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 Author Topic: What is the max draw at WALL of a PSU? 80% of 120% of peak output rating?  (Read 1086 times)
ben8jam
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 July 08, 2017, 04:42:40 AM

I seem to have gotten the math backwards on measuring the safe and efficient max draw measured at the wall for my PSU.

On a 750W PSU, is the wall max 937W measured at the wall, as 80% (efficiency) of 937 is 750W? But to be safe, I should stay under 80% of the efficiency rating, which would be 600W draw from PSU, or 720W (600 *1.2) at the wall?

Can someone confirm this? In order to have four 470/570 gpus dual mining SIA, they hit 720W on my Kill-a-watt on a 750W EVGA G2 PSU.
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ben8jam
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 July 08, 2017, 05:03:51 AM

But is that 600W at the wall measured using a Kill-A-Watt or 600W output (meaning the cards are pulling 600W, but at wall it measures 720W)?

From what I've been reading, 80% efficiency PSU means for every 100W at the wall, it's only outputting 80W, as 20W is being lost to heat. So if your accessories had fixed power draw written on them, you could use up to 600W of accessories, but if you actually measured the amount of electricity you were using, it would show 720W.
ben8jam
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 July 08, 2017, 05:23:57 AM

I don't think I am at all. There is just this 80% number thrown around, and I think people aren't understanding what it means. Start reading up on it and you'll see.

And it does matter, because it is the difference between being able to add an extra card to my PSU.

For starters:

http://www.tomshardware.com/forum/343495-28-power-supply-efficiency-myths

"A power supply rated at 500 W can deliver 500 W of power to the system regardless of its efficiency, the efficiency tells you how much power it must draw from the wall to deliver those 500 W it does not affect its output capacity, a 500 W power supply that is 90% efficient can deliever the same amount of power as one that is only 80% efficient. If the power supply is 80% efficient it needs 625 W(500/0.8 ) from the wall, those extra 125 W are turned into heat by the power supply, while a 90% efficient unit would only be drawing 555 W from the wall, meaning it is dissipating only 55 W as heat, or only 44% of the heat the 80% efficient unit was creating."

So going by that write-up and to stay within the 80% rule that you are talking about (the one that is based on output, not input), then 80% of the max 500W output used in the example above would be a max output of 400W. That would mean a 400W output would pull 500W from the wall (because 20% of the draw is lost to heat), which you would say is too high, but in reality it is right on target.

Do a little research and quote your sources if I'm wrong.
Apneal
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 July 08, 2017, 06:12:30 AM

I never understand people who ask a question and then correct people answering them lol. If you have the technical answer already, why the fuck are you asking people with years of experience that have blown up more PSU's than you've ever bought?

If you dont care how long the PSU last, then just go nuts on it. No one knows how much voltage and electrical noise you're drawing or how close to capacity you're drawing or the quality of your PSU components etc. This shit makes a difference. 650W rated PSUs are NOT created equally. If you have a G3 series PSU, you can push it a hell of a lot more than a GS or G2 for example. Source: I've blown up 3 G2 PSUs instantly under the same conditions that G3s run smoothly under. I've had one PSU with subpar components blow up at 80% load, while I've ran another PSU with better components at 130% capacity for months.
ben8jam
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 July 08, 2017, 06:48:46 AM

That's why I'm asking the question. I'm reading conflicting information and wanted to know the actual technical answer. Don't be a jerk.

80% of output measured at the wall makes no sense as the two are (as you say) not related. Input draw is higher than output.

I'll let someone with some actual knowledge of the subject answer and provide some facts on how to determine an actual number based on efficiency rating.
Frippster
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 July 08, 2017, 07:16:34 AM

No, youre right. A PSUs wattage is the converted output, and depending on the efficiency rating it will draw a different amount of juice from the socket. A Gold rated PSU will have a 12% spill at 100% capacity. So when you are pulling roughly 850W from the wall you would be at its max delivery. I would however not suggest you do this, i would pull the brakes when the killawat reads just above 750W which would put you at roughly 90%... which is fairly high as well.

thesavoyard
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 July 08, 2017, 07:46:07 AM

80% is the law in most countries for datacenter continuous power draw (not peak). If you feel you know better, feel free to max out your PSUs. We'll see you back here soon complaining they are dead.

Gotimour
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 July 08, 2017, 09:28:10 AM

I seem to have gotten the math backwards on measuring the safe and efficient max draw measured at the wall for my PSU.

On a 750W PSU, is the wall max 937W measured at the wall, as 80% (efficiency) of 937 is 750W? But to be safe, I should stay under 80% of the efficiency rating, which would be 600W draw from PSU, or 720W (600 *1.2) at the wall?

Can someone confirm this? In order to have four 470/570 gpus dual mining SIA, they hit 720W on my Kill-a-watt on a 750W EVGA G2 PSU.

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.

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ben8jam
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 July 08, 2017, 01:55:04 PM

I'm so tired of these known it all retards.  Let them burn.

Don't blame us when you wreck your hardware or burn down your house and kill somebody.

80% is 80% fucktard.

Dude settle down.
YIz
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 July 08, 2017, 02:15:03 PM

I agree with the people above me, keep it 80% and have a clear mind, don't push capacities to the limit.
ben8jam
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 July 08, 2017, 02:25:07 PM

80% is the law in most countries for datacenter continuous power draw (not peak). If you feel you know better, feel free to max out your PSUs. We'll see you back here soon complaining they are dead.

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.
ben8jam
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 July 08, 2017, 02:35:45 PM

No, youre right. A PSUs wattage is the converted output, and depending on the efficiency rating it will draw a different amount of juice from the socket. A Gold rated PSU will have a 12% spill at 100% capacity. So when you are pulling roughly 850W from the wall you would be at its max delivery. I would however not suggest you do this, i would pull the brakes when the killawat reads just above 750W which would put you at roughly 90%... which is fairly high as well.

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.
chiwalfrm
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 July 08, 2017, 02:37:21 PM

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.

ben8jam
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 July 08, 2017, 02:52:54 PM

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.

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?
jmumich
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 July 08, 2017, 03:29:57 PM

If you dont care how long the PSU last, then just go nuts on it. No one knows how much voltage and electrical noise you're drawing or how close to capacity you're drawing or the quality of your PSU components etc. This shit makes a difference. 650W rated PSUs are NOT created equally. If you have a G3 series PSU, you can push it a hell of a lot more than a GS or G2 for example. Source: I've blown up 3 G2 PSUs instantly under the same conditions that G3s run smoothly under. I've had one PSU with subpar components blow up at 80% load, while I've ran another PSU with better components at 130% capacity for months.

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...
chiwalfrm
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 July 08, 2017, 04:07:06 PM

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.

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?

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.

philipma1957
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 July 08, 2017, 04:39:10 PM

forget

I mine alt coins with https://simplemining.net...
I see BTC as the super highway and alt coins as taxis and trucks needed to move transactions.
thesavoyard
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 July 08, 2017, 05:29:00 PM

80% is the law in most countries for datacenter continuous power draw (not peak). If you feel you know better, feel free to max out your PSUs. We'll see you back here soon complaining they are dead.

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.

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%.

ben8jam
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 July 08, 2017, 08:03:03 PM

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.

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

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

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%.

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.
darkomega69
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 July 08, 2017, 11:06:23 PM

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?

Of course you can run it higher.

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.

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.

All four of my 750W psus are currently running at max 575W and below.
Awesome.
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