Personal choice, I would look at the 50% load efficiency. Example, system consume 400-450w during load, would recommend something 750-800w PSU (Bronze, Silver, Gold). If you get a 500-600w, not an issue but your PSU won't be running at 50% load rate and you will also generate a lot of heat in the PSU.
Just wasting energies.
My system runs 400w, PSU is 850 G2 gold. It runs warm, the fan barely turns on.