Here's How the 80+ Power Supply Rating System Works (aka PS Efficiencies)

I'm not sure if testing standards for 80+ ratings have changed, but in the past some manufacturers were found out testing their units at sub-ambient temperatures to pass certification for the 80+ label to boost sales/increase pricing. My advice is to always look for more information from third-party testers regarding real-world ratings.
 
Usually these days there isn't a problem with hot 80+ testing as most new PSU designs are actually as or more efficient hot rather than cold. It is their peak power output that suffers with heat. So you'll get cheap PSUs that'll advertise they are "500 watt" but can only deliver that at 20 degrees or something completely unattainable. Good ones should deliver full power at 40 degrees or so.

In terms of independent reviews, https://www.jonnyguru.com/ is a good site. Does good load and regulation testing on all PSUs, as well as tearing them apart and seeing who actually built them.

Also in terms of looking at standards some PSUs are getting Cybernetics ETA ratings as well, which is a more controlled test and so tends to be more useful. Still always good to verify from an independent reviewer though.

Finally something to note is that when it comes to saving money, more efficient PSUs really don't help because a single computer just doesn't use that much power. Getting a Titanium PSU instead of a Gold one is unlikely to end up saving you money in the long run as you just don't spend that much in powering a desktop. It is servers where there are lots of them and loads are high that it matters. That said, I'd still get a more efficient PSU for two reasons:

1) The same things that make a PSU more efficient also usually make it last longer/higher quality. Better VRMs, tighter tolerances on components, that kind of thing not only make it more efficient but just make it overall a better quality product. Likewise, since they end up costing more, companies can afford to use better caps and such, which again helps longevity. A good Titanium unit will often have a 10+ year warranty, and will easily outlive that.

2) Less heat. When you have a PSU that is less efficient, that inefficiency goes straight to heat. If you have a PSU that is taking 500 watts AC input and delivering 400 watts DC output (80% efficient), that means 100 watts are bleeding off as heat. Get a PSU that only draws 430 watts AC to deliver 400 watts DC (93% efficient) and as you'd expect, only 30 watts of heat bleed.
 
The thing I find difficult to find is what the sub 50w performance is, that is, when a pc is idling, how efficient is the power supply?

They have 15-20% load figures typically but this is more like 100-200w which is crazy high for idle.
 
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