While choosing a power supply, consider its physical dimensions. Ascertain that the computer case can accommodate the height, width, and depth of the power supply. Check the connectors needed for the power supply. Choose connectors that meet your every need. Be sure to pay close attention to the number of pins needed. On some computers the connector contains 24 pins instead of 20. Intel Pentium 4 processors have a slot on the board for this P4 connector. If your motherboard has this slot, then get a power supply with a P4 connector on it to help it run. There are many connectors that supply power to hardware components of your computer as the 8-Pin CPU connector, Molex Connector, Floppy Connector, AUX connector, SATA connector and the PCI Express connector. Know the amount of power needed. Buy a power supply larger than necessary because the power supply will step down as per the system requirements. Power supplies deliver only the amount of power needed. Buying a power supply smaller than the requirements is useless.
The power supply will surely be the last part of your computer that you will look for whenever you are assembling your own computer.
The power supply remains an integral part of your mounting. One thing you should bear in mind whenever you chose to assemble your own machine is a higher wattage does not necessarily mean a better quality power supply.
Points to consider
You should know the real amount of power that your computer needs to run properly. For this reason, it is recommended that you use a power supply wattage calculator, which will give you a clear idea of what power supply you should use. This device will calculate the power density needed by your machine based on components and system configuration and you can make your decision accordingly.
Identify your needs
Below is a website which can identify the power density needed by your computer from a power supply - see the image below: