Katzenjammer

The question: does a power supply always consume its rated wattage, or only the total wattage of whatever's plugged into the secondary side? IOW, if I have (say) a 500W computer power supply plugged in to the mains, but all I have attached to the secondary is the mamaboard and a drive for, say, a total of 100W, how much electricity am I paying for--100W or 500W?

If I had the right equipment I could stick a meter in on the mains side and discover the draw by experiment, but I don't so I gotta ask.

walterk46

The short answer is closer to 100W than 500W.

The long answer is that the conversion from household current to 5VDC and 12VDC costs you some power. Stepping down the AC voltage, rectifying from AC to DC, and then regulating the DC voltage all involve some loss. This is why power supplies often have fans--that heat they are dissipating is the difference between 100% efficiency and the real world. So the electricity you pay for is the load (100W) plus whatever is lost in the conversion.

I just looked at the numbers on the back of the power brick for my laptop computer. It says input is 145-190W, and rated output is 72W.

Katzenjammer

Thanks! That's good to know--it means I needn't replace that honking power supply in my tame-webserver build. I didn't want to consume 400W just to run an old mamaboard, 3 cards, and a small scsi disc, but I didn't want to unpocket for a new, smaller ps if I didn't have to. Not having any hardware training, it seemed like both consumption possibilites were equally likely, so I knew one of them had to be reflecting my ignorance

(Because you mentioned yours, I just checked for the first time the power brick for my laptop -- ca. 150W in, 60W out...I had no idea it was so inefficient!)