Arbie :
Xyriin's statements make no sense to me. First, PSUs are less efficient at the lower end of their range - not the upper. Second, power needs are going down with time, not up. Third, few PCs are designed for a 10-year lifetime, for the very good reason that ten years from now you won't even want such old tech. In short, he's got everything wrong.
They make perfect sense if you look at the efficiency curves. Yes at low power they don't function well...but you're talking less than 20% of the rated load.
If you take the time to look it up you can see that efficiency CURVES spike quickly around 0-20% load. Then you have a slight climb in efficiency around 20-45% load, almost a flat peak around 45-80% load and then a drop off around 80% load on. Those are nominal values, each PSU will have it's own unique curve that varies slightly.
The 'sweet' spot is obviously going to be somewhere around 45-80% load. Now it's easy to look at that and say above or below isn't a big deal as they both have similar inefficiency, however you're wasting more power when your peak load isn't in the sweet spot. Who cares if your idle load is in the sweet spot at 90% efficiency...you're wasting maybe 10% power on something like 250W (25W waste). If your peak load is 500W and you're above the sweet spot at 80% efficiency you're wasting 20% power (100W waste). If your peak load was in the sweet spot you'd only be wasting 50W instead.
I'm not advocating buying a 1200W power supply but looking at your system and buying a 500W PSU for a 450W system is a BAD idea unless you want to risk buying a new PSU every time you upgrade a CPU or GPU. Additionally, the closer to the max rating you operate your PSU the faster it's components will die. Then of course you have overclocking as well...that also ups the power needs compared to their listed ratings. That's not anecdotal evidence or a guess, it's a simple fact consistent with electronic circuit and component failure rates. It's also the reason OEM boxes have power supplies that die at frighteningly high rates. Going cheap is going small and they min/max the PSUs in those boxes to save every penny. End result is that the small PSUs operate closer to their max rating and die faster.
Power needs do not only go down after time. They have regularly cycled based on thermal limits. When a smaller manufacturing process happens it immediately drops the power needs, however to improve speed in the new process size they start pumping more and more power through the unit until thermal limits are reached. That cycle continues over and over again. However, we're very close to maxing out the minimum process size and it's also why Moore's Law has already failed. With Moore's Law failing we're not going to get the those power drop cycles as much as we have and further gains will only come from pumping more power into existing circuits to increase clock speeds and improving cooling to compensate. This too has a fail point as I'm sure you've noticed in CPU speeds compared to previous generations.
So yes you're correct, but only if you're looking at the small picture in front of you instead of the big picture and without all the details on top of that.
JamesSneed :
Actually most PSU's do better near full load than if they are way under utilized excluding the Titanium rated PSU's. Of course you don't want to run things at max wattage but I think you missed the point as that is not what is going on most of the time. If anyone has built a PC recently with a single GPU you will realize a 500 watt PSU covers most needs with room to spare and a 650 watt PSU gives a lot of overhead. An Intel 6700K along with the new GTX 1080 is only going to pull about 350 watts peak with both at stock clocks.
I think most enthusiasts would be better served going with higher quality PSU's than higher wattage lower quality PSU's.
I'm not advocating a low quality power supply either. My best recommendation is to get a power supply that can handle around twice the rated load of your CPU and GPU. First off you're going to waste some wattage on the inefficiency, then you've got other components like storage, USB devices, etc. Then you have to take into account common future considerations like a second GPU or overclocking. All that significantly boosts the 'rated' load of your CPU and GPU without extreme upgrades like a new CPU or GPU.