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How We Test Power Supply Units


The 80 PLUS certification measures efficiency at 20-, 50- and 100-percent load of the PSU's max-rated capacity up to the Gold efficiency certifications. For the Platinum and Titanium levels, they also measure efficiency with 10 percent of the PSU's max-rated capacity load.

Simply put, if a PSU has an 80 PLUS certification, then it must have the equivalent efficiency required by the corresponding certification. However, 80 PLUS measures at a mere 23 °C (73.4 °F) ambient, whereas we measure efficiency at a higher ambient temperature. This means that, in many cases, a PSU that is certified to a certain efficiency category fails to deliver the same efficiency at higher temperatures in our tests.

Also, many PSUs are tuned to deliver high efficiency at or above a specified load percentage, usually the minimum that 80 PLUS measures at the corresponding efficiency level. But at loads lighter than that, their efficiency is pretty low. Since many of us run our systems for long periods at low-energy consumption modes, efficiency at light loads can be highly important. So it's wise to pay special attention to our light-load test results.

In our reviews, we measure efficiency at four different light loads: 20, 40, 60 and 100W.

The ATX specification also states that the efficiency of the 5VSB rail should be measured, too. In the table below, you will find the minimum 5VSB efficiency levels that the ATX specification recommends.

≤0.225W< 0.5W to meet 2013 ErP Lot 6 requirement (100V~240V)
≤0.45W< 1W to meet ErP Lot 6 requirement (100V~240V)
≤2.75W< 5W to meet 2014 ErP Lot 3 requirement (100V~240V)

Testing in Standby Mode

In 2010, the European Union released a guideline on Energy Related Products (ErP Lot 6), which states that every electronic device should have below 1W power consumption in standby mode. In 2013, this limit was further reduced to 0.5W. The same year, the EU also released the ErP Lot 3 guideline for computers and computer servers.

This is why we measure the consumption of a PSU in standby mode, which is something that would be difficult without our monitoring software since the readings at such low consumption levels have significant fluctuations. We have to average them over a significant period of time to provide enough accuracy.

Aris Mpitziopoulos
Aris Mpitziopoulos is a Contributing Editor at Tom's Hardware US, covering PSUs.