# Will a 300W psu drain more power than a 100W if the system only uses ~80W?

I have a NFS server that is a old mini-itx motherboard with two 2.5" drives and a no optical drive.

Atm. this is powered by a external 100W psu, but I estimate it only need/uses between 60-80W. This old 100W psu is about to die on me (won't always power on), so I have replaced it with a spare 300W psu I had laying around.

So, what i'm asking is: Will a 300W psu drain more power than a 100W if the system only uses ~80W?

I'm guessing that there is no difference, but I know to litte about electronics to be sure :)

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Related – Dennis Williamson Jun 27 '10 at 21:51

Potentially yes. Power Supplies have an efficiency of about 80%. That means that in order to supply 80 W to the PC, a 80% efficient Power Supply will drain about 100W from the Mains. A less efficient Power Supply will drain even more.

Therefore, a 300 W Power supply might drain a bit more power simply because at a low load it's not as efficient as the 100 W one. However, 100 W may be a bit underpowered if your machine really uses 80 W.

That being said, the difference shouldn't be that big. It really depends on the individual power supply as each is different. When in doubt, grab a Kill-a-Watt (device to measure how much power is drained from the mains) and measure.

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Thanks for a informative answer, and most of all for reminding me that I have a Kill-a-watt somewhere in my office :) – eple Jun 27 '10 at 21:49
A modern, highly efficient power supply with 85% rating is required to have a minimum efficiency of 85%. That means that for a suitable load it could be a lot more efficient. If your "300W" power supply does not provide any specific efficiency, you can guess it has efficiency of 50% for anything but close the full load. In short: if both are low quality power supplies, then the "300W" model will probably eat more electricity. For high quality power supplies, the power drain should be close to equal but "300W" model is probably more costly to purchase as a new. – Mikko Rantalainen Jun 25 '12 at 8:50

Michael is right. However if the old psu is really dated and the spare is recent and less used, You could come out similar or better due to aging of the old PSU and better efficiency in current psu's.

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Thanks for the tip. It's probably a decade between them, so it will be interesting to compare with my Kill-a-Watt! – eple Jun 27 '10 at 21:50
Please post results if possible, very interested in outcome. ;) – deploymonkey Jun 27 '10 at 21:52
Will try to remember that, just have to find the powermeter first. – eple Jun 28 '10 at 18:29