How to calculate electric power every GPU?
you can check TDP of GPU in
Techpowerup How much power (Watt) need to run every GPU?
e.g1x RX580 has TDP 185 W
actually, 500W is enough to run mining Rig (CPU) with one GPU although for later you can do under-volt.
it most important is a mining rig must have power remain 30% or 40 % so as not to lack power