# How to calculate Power Supply Watts

I am trying to figure out how the Watts is calculated based on the Volts and Amps on power supply stickers.

I am told that the 12V rail is the most important, but I don't know how they calculated the 41A? I understand 41 * 12 = 492 for the Watts, but how do they get 41A from the four 18A?

Update - Sometimes they don't specify the combine Amps number like in this power supply. I guess if the manufacturer doesn't provide the numbers there would be no way to tell the amount of watts the 12V rail provides?

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Each 12V rail is probably rated at 18A max but, in total, the PSU is only rated to supply 41A from all 12V rails. – sblair Sep 7 '11 at 14:50
The +12V combined current rating for the second PSU would simply be: 380W / 12V = 31.67A (less than 16A + 18A). – sblair Sep 7 '11 at 15:11
Remember that watt = voltamp. manufacturers don't always list both watts and amps, because given the voltage you can trivially calculate one from the other. you might need to do this to get some information. – jcrawfordor Sep 7 '11 at 16:09
@jcrawfordor For DC, yes. But just to be pedantic, the volt-ampere is a different unit from the watt. – sblair Sep 7 '11 at 16:38
I should clarify that difference, because it's relevant when discussing power supplies. Formally, the Watt is a derived unit whose derivation can be simplified to W=v*A - in theory, the units do perfectly correspond. In DC power, this holds true in real usage. For AC power, though, VA is used to refer to the apparent power in a circuit. Because of various factors real power (expressed in W), or the power actually in use, is lower by a a ratio called the power factor. AC equipment will often be labeled with both a VA rating and a W rating, both should be minded. – jcrawfordor Sep 7 '11 at 19:15

Designing a good PSU is quite an art, especially when balancing power output, stability, efficiency, cost and durability.

Often cheap power supplies will use two or more cheap 12V circuits, which are rated for a relatively low power draw. Better quality PSU's might use multiple higher quality 12V circuits, but others might have a single high quality/power/efficiency 12V power circuit.

Even high quality single rail PSU's may break out that one rail into multiple rails with individual overload protection on each supplied rail.

So, the `be quiet!` PSU is probably of the last type, it has a single 41A circuit, providing power to 4 rails, where either a total draw of more than 41A or a draw of more than 18A on any one rail will trip the over-current protection.

The `Power LC` is either an old or cheap PSU which probably has two different cheaper 12V circuits. Don't expect either rail to perform anywhere near the limits suggested.

Personally, I prefer pure single rail PSU's (where you might see 12V 41A as the only spec). You don't need to worry about balancing power over each rail and you don't risk overloading one rail when you add a new device because you've plugged it into a power connector that uses the same 12v rail as your graphics card.

The only problem with a single rail PSU is that since they can potentially supply more current than the cables are rated for, if you have a short circuit you could end up burning out the cables before the overload protection kicks in. This is still very unlikely though, so much so that respected PSU makes are happy to sell single-rail PSU in the consumer market.

If you are unsure about whether your PSU is suitable for your system or not, I've found the eXtreme Power Supply Calculator Lite v2.5 to be an interesting first port of call. Just don't forget to add in capacitor aging factor (see note 4) if you intend to run your computer 24/7 or want it to remain reliable for more than a year.

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41A is probably the total possible amps available to the 12V rails. Each rail can go up to 18A, but not all at once.

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If the combined amps is not reported, it's the sum of all the lines together. However, I would double-check that, at least so that you don't go above the rated maximum wattage..

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