Psu calculator, believe it or not, this time is right on the money.
Using OC.
Cpu 200w
Gpu 300w
Mobo, accessories & other junk 100w
That's 600w flat out, maxed out, never happen values. Nobody ever runs the cpu, gpu at 100% on both, with hdd/ssd/ram/fans etc also at 100%. So, realistically a 600w psu will work. However, there are no decent quality 600w psus at all. This would require a move to a 620w Seasonic platform or 650w for most other good quality platforms.
Gaming use is generally @60% of total, so roughly 350w, on a 650w psu puts you right in the 50-70% sweet spot for efficiency and performance of a decent psu.
If you aren't pushing that cpu to close to 200w OC, you could easily take off a huge chunk of wattage there, lowering max outputs close to 550w instead of 600w, which would still put gaming values well within the tolerances of a decent 550w psu.
Oh, if you have an eco setting or thermal setting on the psu, the fan won't spin at all until the interior thermistor reaches a certain temp. This temp has relatively little to do with any cpu/gpu /case temps usually as the psu is generally upside down, drawing air from outside, exhausting outside. So the thermistor temp will only be relative to the ambient temp and load. If the fan didn't even engage, you weren't pushing the psu hard enough to get it hot enough to kick the fan on.