According to their respective websites the Seasonic X series is fanless up to 20% load (+/- 5%). While the LZP series is fanless up to an internal temperature of 65/70C.
That info is a bit misleading -- not you Garak, but Seasonic. If that is indeed what Seasonic states, the first statement is based on extrapolation from temperature/load data. The second statement is probably accurate.
AFAIK, every PSU uses thermal monitoring to control the fan, not power or current. The latter is used for overload protection, but for fan control, thermal sensor control is the rule. There are no exceptions among consumer computer PSUs. (Among enterprise server/workstation PSUs, there may be exceptions.... but I don't know of any.)
Hello Mike C.
This is the image which Seasonic uses to describe their “Hybrid Silent Fan Control” design, and which I based my statement on:
And a survey of X-bit labs reviews of the X series (X-750, X-560, X-660, X-760, X-850) shows them starting up at ~150W, my maths tells me that this is 20% (+/- 5%) of each unit's capacity.
It also seems likely to me that Seasonic is using thermal fan speed control, but it still coincides with 20% load (rather than 50% load) based on the info I have seen.