In article , Tony sayer wrote:
... and the efficiency will be awful.

Why should that be then?..

Why?

I can't tell you why -- I'm not an electronic engineer -- but I can
tell you that the efficiency graphs you see for modern PSUs drop off
noticeably as the current drawn drops below about 20%. Typical PSUs are
most efficient at around 50% load, and the efficiency drops off
slightly as load increases to 100%, and more rapidly as it drops toward
0.