Originally Posted by TwoCables
Too much overkill is not good for the PSU nor is it good for the system because the quality of the power delivery can suffer. You also get lower efficiency, and the efficiency is very important to some people.
Agreed. I wouldn't go out and get a 1500W PSU if you are only drawing 300W. But if you can get a good sale, and you can get a 550W or 650W from a great manufacture, I'd go that route for a 300W draw.
I also factor in a good amount of overhead for myself for a general purpose computer since I often will have to have another 250-300W of power when I eventually add a second video card to help extend the life of my build. I typically build up a computer that I will only have to do a minor upgrade (ie more RAM and a second video card, and maybe adding an SSD if I built it with a hard drive) at the 18 month mark. That way I can not have to do a major update (ie mobo, cpu, ram, etc) for every 3.5 to 4 years (or more). I also stay with major names like Seasonic, Enermax, PC Power & Cooling, etc that offer 7 year warranties ... so I can get 2 major builds out of each power supply.
For me, this is my logic. For others, it may not make sense.