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How to calculate wattage on 12v rail?

post #1 of 14
Thread Starter 
I've tried searching for this, only see stuff about amperage, which is kind of skewed when you get into multiple 12v rails.

I assumed the equation was:
12v1 x Amperage + 12v2 x Amperage = 12v rail PSU Wattage

But someone told me that was wrong, and I know that rails aren't exactly added like that, although practically it is? Just a bit confused, i thought I 'knew' the answer already but apparently not, according to some people

Now obviously you can just look it up. But sometimes you can't. I know my Antec Earthwatts PSU doesn't display the 12v rail wattage on it, and many other reliable PSU's don't either. So it's not that simple. Plus, knowing the actual math behind it would be nice...
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post #2 of 14
Power=Voltage*Current, then you add up power from all the respective rails to get the total output of the PSU.
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post #3 of 14
Watts = Amps x Volts

Just to provide an example I have two 12v rails in my PSU each rated at 500 watts.

500 watts = (Amps) x 12

500/12 = ~40 amps.
Edited by Lampen - 4/22/11 at 8:42am
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post #4 of 14
I need to talk to Christoph, I have an idea that will make everyone's life easier...


Forget "Amps". Forget "rails". Look at the PSU's label. Somewhere in or below the load table will be the figure: "Total +12V wattage" or something like that. That's what matters. If you want that in amps, divide by 12.
post #5 of 14
Quote:
Originally Posted by adizz View Post
Power=Voltage*Current, then you add up power from all the respective rails to get the total output of the PSU.
Definitely not. You need to look at the source/primary +12v... not the rails.



Quote:
Originally Posted by Phaedrus2129 View Post
I need to talk to Christoph, I have an idea that will make everyone's life easier...


Forget "Amps". Forget "rails". Look at the PSU's label. Somewhere in or below the load table will be the figure: "Total +12V wattage" or something like that. That's what matters. If you want that in amps, divide by 12.
...but lower-quality PSUs don't have this combined value.
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post #6 of 14
On the label of your Antec EA-430D, it says +12V1@17A and +12V2@16A. You already know you cannot add those up to get your total, that's not how it works.
Still on the label and underneath where it says those amperage ratings, there's an inconspicuous line that states: +12V1, +12V2 max. load: 384W. That's what matters. If you want it in amps, divide by the voltage, i.e. 12, you get 32A.
post #7 of 14
Quote:
Originally Posted by DuckieHo View Post
...but lower-quality PSUs don't have this combined value.
In which case you don't buy said lower-quality PSUs. If they don't list it it's because they don't want you to know it.
post #8 of 14
Thread Starter 
I don't think anyone is really reading my post correctly.

How do you figure out the wattage on the 12v rail when you don't have a convenient sticker that tells you explicitly the wattage on the 12v rail or the PSU.

Assuming you just have the 12v rail amperage and the number of 12v rails (ie the Output specifications), how do you get the 12v rail amperage. The formula I listed in my OP is what I 'thought' was correct, but a few people told me that was wrong, which I why I ask. Apparently as someone said, the lower quality PSU's you can't just add it up like that, but on the better quality PSU's you can. How exactly is that determined? I thought low quality PSU's simply had a lower amperage on the 12v rail relative to the other rails.
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post #9 of 14
Quote:
Originally Posted by Belial View Post
How do you figure out the wattage on the 12v rail when you don't have a convenient sticker that tells you explicitly the wattage on the 12v rail or the PSU.
You can't.
post #10 of 14
Quote:
Originally Posted by Belial View Post
I don't think anyone is really reading my post correctly.

How do you figure out the wattage on the 12v rail when you don't have a convenient sticker that tells you explicitly the wattage on the 12v rail or the PSU.

Assuming you just have the 12v rail amperage and the number of 12v rails (ie the Output specifications), how do you get the 12v rail amperage. The formula I listed in my OP is what I 'thought' was correct, but a few people told me that was wrong, which I why I ask. Apparently as someone said, the lower quality PSU's you can't just add it up like that, but on the better quality PSU's you can. How exactly is that determined? I thought low quality PSU's simply had a lower amperage on the 12v rail relative to the other rails.
Watts ÷ 12 = Amps
If you don't know the watts, you can not figure out the amps.

You can not add the amps of 2 (or 3 or 4) 12v+ rails to get the total amps. The example above showed that. You must know the total watts of the entire 12v to find the total amps

If you added them, you would get 33 amps (17 on 1, 16 on the other), but is actually has 32 amps available on the 12v. It does not matter if it is a high quality or low quality, you just can't add the amps together to get the amount.
What was said is that lower quality PSU's tend to not have the information available to figure it out.

HERE is a good read on it
Edited by blupupher - 4/23/11 at 7:24am
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