PC Power Supplies Explained

Posted by Shopinet Collaborator on

Often overlooked, the PSU has a tremendous impact on how loud your PC is and how many devices you can install in it.


you want in a new PC, the power supply probably isn’t one of the first components that you’d pick out, but it should be, and its well worth spending a bit extra to get something decent.
We have tested a range of supplies to ensure their stability and quality. Overleaf, you can find our recommended power supplies for all types of PC’s. Before we can delve into that, here is everything that you need to know about choosing the right PSU for you.


All PSUs are rated by their maximum output power, written in watt. The good news is that PC’s and components have become a lot more efficient over the years, and require less power. So, while 1.2kW PSUs were on sale a few years ago, you see nothing of the kind available now. In fact most PC owners would be fine with a 500-550W PSU; this will cope with one or two graphics cards. If you have a lot of hard disks and a high power graphics card, then a 750-850W PSU is right for you.


A PSU outputs its power on different rails, which run at different voltages and have a rating in amps that shows the total current they can provide. Different parts of your PC draw on various rails for their power. Power-hungry components such as the processor and graphics card, for example, draw a significant amount of power from one of the 12V rails. A rail’s total rating figure is the total amount of current (amps) that can be drawn by that single rail: multiply that by the voltage to find the total power that the rail can provide.

A lot of power supplies have a single 12V rail for the entire PSU, whereas more expensive and higher capacity models have multiple rails. The total power available for each rail may exceed the total capacity of the PSU, so you can’t use each rail to its maximum. What’s important is the over current protection (OCP). If a rail draws more current from a rail that is allowed, the PSU’s OCP shuts the unit down. With a single rail, the current protection has to be set high, so a single device that fails can pull an exceptional amount of power through the rail, causing damage. With multiple rails, each with their own limit, OCP is triggered at a lower level and your devices are separated, giving you better protection. Typically a single-rail PSU is fine with regular builds; multi rails make more sense in dual graphics card systems that have been over clocked.

When buying a branded power supply, you shouldn’t have to be worried, as they’ll be able to provide close to their rated maximum wattage across their 12V rails.


The efficiency of power supplies has dramatically improved, particularly with the branded models that we’ve recommended. Efficiency describes how much power is wasted when it is converted from input to output. Any ‘lost’ power is converted into heat. For example, a 500W power supply that is 85% efficient would need to draw 394kw to power a 335W PC. By comparison, a 94% efficient PSU would need to draw 356W -38W less to power the same computer.

At an average cost of 13.86 per kilowatt hour (kWh) at the standard rate, the first power supply would cost 5.4p per hour to run. Assuming eight hours of use per day, the first power supply would cost £157.68 a year to run. By comparison, the second supply costs 4.93p per hour to run or £144.08 a year. That’s a difference of £13.60 a year. On top of that, the more efficient supply generates less heat, keeping your PC running cooler. Quality power supplies also comply with the 80 Plus certification, the Bronze, Silver, Gold, Platinum and Titanium awards for minimum efficiencies.

Make sure that your PSU has enough power connectors for all your PC’s components. To run two powerful graphics cards, you’ll need four 6 + 2 pin PCI Express connectors, for example. Plenty of SATA connectors are useful for mechanical hard disks, SSD’s and optical drives. You may need a molex connector or two for old devices and some case fans, although this is increasingly unlikely. Likewise some older devices, such as fan controllers and front-panel card readers, need a floppy connector. Buy a modular PSU if you can, as you have to connect the cables you need, leaving your PC otherwise clutter-free.


Buying a quiet PSU can really help keep your computer’s noise down. We recently upgraded a PC with a new PSU, as the old one had a rattly fan. Picking a semi-passive model (where the fan spins if the PSU is under heavy load), we managed to get a PC that’s a lot quieter.

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  • “For example, a 500W power supply that is 85% efficient would need to draw 394kw to power a 335W PC”

    REALLY 394 kW !! That’s a LOT of power…. ;-)

    Tim on

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