most psu brand tell they got 80plus certification or their efficiency is 80 or 75%,but that's important term in context of saving electricity bills right?
now i don't care about bills but importance is how know that 500w psu will be able to deliver full 500w to the system?no manufacturer lists any numbers about that?
to me importance is of max watt delivery while buying,how know that?
i know it's a stupid one...but....
Check reviews of the units you are considering to see if they can deliver what they claim. Places like jonnyguru put them in a hot box to stress test them under some pretty harsh conditions and make sure they can do what they say.