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Why do they recommend 750+ watt PSu's for SLI/Xfire? - Page 4

post #31 of 44
Quote:
Originally Posted by spaceballsrules View Post
No. At 90%, 800W from the wall is 888.89W total.
The power draw at the wall is always higher than the power that the system is pulling from the PSU because the efficiency is less than 100%.

So exactly 800W at the wall with exactly 85% efficiency is exactly 680W. Exactly 90% efficiency is exactly 720W.
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post #32 of 44
Quote:
Originally Posted by guyladouche View Post
Likely mainly because power supply efficiencies change depending on the load. You might have a 750watt PSU in a system that is drawing 500 watts, but you have to consider the fact that the PSU at that load might only be 85% efficient, which means it can only reliably supply 85% of the max rated power (in theory).

You don't want to live too close to your PSU's rated power, but it would also be dumb to get a 1kw PSU to power a 300w rig because--for the same reason--psu efficiency.
you don't know what you're talking about.

80 percent efficiency means the rated watts is actually ~80% of the total draw from the outlet.

So if you have a 650 watt unit (like i have), then you are actually drawing

(usage watts) * 1/.8

So yes, if you buy a reputable unit, you will get what you see, however keep in mind you are actually pulling power from the wall (hence heat...) than your system is actually using.
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post #33 of 44
Just multiply the power draw at the wall by numbers like .8 for 80%, .85 for 85%, and .9 for 90%.

It really is as simple as that.
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post #34 of 44
i think the original question at hand was whether someone's 1kw rated unit is actually pulling 1kw FOR the system, and yes it is. BUT it actually has to pull more from the wall than 1kw to supply 1k to the pc.
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post #35 of 44
Quote:
Originally Posted by TH3 original View Post
i think the original question at hand was whether someone's 1kw rated unit is actually pulling 1kw FOR the system, and yes it is. BUT it actually has to pull more from the wall than 1kw to supply 1k to the pc.
Well, that's if the system is pulling 1000W from the PSU.
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post #36 of 44
Quote:
Originally Posted by TwoCables View Post
Well, that's if the system is pulling 1000W from the PSU.
Did you see my question? what would be a correct device to measure draw? Ive used my UPS, Belkin "kill-a-watt" type device and a zalman mfc2 and they all read about the same so what would i need to find the true value, or should i wait for an answer from Phaedrus2129
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post #37 of 44
Quote:
Originally Posted by Blindsay View Post
Did you see my question? what would be a correct device to measure draw? Ive used my UPS, Belkin "kill-a-watt" type device and a zalman mfc2 and they all read about the same so what would i need to find the true value, or should i wait for an answer from Phaedrus2129
I would wait for him. I've only used the Kill A Watt so far, and I always assumed that it was more or less accurate. I mean, I didn't know that Active PFC came into play.
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post #38 of 44
Quote:
Originally Posted by Soggysilicon View Post
Sure it does, for all the reasons mentioned in this thread.

Of course you are certainly welcome to go and buy some knock-off 850w HEC power supply and toss the dice for yourself.

Am I implying that you want to build your system around some sticker on a box? Hell no. It just happens to be the first thing I look for when shopping psu's for the purpose of an SLi/Xfire build, beyond that there are plenty of things I want to see to determine if the psu is of quality or not, as well as make some predictions as to where the system efficiency is going to fall when its all said and done.
There are plently of top tier Powersupplies without the certification... as there are many poor/average Powersupplies WITH the certification.

It. Means. Nothing.
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post #39 of 44
Quote:
Originally Posted by Blindsay View Post
umm no, those numbers are measured at the wall, thus what is actually going to the components on the inside is less
Quote:
Originally Posted by TwoCables View Post
The power draw at the wall is always higher than the power that the system is pulling from the PSU because the efficiency is less than 100%.

So exactly 800W at the wall with exactly 85% efficiency is exactly 680W. Exactly 90% efficiency is exactly 720W.
OK I thought you were talking about the 10% overhead you would want from the PSU. I am on the other side of the equation....

720W - What the power supply is providing to the components @ 90% efficiency

800W - Power being pulled from the wall, shown by Kill-A-Watt

888.89 - Minimum PSU wattage with 10% overhead

My bad.
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post #40 of 44
What's this about a 10% overhead? I haven't heard about it yet - not even from Phaedrus2129.
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It's a computer!
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i5-2500K @ 4.5GHz (1.368-1.384V fixed voltage) ASUS P8P67 EVO B3 (UEFI ver. 1850) GTX 780 ASUS DirectCU II (1228 / 6300, 1.180V) G.SKILL Ripjaws X 8GB (2 x 4GB) 1866MHz, CL9 
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250 GB Samsung 840 EVO (C:\) 250 GB Samsung 840 EVO (D:\) 150 GB WD VelociRaptor Samsung SH-S243N 24x DVD Burner 
Optical DriveCoolingOSMonitor
Samsung SH-S203N 20X DVD Burner Thermaltake Frio Win 7 Home Premium x64 SP1 Retail AOC G2460PG (24" 1920 x 1080 144Hz G-SYNC) 
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Filco Majestouch 104-key Cherry MX Blues w/NKRO Corsair HX650 (Bronze, ordered on 12-12-2009) CM 690 Intellimouse Optical (1.1A) 1000Hz polling rate 
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