# Actual PSU Amperage.

#### xFlankerx

##### Limp Gawd
So I know that the actual Power output from the +12v rails is not the same as the sum of the two or more rails, so how do you calculate what the actual power output is?

For example, the Antec True Power Trio 550W has 3 +12v rails with 18A on each of them. That's a total of 54 Amps, but that's obviously not how much power it actually puts out. Someone estimated that the power on that would be 42A, but declined to explain. So I'm wondering if there's some sort of mathematical formula to figure it out.

No, you do not add rails to get your amperage rating. You look for a combined voltage rating, then you do your psu math.

In the case of the Trio, it's even easier: http://www.newegg.com/Product/ShowI...0W+Power+Supply+with+Three+12V+Rails+-+Retail

You will see, the "12v1 12v2 and 12v3 combined amperage is 42a". Easy as pie.

In a different case, let's say, A Silverstone Element 500w. 12v1 = 18a, 12v2 = 18a. Total combined wattage on +12v, 432w. This is the info they give you. 432/12 = 36a on +12v

No, you do not add rails to get your amperage rating. You look for a combined voltage rating, then you do your psu math.

In the case of the Trio, it's even easier: http://www.newegg.com/Product/ShowI...0W+Power+Supply+with+Three+12V+Rails+-+Retail

You will see, the "12v1 12v2 and 12v3 combined amperage is 42a". Easy as pie.

In a different case, let's say, A Silverstone Element 500w. 12v1 = 18a, 12v2 = 18a. Total combined wattage on +12v, 432w. This is the info they give you. 432/12 = 36a on +12v

True. However, some brands or lines of multi-rail PSU's still give neither the total combined +12V amperage spec nor the total combined +12V wattage spec. In this case, then, a rough (ballpark) estimate would be to add up the amperages, then multiply by .8 (80%) to get the combined rating.

I see, thanks for that. So you pretty much have to look at the label on the side of the PSU as the only way to figure it out. But what if Newegg does not show the label? The Silverstone 500W on Newegg has no picture of the PSU label. Would this mean that there is no way to find out the actual provided amperage of the PSU?

True. However, some brands or lines of multi-rail PSU's still give neither the total combined +12V amperage spec nor the total combined +12V wattage spec. In this case, then, a rough (ballpark) estimate would be to add up the amperages, then multiply by .8 (80&#37 to get the combined rating.

Ah, I see. Thanks, I'll keep that in mind.

I see, thanks for that. So you pretty much have to look at the label on the side of the PSU as the only way to figure it out. But what if Newegg does not show the label? The Silverstone 500W on Newegg has no picture of the PSU label. Would this mean that there is no way to find out the actual provided amperage of the PSU?
You could check the manufacturer's product page. That particular PSU provides 18A on each of 2 12V rails.

In this case, then, a rough (ballpark) estimate would be to add up the amperages, then multiply by .8 (80%) to get the combined rating.

This isn't really accurate for most high-end designs, though.

For example, our two HX series PSUs are spec'd at 3 rails with 18A each on them. Seemingly identical, right?

By your method, that 54A x .8 would be 43.2A on the +12V rails.

But since we have a 520W and a 620W, and they're rated at 40A and 50A, respectively, that methodology gives you a ballpark but not really any accurate number. It's almost 20% off on the 620W, where 600W of it's 620W of power can be delivered over the +12V rails.

But then again, our spec label is a bit inaccurate since it kind of lowballs the actual power the thing can put out.