“How Much Do I Get Out, When I Put This Much In?”
While this is a valid question, we should probably rephrase it a little. Usually, you call the ratio between the amount of power drawn (from the wall wart) and the amount of power that is put out (to the computer) efficiency. The lower the amount of power a PSU has to draw in order to output a specific target power, the higher its efficiency.
While we’re at it, we’d like to clear up a very common misunderstanding regarding efficiency. If you have a 500 W power supply with an efficiency of 75 percent, that doesn’t mean it can only output 375 W to the PC. Instead, it has to draw 666 W from the wall in order to provide 500 W to the computer. So, the correct version of our question is, “How much power does my PC draw from the wall when it requires a certain amount of power?”
Example:
Let’s assume we’re really pushing our PC and it needs 600 W. Our PSU is rated at 80% efficiency. Here’s what it’s really drawing from the grid:
600 W / 0.80 = 750 W
Ideally, our PC will draw about 750 W from the wall under load. The remaining 150 W are, quite simply, wasted, and usually dissipated by the PSU as heat.
Nothing Is Constant, Not Even Loss
Our example above only holds true in an ideal world though, and since we don’t have super-efficient Star Trek technology, things usually don’t end up being that straightforward. A computer is used in various states, ranging from idle to full throttle, if you will, with every shade in between. Obviously, it will use the least power while idling on the desktop, more in casual use, and most under full load (3D graphics or intense calculations). Thus, we can’t expect to see constant power usage. Instead, we have to assume at least two states, namely idle and load. Now, let’s take a look at the efficiency of our hypothetical 600 W power supply under various loads.

Uh-oh; what’s this? Our nice, straightforward explanation seems to get bent out of shape in that graph. Looking at the curve, we can see that the PSU reaches its peak efficiency at about 50% of its nominal capacity.
Now, a clever observer would suggest that simply making the PSU twice as powerful should solve that problem. While this is correct in principle, that same clever observer would be forgetting something: the idle state. And this is where modern switching power supplies run into trouble. If their load drops to below 20%, their efficiency plummets to 60 or 50%, possibly even less. Ironically, this situation is only exacerbated by the power-saving mechanisms implemented in modern PC components. For example, a powerful system with a good graphics card can get by with as little as 65 W when idling, but draw a good 500 W under load. Thus, you have to ensure that the PSU is neither overtaxed nor under-challenged.
Example:
This time, let’s say our 600 W PSU is supplying 65 W to the system. What load does that correspond to?
(100% / 600 W) * 65 W = 10.83%
Now, take a look at our chart, and you’ll see things aren’t looking too good. Houston, we have a problem. Let’s repeat our calculation, this time assuming a 68% efficiency, as suggested by the chart.
65 W / 0.68 = 95.6 W
Despite the fact that the system really only requires 65 W, the PSU is drawing almost 100 W from the wall and turning the remaining 30 W into heat. And these are the numbers for the more efficient of the two hypothetical power supplies, too! Not to get ahead of ourselves, but there were two efficiency curves in that diagram, one for a cheap power supply and another for a more expensive one. And wouldn’t you know it, the supposedly cheap (and fictitious) DragonMegaHyperCombatUltra PSU for 30 bucks turns out to be a real power hog when the system is idling, driving up your power bill in the long run.
Again, this is only a hypothetical example. For our next trick, we’d like to show you what actually happens. As it turns out, we can easily allow for the impact of efficiency in our calculations. Oh, and it’s just as easy to prove that cheap PSUs will often turn out to be a lot more expensive than you might think in the long run.
- Brought To You By Granny’s Radio
- How A Switching Power Supply Works
- Efficiency, Efficiency, Efficiency!
- Of Power Factors, Apparent Power, And Effective Power
- How To Spot An Efficient PSU?
- Don't Get Burned: Safety Before Stinginess
- How To Determine Your Power Requirements
- The Power Window Is Important
- Example 1: The Office PC
- Example 2: Mid-Range Gaming PC
- Example 3: The Enthusiast’s System
- If You Don't Like Our Advice, Buy A Fire Extinguisher
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.
i want
Would like to see more articles written like this.
Well written, and well done !!!
i want
No one really listens to this.
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.
Now I got a CM PSU and no problems since!
Helps keep the Phenom under control.
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.
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.
Would like to see more articles written like this.
Well written, and well done !!!
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.
CM PSUs aren't that amazing either
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.
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