Depends a lot on the specific power supply - my Seasonic X1250 and EVGA 1300 G2 PS2 are typically over 90% efficiency with 110v in even very close to MAX rated power out, but they're VERY close to meeting Platinum level 80+ specs (as a few review sites have noted).

 I suspect you meant "0.90 efficiency" not "0.99 efficiency" though at half rated power load - and again, that VARIES with the specific power supply as to it's most efficient point of operation, and a little on the input voltage, though 50% is usually very very CLOSE to that "most efficient" level if it's NOT actually that point, and the drop-off on efficiency above that point on current 80+ power supplies tends to be pretty slow.