basic knowledge question that I don't know the answer to

[Reality|Bites]

How does a PSU know how much power to draw from the mains at any one time?

 

[The Halk]

It doesn't. It's caused to draw power from the mains by the power being drawn from it.

 

[geuben]

This might clear a few things up...it also might not as I haven't read it.

http://computer.howstuffworks.com/power-supply.htm