The chart above contains power-supply efficiency results from our last roundup of models at up to 500 W. As you can see, all of them reach at least 75% efficiency when running at a high workload, but many products drop below 70% if the workload is low. The higher the energy efficiency rating, the less power will be uselessly converted into heat. Depending on how much load we're talking about, the difference between 75% efficiency and 85% efficiency at a 20% load of the specified PSU output can translate into a huge difference in system power consumption. In case of a 60-W idle power requirement of a hypothetical system, the more efficient PSU could effectively run this setup at a total power requirement of 70-75 W, while a less- efficient power supply unit may require 80-85 W or more when the system is idle.

We recommend power supplies rated at 80% energy efficiency and up. Be sure the rating applies to low, medium and maximum loads for the PSU.