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How do I properly read PSU specifications?

P

penn919

Weaksauce
Joined
Feb 10, 2014
Messages
85
Hi,

Initially I was under the impression that the rated wattage for PSUs were sufficient for determining whether or not they are capable of powering all the components in your computer, but it's become clear that that is not the case. I read that I should only be concerned about having enough power on the 12v rail, but based on the specifications I'm not sure how I can determine that. let's use this as an example:

51AhzPDq5-L.jpg


How would I go about determining if this PSU could support a GPU with 300w minimum requirement? I'm sorry if I sound stupid, but GPU specs usually don't mention anything about 12v rails.

btw, this is my first post. Nice forum!
 
this is not a good psu, only 16 amps on the 12v rail a doable one needs to have at least the minimum of 18amps but most of the good ones have 40+. and second the name, its not particularly a good one stick with Corsair, Seasonic, PC Power and Cooling, XFX, Silverstone, Enermax, OCZ and Antec. some of these brands are made by seasonic. never skimp on a psu.
 
techtechie said:
this is not a good psu, only 16 amps on the 12v rail a doable one needs to have at least the minimum of 18amps but most of the good ones have 40+. and second the name, its not particularly a good one stick with Corsair, Seasonic, PC Power and Cooling, XFX, Silverstone, Enermax, OCZ and Antec. some of these brands are made by seasonic. never skimp on a psu.
Click to expand...

Ok, so does that mean I should judge PSUs by how many amps they have? The specs on graphics cards don't say anything about how many amps are required.
 
Watts = Volts x Amps. We know that Volts = 12 for video cards. In your case, you need 300 Watts. Therefore, 300W divided by 12 Volts = 25 Amps.

If you're looking at a PSU and wondering if it's any good, use as much identifying info as you can on it and Google for "HardOCP (psu name)" and "jonnyguru (psu name)" to find out what reviewers and forum posters at those two places think of it.

A lot of cheap PSUs have outright lies on their labels. Any PSU under $40 is not to be trusted unless proven good at a reputable review like like HardOCP or JonnyGuru; such cheap PSUs are very often found to be not actually capable of delivering more than half of their advertised power rating. They get away with this because most users don't actually try to use that much power.
 
penn919 said:
Ok, so does that mean I should judge PSUs by how many amps they have? The specs on graphics cards don't say anything about how many amps are required.
Click to expand...

That's one aspect. You also have to judge them on quality, manufacturer, warranty, performance, etc as well. Not all PSUs are the same and many PSU manufacturers actually over-rate their PSUs capabilities to sucker in those who don't know any better to buy their shitty PSUs. I.e the Logisys: There's no way that PSU is capable of 300W under real world conditions.

You should read this PSU FAQ if you want to know more about how to choose a PSU:
http://www.jonnyguru.com/forums/showthread.php?t=1036
 
evilsofa said:
Watts = Volts x Amps. We know that Volts = 12 for video cards. In your case, you need 300 Watts. Therefore, 300W divided by 12 Volts = 25 Amps.

If you're looking at a PSU and wondering if it's any good, use as much identifying info as you can on it and Google for "HardOCP (psu name)" and "jonnyguru (psu name)" to find out what reviewers and forum posters at those two places think of it.

A lot of cheap PSUs have outright lies on their labels. Any PSU under $40 is not to be trusted unless proven good at a reputable review like like HardOCP or JonnyGuru; such cheap PSUs are very often found to be not actually capable of delivering more than half of their advertised power rating. They get away with this because most users don't actually try to use that much power.
Click to expand...

That was actually the must helpful post I've ever read on this subject. So if I'm understanding you correctly, all video cards need to be supplied with 12 volts and I can always calculate how many amps I need on the 12v rail by dividing the total minimum wattage requirement by 12. Right?

Also, is the 12v requirement for video cards some sort of industry wide standard or are you speaking generally? And Is there some sort of resource you can point me to that details how many volts the various computer components require?
 
Dangman said:
That's one aspect. You also have to judge them on quality, manufacturer, warranty, performance, etc as well. Not all PSUs are the same and many PSU manufacturers actually over-rate their PSUs capabilities to sucker in those who don't know any better to buy their shitty PSUs. I.e the Logisys: There's no way that PSU is capable of 300W under real world conditions.

You should read this PSU FAQ if you want to know more about how to choose a PSU:
http://www.jonnyguru.com/forums/showthread.php?t=1036
Click to expand...

Thanks for that link! I'll definitely check it out.
 
penn919 said:
GeForce GTX 750 Ti
Click to expand...

Then the "300W required" you saw was not just for the video card, but for the entire system. You can find out how much a particular video card actually uses by Googling for its max TDP. For example, Google for "GTX 750 Ti max TDP", and the first hit for that card happens to be Nvidia's official spec page, which notes "Graphics Card Power 60W" and "Minimum System Power Requirement 300W". For older cards you will more likely find third-party reviews just noting what Nvidia or AMD said the max TDP was.

The max TDP is for Nvidia's reference card at stock clock speeds. Third-party makers like EVGA and MSI and others usually offer overclocked cards that do use a bit more power, but generally not an extreme amount more. I see in the HardOCP video card reviews an overclocked 780 Ti using 40W more than a reference 780 Ti, and those have a max TDP of 250W.

We know video cards use 12v because that's what's supplied to the PCI-E slots that video cards plug into, and (if they are needed) to the PCI-E power cables that plug into the video card. Video cards aren't the only things that use 12v, and there are also things that use 5v and 3.3v.

The good quality PSUs start at around 400W or 500W, and there are very few good ones below that; foozy lists one of those few. Nvidia states a minimum of 300W for the 750 Ti because anything below that is certainly crap.
 
evilsofa said:
Then the "300W required" you saw was not just for the video card, but for the entire system. You can find out how much a particular video card actually uses by Googling for its max TDP. For example, Google for "GTX 750 Ti max TDP", and the first hit for that card happens to be Nvidia's official spec page, which notes "Graphics Card Power 60W" and "Minimum System Power Requirement 300W".
Click to expand...

Very helpful tidbit of information right there. Previously I thought I was supposed to use the system power requirement when picking adequate PSUs for my graphics cards.

evilsofa said:
Video cards aren't the only things that use 12v, and there are also things that use 5v and 3.3v.
Click to expand...

I've been reading over that PSU FAQ in your link, and that makes a lot of sense now given that there are also +3.3v and +5v rails. So I guess the only remaining question I have would be how to determine which of the PSU rails each of the other components use. For example, do SATA/PATA ports also utilize the 12v rails or do they use one of the others? I'm guessing the ram also utilizes one of the rails; perhaps maybe the 3.3v rail because it shouldn't require much power?
 
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foozy said:
http://www.ebay.com/itm/FREE-SHIPPI...-/251500245094?pt=PCA_UPS&hash=item3a8e953066

Perfect PSU for what you need. I've used them countless times, never had an issue. Currently in my rig, Q9550, GTX 460(2 molex connections), 4GB RAM. Definitely Supplies more power then rated.
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not too sure I trust that, listing says 80 plus bronze, psu clearly says 80 plus only, no bronze. Person has decent feedback, but who knows if these are actually seasonic psus, he obviously doesn't look to close at his listings as hes posting wrong specs, could be some other company slapping seasonic labels on psus.

Could be legit seasonics, who knows how long they have been used as well since they are system pulls.
 
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penn919 said:
Very helpful tidbit of information right there. Previously I thought I was supposed to use the system power requirement when picking adequate PSUs for my graphics cards.



I've been reading over that PSU FAQ in your link, and that makes a lot of sense now given that there are also +3.3v and +5v rails. So I guess the only remaining question I have would be how to determine which of the PSU rails each of the other components use. For example, do SATA/PATA ports also utilize the 12v rails or do they use one of the others? I'm guessing the ram also utilizes one of the rails; perhaps maybe the 3.3v rail because it shouldn't require much power?
Click to expand...

Generally, you don't need to worry about that stuff. There's usually enough 3.3v and 5v amperage available for the average system. The biggest consumers of 5v are USB and hard drives/SSDs, in general. 3.3v powers some other low power items, generally not enough to matter. That's why the important thing is to have enough 12v amperage, as the most power hungry components (CPUs, GPUs, and fans) use 12v.
 
munkle said:
not too sure I trust that, listing says 80 plus bronze, psu clearly says 80 plus only, no bronze. Person has decent feedback, but who knows if these are actually seasonic psus, he obviously doesn't look to close at his listings as hes posting wrong specs, could be some other company slapping seasonic labels on psus.

Could be legit seasonics, who knows how long they have been used as well since they are system pulls.
Click to expand...

i normally excuse stuff like that when i see that the seller has so much feedback/rating. im sure he's very busy because he's selling a lot. if the issue concerns something big ill ask the seller a question but generally i wouldnt sweat the small stuff (which is what i consider that to be)
and ebay+paypal has you covered for 45 days after purchase if it cheap take the risk the only thing you potentially could loose is a bit of time
 
Tsumi said:
Generally, you don't need to worry about that stuff. There's usually enough 3.3v and 5v amperage available for the average system. The biggest consumers of 5v are USB and hard drives/SSDs, in general. 3.3v powers some other low power items, generally not enough to matter. That's why the important thing is to have enough 12v amperage, as the most power hungry components (CPUs, GPUs, and fans) use 12v.
Click to expand...

Alright. I think I get it now. I'm supposed to focus all attention on how many amps are on the 12v rail. I guess that does make a lot of sense because components like HDD, Ram, and USB don't really vary all that much in terms of power consumption. In fact as more people switch to SSDs, the power usage of the 5v rail will probably go down. However, the 12v rail powers the CPU, motherboard, GPU, etc that can vary quite a bit in power consumption so it's why builders should be more mindful of it.

Thanks all! I think I learned a lot here
 
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