Topic: What is the max draw at WALL of a PSU? 80% of 120% of peak output rating? (Read 1236 times)

I usually run at 80% of the rated power output to be safe and efficient.

you are correct.   it is a math game

 if a psu is sending  70% of it 1200 watts   in dc it is 840 watts

but  that is the dc at the end of all the 12 volt cables  attached to mobo/cpu/gpu.

the ac load  at the kwatt meter  would be 840/.875 = 960 watts..


or 840 dc/ 960 ac = 87.5% eff  .

Correct me if I'm wrong, but 70% dc load is 70% of 1200w, 840w.

Example, take a 1000 watts psu, if you psu is 90% efficiency gold then it means when your computer is at the wall 1100 watts, it means your psu is using its promised 1000 watts but the thing is, as the psu gets close to 1100 watts at wall, 100%, efficiency drops a little, the best efficiency is at 45% offered. If you want the best efficiency at full load then get a platinum or titanium, best brands are corsair, superflower, evga, seasonic, coolermaster.

check out jonnyguru.com

he is pretty much the guy with the numbers.

http://www.jonnyguru.com/modules.php?name=NDReviews&op=Story&reid=523

http://www.jonnyguru.com/modules.php?name=NDReviews&op=Story3&reid=523

http://www.jonnyguru.com/modules.php?name=NDReviews&op=Story4&reid=523

above is a review of a corsair hx 1200 plat.

it has a 100 amp 12 volt rail

at hot and cold box tests.

92 amp   is  89% hot  and  89.4% cold.    since we run 24.7  just look at hot numbers
70 amp   is  90% hot  and  90.6% cold.
46 amp   is  92% hot  and  91.8% cold.

he has numbers for hundreds of psu's

I try to pull 70% on my psu's  so 1200 watt dc = 90.5 % efficient or  1200/.905 =  1326 max x 70% = 928.2 watts  at the kwatt meter

look at 1200 x 80% and you get 960 watts 

very close to the same  960 and 928.2 

if the psu was 87.5%  eff  at a 70% dc load  you get

1200/.875 =1371.4285  x .7 = 959.999

which is 960 = 960

So  a  1200 watt psu  on a 70% dc load  with 87.5% eff  is  the exact same as   80% of 1200  or 960 = 960

so  both people would be exactly correct  some of the time. 

Maybe it's confusion...or lack of critical thinking.

Absolutely. I think the part that is confusing to everyone, is a 750W PSU registering 750W on a Kill-A-Watt is actually only outputting approximately 637W (given say a 85% efficiency rating at that higher draw). To run it at the full 750W output, you would see about 930W on the kill-a-watt (assuming it only runs at a decreased 80% efficiency at max output).

Of course you can run it higher.


I actually looked and couldn't find any research on it, so I agree with you, the 80% for continuous load for a circuit is probably being conflated to "ideal" continuous load on a computer power supply.  However, to me, it does make sense that a power supply ran <100% would last longer than a power supply ran at 100%.  With that said, getting it around the  80% mark is not a bad idea.  I wouldn't sleep over it if I go a little bit higher.  I have a 2 power supplies, 900w, with 6x125w load, and a 750w, with 475w load, that comes to a load 83% and 63% respectively.


Awesome.

Thank you actually writing something meaningful. Looking at the efficiency graph (see links to chart below) for my PSU, it looks like at 80% of max output (600W output) it is still ~90% efficient. So at 600W it would register 666W on a Kill-A-Watt while 300W (or 40% of max output) it is the most efficient, does that necessarily mean it needs to run that low?

I guess my question is, where is there research that states 80% of max INPUT is the magic number? I know you might feel like I'm beating a dead horse here, but there is obviously a lot of misinformation about this. The 80% number is a percentage that datacenters use in terms of the max DRAW on a circuit, not anything that has to do with a PSU. So, I wonder if people have conflated these things, while really they are unrelated.

And then people "props" who just calls everyone morons, and has no actual empirical evidence (if you look at his 16 pages of comments, they are 90% calling people morons or dummies or just trolls comments in general.

I will call EVGA and get it straight from the horse's mouth tomorrow as to what the best output is for long life while also hard use. And for everyone saying I'm burning my house down, the reason I'm asking this is so I don't do that. But I like science and actual fact over generalized statements. All four of my 750W psus are currently running at max 575W and below.

Efficiecny charts for the G2 750W PSU
http://www.tomshardware.com/reviews/evga-supernova-750-p2-platinum-psu,4566-5.html
https://www.techpowerup.com/reviews/EVGA/SuperNOVA_G2_750/6.html

----------------


Thanks. How did you determine your output draw to know you're at 50% and thus the max efficiency? Feel like there needs to be a kill-a-watt type device that is in between the PSU and the cabling. But obviously that would require quite a setup.

Well, he is right. The PSU is rated at the output, not input. So I'm running my platinum PSUs at about 50% load, I should be getting the max 93%.

forget

Yes 522W on a 750W PSU, that's already 70%.  That 87% rating you quoted is the maximum efficiency, it is not the same from 0 to 100% load.  See this article:

Quote: "Typically, your power supply's maximum efficiency is between 40% and 60% of it's maximum capability."
Source: http://www.corsair.com/en-us/blog/2013/october/how-to-build-a-pc---the-power-supply

Running a PSU closer to its limits will also put more wear and tear on the components, it will run hotter for most of its service life, and what's the big deal starting out with a bigger PSU?  You pay more upfront but a PSU lasts a long time.

This is the best answer. It depends heavily on the quality of the PSU. I have a bunch of high quality Corsair and EVGA PSUs that I've run at close to 90% the rated capacity for years without an issue. Before that, I tried to save a few bucks on lower-quality PSUs and learned my lesson quickly - cheaper PSUs are more expensive in the long-run.

Keep in mind also that PSUs are more efficient closer to 50% load - that could save some $$ in the long run over running them at 80-90% rated capacity (along with less chance of damage and higher resale value).

But if OP is going for density - why not contact the manufacturer and ask if you can run at 80% of 120% peak output rating without voiding the warranty? That should tell you all you need to know - if the manufacturer will stand behind it, it should be good. I'd still make sure everything is as fire-proof as possible - no matter what load. All it takes is a bad riser and some dust...

So what you're saying is because most people don't know what PSU efficiency ratings are and/or basic math, that the blanket 80% rule is safest for them to use?

But if we actually look at the numbers. Based on a 750W 87% efficient PSU, 600W at the wall means it is only outputting 522W (600 x 87%) which is 69.6% of its max (522W / 750W). Is that essentially what you are saying? That a 750W PSU can only run safely outputting 522W? Because doesn't that seem wrong?

PSU's ratings are there to show you the limits.  You shouldn't be running them at 100% load 24x7.  That is why people are telling you 80% of their rating as measured from the wall because while there are different interpretations, this is the safest.  Safety is more important than that last GPU you are trying to load.

Yes, this is what I'm finding to be the truth as well. The other people on here don't actually know what they're talking about and are applying a PDU rule to a PSU.

The 80% rule for data centers that you're referencing is based on the circuit load not the PSU draw. You can't pull more than 80% of the circuit max. And the same should apply to your home. If you have a 15A circuit then don't pull more than 12A continuous, or 1,440W (based on 120V). In THIS case the measured wall draw using a Kill-A-Watt would apply. But that's not the question I'm asking.

A 87% efficient Gold rated 750W PSU will pull 862W at the wall while outputting 750W (hence why higher efficency psu's are better, becuse you lose less wattage to heat).

The more I research it, the answer is to keep your output below 80% of the OUTPUT (not the input) which would be 600W. And to do so, you should measure your input using a kill-a-watt, then caclulate the actual output based on your efficiency, and not go over 689W (600W / 87%).

I'm getting the impression you all don't really know what you're talking about. If anyone who is good at math, and has actual insight into this cares to reply, please do.

I agree with the people above me, keep it 80% and have a clear mind, don't push capacities to the limit.

Dude settle down.

I have a 1250W, gold 90% efficiency. I run at at 1150W at the wall (using at 80% of the rating). I think it will be high efficiency and last long.