I'm not sure I agree with the conclusion you arrive at.
Now I will agree that if you are below 20% utilisation of your PSU, that can be quite inefficient. 30% or so is typically about as efficient at 80% load, and 50% is the maximum efficiency on modern power supplies, usually about 3-4% more efficient than 30/80.
I would say that you want peak power consumption - that is to say, an artificial load such as furmark & prime95 running at the same time, to be around 60% load on your PSU. This should put the actual
real-world maximum load you are likely to see from gaming for example, at around 50-55% load on the PSU, where it is most efficient.
On a high powered system, your idle usage will typically be about half of your maximum load which puts it around 25-30%. Not the most efficient, but not inefficient
either. If you are using more efficient parts, your idle load might be lower than this, but while it may be less efficient, it's a lower efficiency at a lower load, so the power lost from this inefficiency is minimal and it may even put your PSU into its silent mode of operation.
Your argument is that you should choose your power supply so that maximum (gaming) load is around 80%. Now that, I can't agree with.
It is true, that if you are only looking at efficiency as a percentage
, you end up with 0.25% better efficiency overall with the numbers that you have used. Which, honestly, is insignificant.
With this setup though, you are stressing your power supply under load, causing it to put out a lot of heat and noise - assuming that your PSU has a variable fan, as most models do now.
So your overall efficiency is 0.25% higher now - but these efficiency gains are at the lower end of power consumption rather than the higher end, where the power savings from greater efficiency are at their smallest. A 3% gain at idle usage might save you a few watts but a 3% gain under maximum load would save you 2-3x that amount.
Not only that, but up to about 50% is where most variable fan power supplies stay in their quiet mode, which keeps the fan running at a low speed, and as you go higher than that, the amount of heat and noise goes up significantly.
Here is the chart from Corsair's AX850 power supply for example.
Now while it may be less efficient below 20% load, the power supply is not only cool under that load, it is silent
, and up to about 55,60% it is very quiet
If you bought one of these power supplies (or a lower wattage version) and kept it under 80% load when gaming, you will have more fan noise at idle, it would be hot & loud under load, and you will be cutting its life short. Not only that, but power supplies lose efficiency over time, and they lose efficiency at a greater rate when they are under more heat/load, so if you are operating your power supply under higher loads for longer periods of times, that 0.25% efficiency you saved overall (even though it will be using more energy if you looked at power consumption
) will disappear in a short amount of time.
I don't think that you should choose a power supply so that an artificial load is at 50% utilisation on your PSU, because real-world gaming load would be lower, but you absolutely should be shooting for 50-55% load when gaming
on any power supply you buy.