This is a typical picture. You are correct about the efficiency. Its often best at ~90% (for many reasons)
However talking about actually using 50% of your PSU's nominal capacity sounds like a completely absurd thing to do.
also to your second point, that's exactly what I meant. Just was typing from mobile via VNC, so it was kinda hard:)
(1KW DC pulls 1.1KW AC with efficiency of 0.9)
EVGA MAKES GOOD EQUIPMENT--
And they stand by their warranties. There are other known, popular brands with "80+" labels that will simply die if placed at full load. A PSU will last longer if used at 50-70% of full load. The capacitors degrade more rapidly at full load.
After having several failures, I always purchase a PSU with a higher rating than my expected load. --scryptr