ramon zarat :
1-Buy Seasonic Gold or Platinum, exclusively.
2-Power supply are most efficient @ ~80-90% charge under load.
3-The end
So if you calculate a requirement of say 450W @ full load on paper, a 500-550W unit would be optimal. 600W top if you want a bit of margin. Many are getting 800W or even 1200W unit, which is a complete waste in most scenarios, even with dual GPUs.
Every graph I've seen of efficiency vs load show highest efficiency at 50-60% load. After that it falls off. That being said, the difference between 60% and 80% on average is only maybe 10%, not enough to really worry about.
As for buying Seasonic Gold/Platinum, again, for the price difference between a solid Bronze/Silver rated PSU and the higher rated Gold/Platinum/Titanium ratings, the price seems to go up almost exponentially with the higher ratings, while the actually wattage saved is so minimal that the payback time for going with the higher ratings just doesn't justify the costs.
Unless they're running 8-core AMD units with older GTX non-Maxwell based GPUs or AMD 290X equivalents, or a high end Intel X99ish package with SLI something the average I will agree that the average user is just fine with a 550-650w PSU from a good manufacturer with a minimum of a 80+ Bronze rating.
I'm running a Xeon X3470 OC'd to 4.2ghz, extra dual NIC card, SLI'd GTX 960's OC'd, RAID card, 3x Seagate ES 4TB HDDs all with a Strider Gold 750w and the PSU barely ratchets up the fan to the point where I can actually hear it. Hmm... maybe tomorrow I'll put the Kill-A-Watt meter on it and see what it pulls.