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Author Topic: What is the max draw at WALL of a PSU? 80% of 120% of peak output rating?  (Read 1066 times)
ben8jam
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July 08, 2017, 04:42:40 AM
 #1

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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July 08, 2017, 04:56:29 AM
 #2

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.

600w max if you want it to last.
ben8jam
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July 08, 2017, 05:03:51 AM
 #3

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.
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July 08, 2017, 05:11:03 AM
 #4

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.

You are way over thinking this.  600w max at the wall for a 750w psu.

All electrical should be 80% for safety.
ben8jam
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July 08, 2017, 05:23:57 AM
 #5

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.
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July 08, 2017, 06:12:30 AM
 #6

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
 #7

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.
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July 08, 2017, 07:16:34 AM
 #8

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.

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

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July 08, 2017, 09:28:10 AM
 #10

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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July 08, 2017, 01:46:47 PM
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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.
ben8jam
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July 08, 2017, 01:55:04 PM
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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.
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July 08, 2017, 01:59:23 PM
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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.

No, how many people have given you the correct answer?  How many times have you re asked the same question looking for a different answer?  You're an idiot plain and simple.  Now, go burn down your house.
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July 08, 2017, 02:15:03 PM
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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
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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.
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July 08, 2017, 02:28:42 PM
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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.

If you don't want to listen to us, go read some psu reviews on the internet dumb dumb.

But like I already said, go burn down your house.  You seem like the type of person that already has though.
ben8jam
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July 08, 2017, 02:35:45 PM
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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.
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July 08, 2017, 02:37:21 PM
 #18

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.

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July 08, 2017, 02:39:13 PM
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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.

You seem the be the one who doesn't know what they are talking about.

Have fun destroying equipment.  Bye bye.
ben8jam
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July 08, 2017, 02:52:54 PM
 #20

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?
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