Learning about PSUs

[xFlankerx]

So my eyes are just being opened to the fact that you don't need 700W PSUs to power most high-end systems. The thing that basically triggered this enlightenment was this article on DailyTech about the 8800GTX, and the total system power consumption shown in a small chart near the bottom. As you can see, the Kill-A-Watt Power Meter shows that the PSU is pulling 321W from the wall socket. At 85% efficiency, it would mean that the system was actually being fed 273W.

So I started to wonder, what kind of a power supply would actually be needed to power this 8800GTX system? My original belief was that you would need a (273W/.7) = 390W PSU @ 70% efficiency to supply that. But then someone came along and told me that the power supply actually supplies just as much power as it's rated to, and it pulls extra from the wall socket, which is then lost as heat. A scenario came up;

Assume that we are testing a system that has 340W being supplied to the system. This person told me that the power supply that would be needed for this system, would be one that is called a 340W Power Supply. The actual draw from the wall would be 485W at 70% efficiency, and 400W at 85% efficiency. But in both these cases, it would still need to be rated a 340W PSU.

So if the 8800 system draws 321W at the wall, at 85% efficiency it's only sending 272.85W to the components. Person says that only a 273W PSU would be needed to power this system.

So, accounting for inconsistency and other bad things that happen to PSUs, is it true that you could run a 8800GTX system on a quality 350W PSU with lots of Amps on the +12v rail(s)?

 

[vexeus]

That review is semi vague about the power consumption, but I think they mean that it draws 321 W from the wall, and the PSU has an efficiency up to 85%, so 273 W from the PSU as you stated.

So yea, according to this logic, I'd say that as long as you have 25A on the 12V rails (I'm being super cautious here cause I don't know peak draw in systems when everything is on full blast), that'd be 300W on the 12V rails and maybe a 350 W PSU total.

I'm no expert though, so I could definitely be wrong here. I have seen the Corsair rep state that he has an SLI 8800 GTX system running rock solid on a 620 W Corsair PSU, so this solid 350 W PSU seems reasonable.

 

[Rabid Badger]

Just because you can get it to run on a 350W power supply doesn't mean you should. It's not the best idea to run your psu @ 100% capacity all the time, you'll just be begging for it to fail. Also, you won't be leaving yourself any room for future upgrades.

 

[sardonic.n7]

http://jonnyguru.com/reviews.php

 

[xFlankerx]

Just because you can get it to run on a 350W power supply doesn't mean you should. It's not the best idea to run your psu @ 100% capacity all the time, you'll just be begging for it to fail. Also, you won't be leaving yourself any room for future upgrades.

Yeah, but it won't be running 100%. As you can see, the 273W being fed to the system is at load, so the maximum power draw at startup shouldn't really be that much higher. And even that load won't be 100% of the time, how often do you have to have an 8800GTX running a game at full blast? So that power is actually around 78% of the maximum load of a 350W PSU.

But going even further than that, wouldn't the Forton 400W PSU with a total of 34A on the +12v rails be enough? As far as I know, it doesn't get much better than Forton (FSP) for quality PSUs. That would have the PSU running at 68% at that kind of load. Or in other words, an overhead of almost 50%.
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PLEASE correct me if I'm wrong, but I'm only going by facts that I see. I won't pretend to know everything about PSUs, I'm a relative noob, but I DO think that people grossly over-exaggerate the Power Supply that they need for their system, simply because of the large numbers that PSU companies are pumping out.

http://jonnyguru.com/reviews.php

Thanks for the link, but didn't really answer my question, lol.

 

[Rabid Badger]

According to the [H] review of the 8800 series, a beefy system with one 8800GTX installed uses 263W at idle, it uses 345W under load in a DX9 game.

Given that these cards will probably become even more power hungry running DX10 software and that psu's lose some of their punch over time, I'd say a little more cushion is advisable.

 

[Chernobyl1]

Yes, quite simply, you can power it with a PSU that gives those requirements but as others have pointed out, you shouldnt run a psu at 100% for quite a few reasons such as the specs not being 100% accurate.

Also...
PSU's age and as they do their maximum power drops and the voltage rails get noisier.
The hotter the PSU is, the faster it will age.
More efficient PSU's heat up less.
The harder you push a psu, the hotter it will get so lower power PSU's will age faster.

The higher the load you put on a PSU, the noisier the voltage rails get.
This can make a clocked PC (ie on the limit) unstable before the PSU has maxed its power output.

The efficiency of a PSU depends not only what its made from but how much power you draw too.
There is a range where you will get the PSU's highest efficiency. Outside that range the efficiency drops.
There are some good review sites that measure the optimal range for efficiency.

 

[xFlankerx]

According to the [H] review of the 8800 series, a beefy system with one 8800GTX installed uses 263W at idle, it uses 345W under load in a DX9 game.

From the review;

Here we are measuring the power being consumed by the system at the wall socket.

The DailyTech review measures 321W from the wall. However, DailyTech used a 85% efficiency Power Supply. More than likely (the review didn't list the PSU as far as I could tell), the one used in the [H] review was less than 85% efficient, 70% efficiency is what like 90% of PSUs are.

You shouldnt run a psu at 100% for quite a few reasons.

Yes, but as I pointed out, a 400W Forton with 34A on the +12v rails (408W) has the PSU running at 68% load, or an overhead of almost 50%.

 

[Chernobyl1]

Yes, but as I pointed out, a 400W Forton with 34A on the +12v rails (408W) has the PSU running at 68% load, or an overhead of almost 50%.

Thats good, the PSU wont be running hot and is well within the optimum power efficiency range.
Noise shouldnt be a problem either if overclocking.

 

[MrWizard6600]

yeah, a large chunk, often the majority of the power your computer is using is being used via CD drives and hard drives.

 

[unhappy_mage]

yeah, a large chunk, often the majority of the power your computer is using is being used via CD drives and hard drives.

... no. CD drives only draw ~1A from the +5 and +12 rails; hard drives may get up to 1A on each of those as well, or exceed that on spinup. But the CPU draws many watts from the 12V line even at idle - with smaller process sizes, the difference in current draw of just being on versus at full load gets smaller and smaller. So, since the TDP is 89W for the x2 line, call it 35W or 3A@12V when idle. Add in the memory (it's always powered up), the motherboard and addon cards, and the graphics card (!) and the hard drives are the least of your worries.