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 :)

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.

On top of that, Power Supplies tend to have their best efficiency at about 70-80% load, so "underloading" your Power Supply might make it less efficient.

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.

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.