Have one RX580 nitro+ 8GB and I flashed the bios.
I got 30.300 mh/s on eth and 909 on Decr. GPU fan is quiet, power draw from wall with GPU : 205w.
On most tutorial the power consumption are between 130-150w.
if your power draw at the wall for your computer with the gpu connected is 205 watts , thats mean your card probably pulling around 130-140 watts at the wall.
You can check easy your pc power draw if you have a integrated graphics in the cpu. Start the pc without your rx 580 and measure the power draw connect your rx 580 start mining and measure your power draw , the different between that 2 number is going to be the closest number for a real power draw for your gpu.
Usually a simple built pc , with ssd, cpu, 4-8GB of mem, no extra cooling fans on idle is pulling between 40-70 watts...