin/XYV/template/LISEXT/1FRAME/index.htm

    Look for the symbol on the PSU label that looks like a backward "R" next to a "U". Under that symbol, you will find the letter "E" followed by a 6-digit number. Put that in the UL Online Certification Directory on the line that says "UL File Number" and it will tell you who that file number belongs to. For example, you would find that some Silverstone PSU's are made by Silverstone, some are made by "Enhance Electronics Co. LTD." and some are made by FSP.
  • 0 Hide
    CaedenV , September 7, 2011 12:41 PM
    First 2 builds were power supplies that were bundled with the case (350W and 450W). After killing several mobo's and hard drives I finally forked over the money for a nice Thermaltake 480W power supply, back when Thermaltake was awesome :-). The difference was night and day between the day to day opperation of my computer. I had no idea that it would cause everything to run so much smoother.
    Fast forward another year sitting in a 100yr old dorm room with a power grid that was not meant to run the amount of draw that we were all pulling, and we were all loosing parts left and right. This was long before active power correction was normal, or even affordable. But I lost my video editing rig right in the middle of finals week, and it was entirely due to the power available (though not the power supply that lasted me several more years without problems when I moved out of the dorms).
    On my last build I bought a very nice 500W power supply that has all of the modern niceties of voltage regulation and power protection (as well as the power connectors that the old supply did not have), but now that I am looking at doing a new build next year I am not sure if it will be big enough. I think it will be fine at first, but I am going to do SLI down the line, and according to the math here I am going to need a peak of ~900W available, meaning I need a ~1000+W PSU... and that is going to hurt the wallet
  • 1 Hide
    julius 85 , September 7, 2011 12:50 PM
    Peak ~900W? You're going to use like 4 GTX570s?
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