We have been discussing the output parameters of the PSU so far but the efficiency counts its input parameters in. It means what percent of the power taken from the mains is converted into the power the PSU yields into the load. The difference is wasted for heating the PSU up.
The current version of the ATX12V 2.2 standard restricts the PSU efficiency from below: minimum of 72% at typical load, 70% at full load and 65% at low load. Besides that, there are optional numbers (an efficiency of 80% at typical load) and the voluntary certification program “80 Plus” which requires that the PSU has an efficiency of 80% and higher at loads from 20% to maximum. The new certification program Energy Star 4.0 has the same requirements as in the 80 Plus standard.
The efficiency of a PSU depends on the input voltage. The higher that voltage is, the better the efficiency. The difference in efficiency between the 110V and 220V power grids is about 2%. Moreover, different samples of the same PSU model may vary in efficiency by 1-2% due to the variations in the parameters of the components employed.
In our tests we are changing the load on the PSU from 50W to the specified maximum in small steps and measure the amount of power the PSU consumes from the mains. The ratio of the output power to the input power is the efficiency of the PSU. This test produces a graph showing the dependence of efficiency on load.
The efficiency of typical switching PSUs grows up quickly along with the load, reaches the maximum, and then slowly lowers. This non-linearity has one interesting consequence: from the efficiency standpoint, it is better to buy a PSU whose specified output power is adequate to the load. If you take a PSU with a large reserve of power, it won’t be very efficient at low loads (like the 730W PSU whose efficiency is shown above is at a load of 200W).