In 2006 Google wrote
Instead of the typical efficiencies of 60-70%, our servers’ power supplies now run at 90% efficiency or better, cutting down the energy losses by a factor of four.
Ten years later, now that 80 Plus Platinum and 80 Plus Titanium are a thing, 90% efficiency and even 94% efficiency can be bought off the shelf in 2017:
I just upgraded from an 80 Plus Gold power supply I bought in 2011 to an 80 Plus Titanium, and here are my results with an i7-7700k and a 1080 Ti:
Prime95 — 252w to 235w (7% better)
Rthdribl + Prime95 — 443w to 426w (4% better)
Efficiency does depend on the load factor and seems to peak at 50% load, but impressively enough, titanium guarantees 90% efficiency even at 10% load.