We're impressed with how well the RM850x does here, as it is up against a couple of Platinum-rated models in the form of EVGA and Fractal PSUs.
From previous testing, we can say the review supply is at the upper end of the 80 PLUS Gold scale, verging on Platinum.
Using a dedicated Chroma load-tester allows us to increase the load a watt at a time. On our sample, the fan's switch-on point occurs at 341W. That's enough to power a decent rig in passive mode.