help understanding PSU wattages

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drdenby

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Jan 21, 2014
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how is it that a PSU can have a Max output capacity of 550W and a "peak" output capacity of 660W ?

How is it that max doesn't mean max? What is the difference between them?

If I did get a 550W PSU, how much power can I truly expect from it?

What will 550W run? will it run a 3ghz processor, 8gb ram, one mid range gpu, and 2 displays (1monitor, 1tv)?
 
Solution
V
Any value with peak in the title is not very useful. The supply can't maintain these values consistently.
In most systems, the most important figure to read is the rating of the +12V rail.
If in amps, multiply this figure by 12 to get the rating in watts.
Multiply by 0.8 because we only want to load this to 80% of the maximum rated load.

Add the following to get the required power in watts:
TDP of the CPU (77W to 95W for most Intel CPUs, 125W for most AMD FX CPUs)
Max power rating of the graphics card (250W for GTX 780, R9 280X, 290 or 290X, 230W for GTX 770, 180W for R9 270X, 170W for GTX 760, etc.)
40W for other components in a typical system

So as an example, a system with a Intel Core i5 4670K and a GTX 760 graphics card would...
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ksham

ksham

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Mar 29, 2013
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There is no standard for what 550W means. There are a lot of PSUs that do NOT perform up to specification. Just because it says 550W, you may not get 550W. You may get only 500W. So it' s important to buy a PSU from a reputable brand and read up on reviews of PSUs before purchase.

Peak rating is not an important metric when evaluating a PSU. The more important one is that it provides continuous current. If a PSU does not provide continuous current, then do not buy it. A peak current means how much a PSU can provide for a brief period of time. If your PSU only has peak current and not continuous, then it isn't useful because there's no standard to what Peak means. It can be 1 minute, 1 day, a few hours, no one knows. At which point, it goes down.

So to answer your question based on "550W" without knowing the brand and model would be a rather impossible task.
 
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BustaRhymes

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Jun 16, 2014
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I dont know the answer to your first question.

Your displays will not be run by your power supply. You will plug those straight into the wall.

550W should be ok for most computers with one graphics card. What graphics card do you have?
 
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VincentP

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Oct 18, 2013
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Any value with peak in the title is not very useful. The supply can't maintain these values consistently.
In most systems, the most important figure to read is the rating of the +12V rail.
If in amps, multiply this figure by 12 to get the rating in watts.
Multiply by 0.8 because we only want to load this to 80% of the maximum rated load.

Add the following to get the required power in watts:
TDP of the CPU (77W to 95W for most Intel CPUs, 125W for most AMD FX CPUs)
Max power rating of the graphics card (250W for GTX 780, R9 280X, 290 or 290X, 230W for GTX 770, 180W for R9 270X, 170W for GTX 760, etc.)
40W for other components in a typical system

So as an example, a system with a Intel Core i5 4670K and a GTX 760 graphics card would require 84W + 170W + 40W = 294W.
Rating on the +12V rail should be 367.5W (294 / 0.8) or 30.625A
A good quality 500W power supply would be ample.

The biggest thing with power supplies is to look for quality. Supplies advertising peak values are usually rubbish.
Have a look at the following:
XFX Pro 550 or XFX Core 550
Seasonic G 550
Corsair CX 500

The Corsair isn't as good as the other two, but still not a bad supply.
 
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Solution
ksham

ksham

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Mar 29, 2013
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What are you talking about? What question(s) are you answering with that post?
 
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ksham

ksham

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Mar 29, 2013
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I never said that there is no standard. I have no idea what you are talking about. And for your information, V x A = W is NOT a standard. That is a FACT.

The only thing that I said about standard is that there is no standard to what peak means. You need a lesson in reading comprehension.
 
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drdenby

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Jan 21, 2014
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This is what I am considering. With $20 rebate, seems like a great deal.


Thermaltake SMART Series SP-550PCBUS 550W ATX 12V 2.3 SLI Ready CrossFire Ready 80 PLUS BRONZE

What do you guys think of this? Is it a good one I can trust?

Using the formula Vincent gives, I figure 400W needed, which is about 73% , but thats if the PSU is quality, consistent, etc?

Do you guys think it is?
 
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ksham

ksham

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Mar 29, 2013
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I wouldn't buy a Thermaltake PSU. What is your budget?
 
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VincentP

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Oct 18, 2013
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I can't find any detailed reviews for this power supply.
Thermaltake have made a few decent power supplies in the past, but mostly rubbish.

XFX Pro 550, manufactured by Seasonic (rated 44A on +12V):
http://pcpartpicker.com/part/xfx-power-supply-p1550sxxb9

Corsair CX 500 (rated 38A on +12V):
http://pcpartpicker.com/part/corsair-power-supply-cx500

Corsair CX500M (modular, rated 38A on +12V):
http://pcpartpicker.com/part/corsair-power-supply-cx500m

Seasonic G 550 (rated 45A on +12V):
http://pcpartpicker.com/part/seasonic-power-supply-ssr550rm

Of these I would choose the XFX Pro 550. Quality is as good as the Seasonic. Efficiency is 80 plus bronze. Efficiency on the Seasonic is 80 plus gold, but the difference isn't massive. The price on the XFX Pro 550 is very good.

As I mentioned in my earlier post, the Corsair is OK but not as good. Probably better than the Thermaltake though.

 
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