Before making any purchases or performing upgrades, first determine the customer’s needs. Ask the customer which devices will be connected to the computer both internally and externally. The computer case must be able to accommodate the size and shape of the power supply.

The computer case holds the power supply, motherboard, memory, and other components. If you are purchasing a computer case and power supply separately, ensure that all components fit into the new case and that the power supply is powerful enough to operate all the components. Many times a case comes with a power supply preinstalled. In this situation, you still need to verify that the power supply provides enough power to operate all the components that will be installed in the case.

Power supplies convert AC input to DC output voltages. Power supplies typically provide voltages of 3.3, 5, and 12, and are measured in wattage. It is recommended that the power supply has approximately 25 percent more wattage than all the attached components require. To determine the total wattage required, add the wattage for each component. If the wattage is not listed on a component, calculate it by multiplying its voltage and amperage. If the component requires different levels of wattage, use the higher requirement. After determining the necessary wattage, ensure that the power supply has the required connectors for all the components.