The primary job of a computer power supply is to take the AC power that is outputted by the socket on your wall and convert it to the DC power that your components can use. When power is converted from AC to DC the conversion is not one to one and some of the AC power ends up being expended as heat. As a result, if your components require 500 Watts to run, then your power supply must pull more than 500 Watts from the wall. The ratio of DC power being produced by a power supply to the amount of AC power it is consuming is what determines its energy efficiency.
A power supply's energy efficiency will vary based on load. For example a 500 Watt unit may be 85% efficient at 50% load and 82% efficient at 100% load (see table below for breakdown). Generally, power supplies are most efficient at mid-level loads and have lower efficiency at low and high level loads.
|Unit's wattage||Load||AC Consumption||Efficiency|
|500||50% = 250W||294.12W||85%|
|500||100% = 500W||609.76W||82%|
Energy efficiency is important to look at for a couple reasons. First and foremost, the more efficient a power supply is, the less energy it is wasting which in turn means a lower electricity bill. Second of all, since the energy that is not converted to DC power is expended as heat, the more efficient power supply is, the cooler it will run. A cooler running power supply will benefit the longevity of your system and also keep things running quietly since the fans will not have to work as hard.
80 PLUS® is a voluntary certification program that rates the energy efficiency of a power supply. There are many different tiers of 80 PLUS® certification: standard, bronze, silver, etc and each of them has a different set of requirements for how efficient a power supply is at different levels of load. At the base level, an 80 PLUS® certified power supply must have 80% energy efficiency at 20%, 50%, and 100% load.
The table below shows the 80 PLUS® tiers and the required efficiency for a given load in 115V. More information on 80 PLUS® certification can be found at www.80plus.org.
|80 Plus Standard||80%||80%||80%|
|80 Plus Bronze||82%||85%||82%|
|80 Plus Silver||85%||88%||85%|
|80 Plus Gold||87%||90%||87%|
|80 Plus Platinum||90%||92%||89%|
|80 Plus Titanium||92%||94%||90%|