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Why do they recommend 750+ watt PSu's for SLI/Xfire? - Page 2

post #11 of 44
Thing is though, not every PSU have enough amps on its 12v rails do they? How is the card manufacturer suppose to know what PSU does people use.
    
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post #12 of 44
P/U=I So they asume that if you got enough watts the current will be fine. And since it`s not theire money you spend they don`t care if you buy an overkill PSU...
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post #13 of 44
The reason why some people recommend PSUs with way more capacity than is actually needed is because they just don't know what they're talking about (and so they shouldn't even be trying to help).

Here's one thing that many people don't know: the power supply requirements that AMD and NVIDIA state are in regards to PSUs with a "peak" rating. So let's look at the power supply requirement for the GTX 470:

550W

So this means that they are recommending a 550W peak-rated power supply to power a system that has a single GTX 470 in it (it's for the entire system, not just the graphics card). In addition, they're likely also taking into account the chances that the person who buys a GTX 470 has the latest system (that is, whatever is the latest at the time the graphics card was released) instead of what I have (see my sig rig).

Therefore, we can summarize it by saying that power supply requirements for graphics cards are always bloated, especially for overclockers. The reason why is because almost every experienced overclocker (or everyone who's experienced in building computers) is using a quality power supply. Most quality power supplies (i.e. recommendable power supplies) have a "continuous" rating. For example: my power supply has a 650W continuous rating. One giveaway is that the +12V's wattage capacity is very close to the wattage rating of the entire power supply (624W, 26W shy of the 650W rating).

If you're wondering why I am using the HX650 to power my little system, it's because I originally wanted the HX450 (I like the look of the HX series), but the cost after international shipping made it more expensive than the HX650 at that time.

Another reason why people recommend an overkill power supply is because they don't know that having overkill negatively effects the PSU's efficiency:

On PSU Efficiency

PSU "50% Load" Myth

Others simply think it's much wiser to have extreme overkill because that means you never, ever, ever, ever, ever have to worry when you upgrade. However, these people are failing to take these things into consideration:
  1. The cost of an overkill PSU
  2. The lower efficiency due to the extremely tiny power draw on the PSU
  3. The chances of having poor PSU performance due to having too small of a power draw on the PSU - even with a top-quality unit.
  4. The chances of others seeing what you have and thinking "Well, if that's what they're using to power their system, then that must be what is needed. So, that's what I'll get". I mean, some people prefer to just look at what everyone else has instead of "bothering" us with a new thread. Heh.


Finally, another reason is because they don't know how to use online power supply calculators correctly.
Edited by TwoCables - 3/25/11 at 4:54pm
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post #14 of 44
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.
    
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post #15 of 44
They say use a 650+ for SLI/CFX and 550+ for single cards. nAs for the 6990/590 they say 1200+ is needed.

(that's what they say anyway. I bet you can run it with just 950-1000watts.
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post #16 of 44
Here's how efficiency works:

If the power draw at the wall were 700W, and if the efficiency during that power draw were 85%, then it means that the system was pulling 595W from the power supply during that reading at the wall outlet.

If the power draw were still 700W at the wall but with an efficiency of 80%, then it means that the system was pulling 560W from the power supply.

To make this even clearer: if the efficiency were 100% at the time of that power draw, then it means that the system was pulling 700W from the power supply.

So, the less efficient a power supply is, the more power it has to pull from the wall in order to give the system what it is asking for.

In other words, let's say that you have a system that pulls 500W from the power supply during completely full load between the CPU and graphics card(s):
  • A 100% efficient power conversion would mean that the PSU draws 500W from the wall (exactly the same, due to 100% efficiency)
  • A 90% efficient power conversion would mean it draws about 556W from the wall
  • An 85% efficient power conversion would mean that the PSU draws about 588W from the wall
  • An 80% efficient power conversion would mean that the PSU draws about 625W from the wall

Of course, there are other factors that effect efficiency (such as how big the power draw is on the PSU), but let's pretend that it's this simple.
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post #17 of 44
Quote:
Originally Posted by TwoCables View Post
Here's how efficiency works:

If the power draw at the wall were 700W, and if the efficiency during that power draw were 85%, then it means that the system was pulling 595W from the power supply during that reading at the wall outlet.

If the power draw were still 700W at the wall but with an efficiency of 80%, then it means that the system was pulling 560W from the power supply.

To make this even clearer: if the efficiency were 100% at the time of that power draw, then it means that the system was pulling 700W from the power supply.

So, the less efficient a power supply is, the more power it has to pull from the wall in order to give the system what it is asking for.

In other words, let's say that you have a system that pulls 500W from the power supply during completely full load between the CPU and graphics card(s):
  • A 100% efficient power conversion would mean that the PSU draws 500W from the wall (exactly the same, due to 100% efficiency)
  • A 90% efficient power conversion would mean it draws about 556W from the wall
  • An 85% efficient power conversion would mean that the PSU draws about 588W from the wall
  • An 80% efficient power conversion would mean that the PSU draws about 625W from the wall

Of course, there are other factors that effect efficiency (such as how big the power draw is on the PSU), but let's pretend that it's this simple.
Understood.

With PSU ratings--let's say a 750w PSU. Is that the theoretical max that the PSU could supply (assuming 100% efficiency--yeah, right...), or is that the maximum amount that the PSU can draw from the wall? I've always been confused about that.
    
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post #18 of 44
Quote:
Originally Posted by guyladouche View Post
Understood.

With PSU ratings--let's say a 750w PSU. Is that the theoretical max that the PSU could supply (assuming 100% efficiency--yeah, right...), or is that the maximum amount that the PSU can draw from the wall? I've always been confused about that.
I think that the rating is the capacity for the computer system without any regard to the power draw at the wall outlet.
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post #19 of 44
Quote:
Originally Posted by TwoCables View Post
I think that the rating is the capacity for the computer system without any regard to the power draw at the wall outlet.
Thanks.
    
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post #20 of 44
So, would a good psu like ax750w be enough for sli gtx 580(mild overclock) with i7 920(mild overvolt)?
Edited by Madstrike - 4/2/11 at 5:57pm
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