Everything you need to know about power supplies

The primary task of a power supply is to take the AC (alternated current) input and convert it to DC (direct current), and to do so at a variety of voltages required by the components of a PC. The specs of a PSU are defined by a number of factors, so how can you tell what to look for?

What’s watt?
The most important function of a PSU is its output power, measured in watts. In simple terms a watt is the rate at which energy is transmitted by a circuit, although other factors play a role (such as resistance). Generally, wattage is measured by multiplying amperage by voltage.

PSUs are commonly marketed by their total output power in watts. The value is a total of all the separate output rails combined (more on this below), but it can sometimes be misleading: firstly, as a combination of all the rails, you don’t necessarily get access to all that power where you need it most. Many PCs these days rely heavily on the +12v rail, and two identically rated 500W PSUs can provide two different output maximums on the +12v rail, for example.

Secondly the rated output power isn’t a hard limit. A 500W PSU can output more than this, but it’s not rated to do so efficiently or stably, and ultimately protection circuits kick in.

Finally, a PSU’s output power is also rated for a given temperature. Any good PSU for a PC will be rated for 50 degrees Celsius. The temperature rating is important – as a PSU heats up, its efficiency can decrease. This is known as the de-rating curve, and we’ll cover this more below.

You will also see some PSUs advertised with a ‘peak’ rating. Ideally, don’t use this as a consideration for purchasing, look for the ‘real’ rating that the PSU can consistently deliver. Any good PSU usually has a ceiling above its rating, but if you need that much power you’d be better off getting a more powerful PSU in the first place. That said, some cheaper PSUs are rated higher than the real output wattage they can reliably deliver. This is a dodgy practice but is usually relegated to low-end yum-cha units.