First of all, what is a power supply? Well, to put it simply, a power supply unit (PSU) is designed to transform 120/230V mains AC electrical power that is present in your home to smaller DC voltages such as 5V and 12V. This is necessary because AC (alternating current) is great for transmission across long distances but not so great for powering integrated circuits like the ones found in your PC, for this we use DC (direct current).
In essence, your phone charger is like a computer power supply except it provides only a single voltage (typically 5V) and a lot less current: therefore is has a lower wattage.
Wattage is a measurement of power over time, namely Joules per second. How this relates to PC gaming hardware is that your CPU and GPU both require a certain amount of power to function correctly. However, depending on system load, the power requirements will vary. So in this case, it's important to have enough headroom to handle spikes in power demand from your components.
So, how do we calculate the required wattage? Well, it should be as simple as adding up the power requirements of all of your components and adding a little margin to handle spikes.
To do this, you can get a rough estimate using the Thermal Design Power (TDP) of a CPU or GPU from the hardwareDB database. This is a measurement of how much energy is produced as heat at maximum load.
It's not exactly an indicator of power consumption, but it's a good baseline.
For example, an Intel Core i9-12900K (241W TDP) paired with an NVIDIA GeForce RTX 3080 (320W TDP) will require at least a 561W power supply. This however doesn't take into account the other components in the system. So add at least 100-200W to this figure. If you plan on overclocking or use very high-end components, you may want to add even more headroom.
Alternatively, you can use an online calculator to find the required wattage.
For instance, the OuterVision Power Supply Calculator recommends 678W for a system with the aforementioned components with RAM and an SSD. In this case, it would be best to use a 750W power supply.
Efficiency (80+ rating)
When converting AC to DC, some energy is lost to heat, as this process is not 100% efficient. Computer power supplies carry an efficiency rating that allows you to choose one that best suits your needs. More efficient PSUs will use less power and emit less heat, however since they are made of higher quality components, they will be more expensive.
Most power supplies are certified with an 80 PLUS rating with different levels:
|Grade||Efficiency (20% load)||Efficiency (50% load)||Efficiency (100% load)|
Deciding on a rating will be mostly preference based. The differences in efficiency probably won't make up for the difference in the cost of electricity, but with a higher rating it's guaranteed to contain higher quality components. A higher quality power supply has less chance of failure and will have more safeguards in order to prevent component damage.
This last point depends on the size of your PC case. Every PC has a form factor: ATX, Mini-ITX, Micro-ATX, etc… which denotes the size and placement of screws. When selecting a PSU, you must make sure that it will be compatible, otherwise it might not fit or be too small.