> It really isn't. Sure, the engineers who design the power supply do that test, but that is not at all what determines the advertised rating of the final produ

> It really isn't. Sure, the engineers who design the power supply do that test, but that is not at all what determines the advertised rating of the final product. At 100% of the advertised load, most computer power supplies are still delivering nominal voltage or slightly above, not 5% below as allowed by the ATX spec. I assumed power supply manufacturers would want to slap the peak power number on their supplies for marketing purposes. If they are actually being conservative, then you are correct. > Yes, on either side of the peak efficiency, you'll have points of equal efficiency. And in the middle, you'll have a few percentage points higher efficiency. My point is that efficiency is a plateau, not a "peak". Once in the plateau the fluctuations of efficiency from one load point to another are not significant. Below a certain minimum load and past the peak power "knee" is a different story, but that plateau is very large. > But more importantly, at the ~30W a typical desktop will actually be drawing most of the time, a power supply with a smaller rating will be substantially more efficient. I never disputed this. I disputed the nonsense that power supplies have a meaningful "peak" efficiency, and that it is a function of it's rating. 30W is probably not enough of a base load for efficient operation of larger computer power supplies, but once that point is hit it no longer matters what the actual load is. I'm also not suggesting it is a good idea to waste money on a larger supply than you really need.