At once time PSU were pretty shitty with narrow "sweet spot" (40% to 60% of peak power) and horribly efficiency outside of that (<70%). Those days have been gone for a long time with most quality PSU having an almost flaw efficiency "curve" from 20% to 100%.
Yup, the
Coolermaster V1000 I bought for my Jupiter has a graph on the back side with their claimed efficiency, which goes from 90% (100% load) to about 93% (50% load) to 92% (20% load) efficiency.
However, there is also a graph for the fan speed which goes from about 1200 RPM at 75% load to about 2300 RPM at 100% load. So there it might matter.