The single figure quoted is a fairly meaningless number.

All power supplies have multiple circuits with different voltages.

For example we stock a 500Watt Sumvision Power supply which we sell for £13.

It has the following characteristics, found on a chart on the power supply.

At 3.3 volts it outputs 28amps
At 5 volts it outputs 38amps
At 12 volts it outputs 17amps
At -5 volts it outputs 0.5 amps
At -12 volts it outputs 0.8 amps
At 5 volts standby it outputs 2 amps

If you multiply the volts by the amps for each empty, ignoring the negative numbers and add each one together as follows.

(3.3*28) + (5x38) + (12*17) + (5*0.5) + (12*0.8) + (5*2), you get 508.50

For convenience it is rounded down to 500Watts.

This is a basic overview of the situation but there are several facets to this subject that can make it more complex.

As far as choosing a power supply then you need to do some research, each component you connect to the PC will connect using 1 or more of the circuits above.

E.g. A floppy drive connects to the 3 volts power supply Molex, some serious graphics card draw 12 volts from the AGP socket on the motherboard and have a hard drive style Molex to allow the card to draw more current from the power supply.

What you need to work out is how much current is being drawn on the individual circuits (known as rails).

If you are drawing more than the power supply is rated for then the PC will become unstable.

Generally as with most things in life then more you spend the better it gets, hard code game players may use an Antec power supply that costs in excess of £100.

If you have any further questions then please visit our eBay shop at : Highridge Computers Ltd.