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)