The 80 PLUS badge appears on nearly every power supply sold, in one of six tiers. It measures one specific thing, and it is routinely read as measuring several others.
What it measures
Efficiency: the proportion of power drawn from the wall that reaches your components. The remainder becomes heat.
A unit at 90 percent efficiency delivering 500 W draws about 556 W from the wall, and the missing 56 W is heat inside your case and your room.
Certification tests efficiency at 20, 50 and 100 percent of rated load. Higher tiers require higher efficiency at each point, with Titanium adding a 10 percent load requirement.
What it does not measure
Ripple, meaning how clean the delivered voltage is. Voltage regulation under changing load. The quality of protection circuitry. Component quality and expected life. Fan noise.
These matter at least as much as efficiency for whether a supply is a good buy, and none of them appear on the badge. It is entirely possible for a poorly built unit to achieve a high efficiency rating.
What the difference is worth
Between Bronze and Gold on a typical desktop, the difference is a few watts of waste heat and a small amount on an electricity bill over a year.
The practical benefits of a higher tier are marginally less heat in the case and often a better built unit, because manufacturers tend not to put Platinum ratings on their cheapest designs. That correlation, rather than the efficiency itself, is the better reason to pay attention to it.
Use it as a tiebreaker between two otherwise similar units. Do not use it to justify choosing a supply from a manufacturer you have never heard of over a Bronze unit from one with a reputation to protect.
Working out the real saving
The arithmetic is simple enough to do for your own situation rather than accepting a general claim.
Take the power your system actually draws, which our calculator estimates. Divide by the efficiency at that load, once for each unit you are comparing, and the difference is what the wall sees. Multiply by the hours you run it and by your electricity rate.
For a system drawing 400 W, eight hours a day, the difference between a Bronze unit at roughly 85 percent and a Gold at roughly 90 percent is around 26 W. Over a year that is somewhere around 75 kWh. Whether that justifies a price difference depends on the gap and on what you pay for electricity, and it is a smaller number than the marketing tends to imply.
The case for the higher tier is stronger for a machine that runs continuously, such as a home server, and weaker for one used a few hours a week.
What to look at instead
Since the badge tells you little about quality, these are more useful signals.
Warranty length. A manufacturer offering ten or twelve years is making an expensive statement about expected failure rates.
Who actually built it. Most brands do not manufacture their own units. Independent reviewers usually identify the original design manufacturer, and some platforms are considerably better than others regardless of the badge on the box.
Independent testing. Reviewers with load testing equipment measure ripple, voltage regulation and behaviour under transient load. Those measurements tell you what the certification does not.
The ATX specification it meets. A unit meeting ATX 3.0 or later is designed around the transient behaviour of modern graphics cards, which matters more for stability than any efficiency tier.