A more accurate answer is that the Kill-A-Watt measures total input power for the system including PSU losses. The PSU spec (850W in this case) may be total DC power output, IE, before losses attributed to the PSU.
If you're reading 855W from the wall on a 80% efficient power supply, the total power delivered to the load would be approximately 684W. If it was a 90% efficient power supply and still drawing 855W then the total load power would be 769W.
A Watt is a Watt, AC or DC is irreverent until you start getting into power factor considerations and VARS
ac or dc is a relevant factor considering they are 2 different types of systems, a watt of dc is not going to be the same as a watt in ac