Originally Posted by Nnimrod
Your PSU doesn't consume power, it only converts it, and loses some in the conversion. Efficiency is what you're talking about, not rated max wattage.
Ex. if system draw is 300W, a 1500W 80+ silver PSU will draw noticeably less power than a 350W not 80+ certified unit. However, power is relatively cheap, so don't spend $100 more for an 80+ platinum PSU with the idea that it will save you money; especially if system draw is relatively small.
I appreciate your enthusiasm but you don't seem to understand much.
A 1500W Silver rated unit will offer 85% minimum(in theory) efficiency from 20% to 100% load. Below 20% efficiency simply plunges, which means that below 300W draw(actual PSU output not the input you measure at the wall) you're going down hill very very fast.. in such circumstances, with a system that constantly draws less 300W your 350W 80+ PSU will draw less power than your 1.5KW Silver PSU, because conversion at very low loads in all modern PSUs is very poor.
In such circumstances(like the system of the OP) a 350W 80+ unit will be better suited than a 1.5KW Silver, it will be more efficient, which translates into lower operating costs, if will put out less heat due to that which subsequently translates into a less noisier rig(providing similar fans and controllers for both PSUs) and also in a longer PSU life providing similar caps user, for heat degrades capacitors.
Now, how exactly did you end up comparing a 350W 80+ PSU to a Silver 1.5KW!?.. I mean, which evolved vertebrate goes buying a PSU and says.. "this two are my options"!?