Computer Forums Power Supply Questions (Amps and Watts!)
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 11-23-2009, 09:15 AM #1 Beta Member   Join Date: Nov 2009 Posts: 1 Power Supply Questions (Amps and Watts!) I have a Dell Vostro 200 tower and it has a stock 300Watt PSU... I have been having some electrical problems and wanted to check the Amps on all my electronics to make sure that I am not overloading any circuits and breakers. The PSU says on the sticker that it requires 9.0A at 120V but to get the amps the formula is Amps = Watts / Voltage which means my computer should pull 2.5A at max. I don't have a kill-a-watt meter but does this sound right? Why should it take 9 Amps??? Should t he computer ever pull anything more than 300 Watts that the PSU is rated at? Thanks! __________________
11-23-2009, 10:16 AM   #2
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Join Date: Dec 2007
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Re: Power Supply Questions (Amps and Watts!)

Quote:
 Originally Posted by Agent56 The PSU says on the sticker that it requires 9.0A at 120V but to get the amps the formula is Amps = Watts / Voltage which means my computer should pull 2.5A at max.
So the 120V PSU can pull a max of 9 amps (!) from the wall? Thats 1080 watts! And lets say the PSU efficiency is about 80% that means a PSU rated at 'about' 850W. Don't know where the 2.5A is coming from????

Quote:
 Originally Posted by Agent56 I don't have a kill-a-watt meter but does this sound right? Why should it take 9 Amps??? Should the computer ever pull anything more than 300 Watts that the PSU is rated at? Thanks!
A PSU may draw 9 amps but damn you gotta be running something heavy. And refer to my equation above. This would be from a 750W-850W PSU maybe?

What is it your trying to grasp?
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11-23-2009, 10:44 AM   #3
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Join Date: Jul 2009
Location: England, UK
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Re: Power Supply Questions (Amps and Watts!)

Quote:
 Don't know where the 2.5A is coming from????
The ~2.5A is coming from 300/120 - what he says the PSU is rated at

There's a few things of things that spring to mind over why the rating seems so high. As said above, efficiency isn't 100% at all - more like 80, perhaps less on a cheap PSU. So while you're getting 300W out of the PSU in useful power, it may be using another 50 (bit of an overestimate but the point stands!) in heat / circuitry / fans.

Secondly, it's possible that the PSU will be able to handle >300W for a brief period of time, such as on startup, when almost everything surges at once. So the 9A figure may well have been derived from this peak value rather than the continuous power rating.

And lastly, manufacturers always overestimate slightly rather than the other way round. If they think a supply will only ever pull 8A in normal use and they tell you to make sure it has capacity for 9, that gives a bit of a margin should the voltage drop slightly or something else untoward occurs. If they say it only requires 5 and therefore you put it on a 5A supply and it trips out constantly, they're going to have lots of complaints and potentially lawsuits on their hands!