If there is any one component that is absolutely vital to the operation of a computer, it is the power supply. Without it, a computer is just an inert box full of plastic and metal. The power supply converts the alternating current (AC) line from your home to the direct current (DC) needed by the personal computer.
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Some computer cases will come with a power supply pre-installed inside them, but most do not. As a result, it is necessary to get a power supply that works with your components and has sufficient power.