A PSU (Power Supply Unit) is what is used to convert the AC power input from the mains
electricity into multiple lower voltage DC lines that the PC can use.
Modern power supplies also often include protection systems that will trip if needed to prevent
damage to the PC, usually by shutting down or blowing a fuse.
High quality units will often also include low-ripple outputs and other things that are useful
or beneficial to a PC
Output
Power supplies are primarily rated by their output, usually a figure given in Watts.
For modern power supplies this is usually in the 300-700W range, although it is possible to
get very high or very low output power supplies, for example a 160W PicoPSU, or a 2,000W
PSU. This figure denotes the maximum amount of power that the power can supply.
Efficiency
Power supplies take their input from mains electricity, and convert that AC power to DC -
since computer components use DC. (Most electronics use DC power)
This process is not 100% efficient - that is to say, you will get out less DC power than the
AC power used to make it, with the wasted electricity being produced as heat in the PSU
system. More efficient units will therefore use less power, and generate less heat, which will be beneficial
to the end-user.
Power supplies are often tested and given a rating of their efficiency, usually according the
the 80 Plus standard, which can be determined as follows:
No rating
Obviously the worst efficiency rating is not having a rating at all.
In truth any power supply that falls into this category because it does not meet even
the 80 plus white spec below is probably best avoided.
80 Plus White
80 Plus (also known as 80 Plus White, due to the white logo of this certification level)
PSUs are rated to be a minimum of 82% efficient at 20% and 100% loads, and 85%
efficient at 50% loads, and with a power factor of 0.9 or higher at 100% load. Higher
power factors reduce peak current draw, which reduces load on the circuit.
80 Plus Bronze
80 Plus Bronze power supplies are 85% efficient at 20% and 100% loads, and 88%
efficient at 50% loads, as well as having a 0.9 or better power factor at these load
levels.
80 Plus Silver
80 Plus Silver power supplies are 87% efficient at 20% and 100% loads, and 90%
efficient at 50% loads, as well as having a 0.9 or better power factor at these load
levels.
80 Plus Gold
80 Plus Gold power supplies are 90% efficient at 20% load, 89% efficient at 100% load,
and 92% efficient at 50% loads, as well as having a 0.9 or better power factor at these
load levels.
80 Plus Platinum
80 Plus Platinum power supplies are 92% efficient at 20% load, 90% efficient at 100%
load, and 94% efficient at 50% loads, as well as having a 0.95 or better power factor at
these load levels.
80 Plus Titanium
80 Plus Titanium power supplies are 94% efficient at 20% and 100% loads, and 96%
efficient at 50% loads, as well as having a 0.95 or better power factor at these load
levels.
