Some cases come with power supply unit pre-installed for you. Likewise, if the case you are using has been used before, it may have a power supply already installed. In that case, you only need to make sure it is an adequate unit for the computer you intend to build. Also, if you are building an ATX machine, you must make sure that the power supply is an ATX power supply.

If the power supply is not already installed, you will need to install it now. Here’s how:

1. Take the power supply unit and line it up for placement into the PC case. The fan should blow toward the rear and the wires should face forward.
2. Insert the PSU into the case. Sometimes this takes a little maneuvering to get it into position.
3. Once the unit is in place, check the back of the case and make sure the holes on the rear of the PSU line up with the screw holes on the case. If they do not, you may need to turn the power supply over.
4. Using your screwdriver, tighten the PSU down using standard chassis screws.
5. Make sure the voltage is set correctly. There is a little switch on the back that lets you switch between 120 or 220 volts. In the United States, its 120. If you are in a country overseas, its most likely 220. If you use 220, make sure the cord is rated for it. It should say on the side of the cord. Its easiest to just check this now while you’re thinking about it.