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 Post subject: Confused about psu specs
PostPosted: Tue Apr 17, 2012 10:44 am 
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Guys iam stuck between two psu specs.

PSU 1:

600W, has two rails with 30A each, combined power 504W. This has a bronze certificate.

PSU 2:

600W, has a single rail with 48A. Delivers a total of 576W. 80plus.

So, when taking the specs into account the second one seems to be more preferable as it provides more power than the first one. Am i right or there is something else i should know?

Secondly, the gpu i installed requires a psu with at least 500W power. If, for example, i choose a psu with 550W which delivers only 462W, which figure should be taken into account, first or the second one?

Thanks in advance


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 Post subject: Re: Confused about psu specs
PostPosted: Tue Apr 17, 2012 11:36 am 
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30A is a lot for a single rail so it's no problem. 80Plus Bronze is nice so not knowing anything else I'd lean towards that one, though there might of course be other factors to weigh in like noise, brand and cost.

There's no single GPU/card that actually requires as much as 500W, the card maker recommendations are usually set with a huge margin in order to leave room for what else the builder might possibly put into the system.

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 Post subject: Re: Confused about psu specs
PostPosted: Tue Apr 17, 2012 3:29 pm 
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Throw your build list out here and we can estimate your actual power use/psu needed. As mkk stated, GPU mfgrs have no idea what other components are going into the PC or how shoddy/good the psu will be. So, they state a very large number to cover it.

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 Post subject: Re: Confused about psu specs
PostPosted: Wed Apr 18, 2012 4:45 am 
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Location: Grenoble, France
Mayalogy wrote:
Secondly, the gpu i installed requires a psu with at least 500W power.

Your GPU doesn't actually require a 500 W PSU, it's just a very misleading recommendation from the GPU manufacturer. They recommend that you use a much higher powered PSU than is usually necessary, because they don't know what your other components are like (so, for instance, they assume that you have a high power, overclocked, CPU - just in case) and because they don't know whether you are going to buy a good quality PSU (which can actually deliver what's written on the box) or a shitty no-name PSU (that will set your PC on fire if you try to actually push it to 2/3 of its 'official' power rating - and I'm barely exaggerating here).

In practice, if you have the slightest idea of what you are doing, you can safely ignore the PSU recommendations from graphics card manufacturers. What matters is the actual power consumption of the graphics card, added to that of your other components.


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 Post subject: Re: Confused about psu specs
PostPosted: Wed Apr 18, 2012 9:31 am 
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What actually I was trying to find out that why there is such a big gap between the combined powers ie 504 vs 576. The second psu (corsair) delivers almost what it says on the label whereas in the first one (high power), the power drops down almost one-sixth of the total power stated on the label. That is a bit too much I guess.

I know no gpu consumes 500W ( maybe a few out there, not sure) but when using those psu calculators on the net giving you a rough idea about the total consumption, would you choose your psu according to the advertised power or the combined power?


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 Post subject: Re: Confused about psu specs
PostPosted: Wed Apr 18, 2012 10:22 am 
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Mayalogy wrote:
What actually I was trying to find out that why there is such a big gap between the combined powers ie 504 vs 576.

There's many different ways to build a PSU and these methods have evolved over time as PC power usage has evolved. In the past, a large part of the power used by the cpu, mobo, and add-on cards came from the 3.3V and 5V rails and very little voltage conversion was done on those boards. Now, most of the draw is from the 12V rail and the mobo VRM circuitry as well as all the add-on cards do whatever voltage conversion they need locally.

So, a PSU rated at 500W just means you add up all the power available from the 3.3V, 5V and 12V rails. Some of the older/cheesier designs follow this. Newer/more reputable designs can add up to more than the rated 500W and nearly make the 500W rating from just the 12V rail.

A better way to figure out how big of a PSU you need is to:
1) figure out how much power you'll need:
- look at review sites with similar builds. They'll do stress tests and you can see the max power used.
- or list your build and ask us.
2) decide on how quiet you want it to be/how much you want to spend/ how efficient:
- Take a look at the recommended PSU list. Compare your estimated power to the noise levels.
- If you go with a fanned design, chances are a quiet PSU will be rated at ~ 2x the power you need....it's how they set the fan speed profiles.
- If you go with a fanless design, be sure to add some margin to your estimated power to account for derating over time, derating over temp.

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 Post subject: Re: Confused about psu specs
PostPosted: Wed Apr 18, 2012 11:41 am 
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Some time ago, I posted my build list already but made couple of changes on it, so here it is:

p8z68 asus mobo
seagate 7200 (and possibly a second hdd with 100gb or so for data backup)
hd6850
8gb ram
i5 2500
dvd-wr
1-2 case fans

I don't think this build will be a resource hog so a moderate psu would possibly be enough, but I am in kinda dilemma whether I should go for something 550W or 600-650w for future upgrades :?


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 Post subject: Re: Confused about psu specs
PostPosted: Wed Apr 18, 2012 1:26 pm 
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Mayalogy wrote:
Some time ago, I posted my build list already but made couple of changes on it, so here it is:

p8z68 asus mobo
seagate 7200 (and possibly a second hdd with 100gb or so for data backup)
hd6850
8gb ram
i5 2500
dvd-wr
1-2 case fans

I don't think this build will be a resource hog so a moderate psu would possibly be enough, but I am in kinda dilemma whether I should go for something 550W or 600-650w for future upgrades :?

Your system and one of mine are almost identical but for the mobo; mine's a P67 -- no significant power diff. My system uses a Seasonic X560 (because it was there) and the maximum peak AC power I've seen on my power meter was around 170W, iirc, in the middle of some 3D game demo. It usually cruises (like now) at 50~75W. I could easily run it with a good 300W PSU. Even 200W would do.

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 Post subject: Re: Confused about psu specs
PostPosted: Wed Apr 18, 2012 1:52 pm 
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Location: Grenoble, France
Mayalogy wrote:
What actually I was trying to find out that why there is such a big gap between the combined powers ie 504 vs 576. The second psu (corsair) delivers almost what it says on the label whereas in the first one (high power), the power drops down almost one-sixth of the total power stated on the label. That is a bit too much I guess.

You seem to be mixing up two different things : the total power rating, and the power rating for the 12 V line(s) only. A PSU doesn't just output 12 V, it also outputs 5 V and 3.3 V (and -12 V, but that's pretty much negligible). In practice, most modern components draw power mostly (or only) from a 12 V line, but the other lines still get some use (USB ports include a 5 V power line, for instance). So, a PSU rated for 600 W total and 504 W combined on the 12V lines isn't "dropping down almost one-sixth of the total power stated on the label" - it just means that the other 96 W are only available to the 3.3 and 5 V lines. In practice, you want a PSU that can output most of its total rating on the 12V line, but both PSUs seem to fit the bill well enough in that regard (500W is quite a lot already).

Mayalogy wrote:
I know no gpu consumes 500W ( maybe a few out there, not sure) but when using those psu calculators on the net giving you a rough idea about the total consumption, would you choose your psu according to the advertised power or the combined power?

First of all, the PSU calculators you can find on-line generally give very conservative estimates (read: they overestimate everything). So keep in mind that you should generally consider them as a worst case situation (if they say you need a 500 W PSU, you really don't need more, and you're probably fine with a 450 or 400 W one...), rather than some kind of average estimate. Of course, I can't say that's true for all on-line calculators - but it seems to be true for most of them at least.
(edit: The numbers from real measurements given by MikeC above show just how much those on-line PSU calculators can over-estimate... Though, for fanned PSUs, most of them get way noisier when you get close to the max power rating, so you may want a bigger PSU than necessary for this reason.)

Second, what they give you is the total power consumption, regardless of the lines used, so you want to look at the total power rating of the PSU. But you also have to keep in mind a few things:
- a modern PSU needs to be able to output most of its power rating on the 12V lines, because it's what most components use;
- obviously, that means that it doesn't need very high ratings on the 3.3V and 5V lines, so you normally don't have to worry too much about those (except if it's for an old PC);
- very cheap, no name PSUs often use very misleading labels (when they don't outright lie about their power ratings) - though you don't have to worry about that if you buy from a reasonably reputable brand.


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 Post subject: Re: Confused about psu specs
PostPosted: Wed Apr 18, 2012 3:38 pm 
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MikeC wrote:
Your system and one of mine are almost identical but for the mobo; mine's a P67 -- no significant power diff. My system uses a Seasonic X560 (because it was there) and the maximum peak AC power I've seen on my power meter was around 170W, iirc, in the middle of some 3D game demo. It usually cruises (like now) at 50~75W. I could easily run it with a good 300W PSU. Even 200W would do.


MikeC: I think that 170W AC could pop up to 250W* with a game that stresses both the cpu and gpu near 100%. I never thought that would happen until I loaded SWTOR on my PC and my power load when gaming went from 145W to 215W AC.

Mayalogy: Anyway, there are lots of decent PSUs in the recommended list that are pretty darn quiet/silent @ 250W AC. How much do you want to spend?


* comparing my e8400/hd5770 system with a ~80% psu to an i5-2500/hd 6850 with a ~90% PSU.

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 Post subject: Re: Confused about psu specs
PostPosted: Thu Apr 19, 2012 9:16 am 
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Quote:
Mayalogy: Anyway, there are lots of decent PSUs in the recommended list that are pretty darn quiet/silent @ 250W AC. How much do you want to spend?


Around 50-60 bucks. A quiet and efficient psu will just do the job.


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 Post subject: Re: Confused about psu specs
PostPosted: Thu Apr 19, 2012 10:44 am 
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That's on the low end of quality PSU pricing. You can get a Seasonic M12II 520W for ~$60. It's a decent, fairly quiet PSU.

If you move up to the $100 range, there are higher efficiency/newer designs that run cooler and quieter..as well as use less power. You can see the recommended/reviewed ones here.

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 Post subject: Re: Confused about psu specs
PostPosted: Thu Apr 19, 2012 12:39 pm 
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I haven't decided yet but I'll likely go for one with the best price and performance ratio.

Thank you all guys for your nice comments, appreciated.


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