How to choose a power supply
The power supply is the heart of your system. It's the one single component that "pumps" electricity into your PC.
AC vs DC
The electricity we get from the grid comes in the form of a wave, or "AC" (Alternative-Current). But a PC works with electricity in the form of direct current - "DC".
So we need some sort of a converter that transforms AC to DC, and that's the main purpose of your power supply.
Power Supply Efficiency
Unfortunately the power supply consumes a little bit of power for itself in the process of converting the current from AC to DC.
For example, a power supply with efficiency of 80% means that the power supply consumes additional 20% for itself. Let's say you have a PC that consumes 300W of power, in that case the whole system (including the PSU) will consume 360W (because of the additional 20% of your power supply).
Usually we'd want a power supply rated with at least 80% efficiency.
How to choose a power supply?
The first factor you should consider when purchasing a new power supply is the brand. You'd want a power supply from a respected brand such as Corsair or EVGA.
The second factor you should look for is the actual power rating of the power supply, for example if you have a system that consumes 300W you should get a PSU that's capable of providing 400W (you want some headspace).
Usually you'd calculate the wattage based on your system's GPU and CPU, the two most power hungry components of your PC.
You can calculate the minimum PSU wattage required for your rig using our calculator.