While the performance and speed of a system might rely mostly on components like the motherboard, CPU, and memory, all of a computer’s hardware relies on the power supply to function. Flashy video cards might look good and have impressive names, but they’re meaningless without the right power supply. The maximum power or wattage might be the single most important specification when looking at a PSU, but there are other elements to consider. Getting enough power and having the connections you need are vital to setting up a computer.
Form Factor
The form factor or type usually refers to the size of a power supply. It’s important to choose a model that fits your computer tower. The most common is an ATX power supply, which should easily fit into a full or mid-size tower. For smaller computer cases, consider a Micro ATX or Mini ITX power supply. The form factor can have an impact on the number and types of connections, as well as maximum power, so you’re generally better off getting as large a model as you can fit.
Maximum Power
A power supply plugs into a wall outlet, bringing power in from a home or office and controlling how it’s distributed throughout your computer. You want to choose a PSU with a high enough maximum power level for the components in your computer. A good standard to remember is that a basic motherboard, processor and memory combo will require about 200W, depending mainly on the CPU, and whether it's a basic, mainstream, or enthusiast-level motherboard.
A single hard drive or optical drive uses up to 30W, and video cards generally take at least 100W - 200W each. So the average PC would require a bare minimum of about 400W. However, you don’t want to run at the bare minimum, so multiply what you think you need by about 1.5 times. If you think you only need 400W, then consider a 600W or 650W power supply. Once you start adding multiple video cards and very powerful CPUs, then you’re going to need an 800W or even more powerful PSU.
Manufacturer
There are quite a few different PSU manufacturers out there, so consider your options carefully. Some of the most common and popular companies include Corsair, Thermaltake, Antec, and CoolerMaster. There are many more companies which manufacture adequate power supplies, so it can be hard to narrow down exactly which model to look at. Keep in mind the warranty offered by a manufacturer and look for a PSU with a five or seven-year warranty. Other considerations, like maximum power and efficiency, however, are going to be more important than the manufacturer.
Efficiency Rating/Certification
The efficiency rating on a power supply indicates how much of its total power is maintained at a steady pace, with little lost during operation. 80 Plus is the bare minimum you should look for, and there are a number of even better rating levels. Each level means the power supply has been tested and is increasingly efficient.
80 Plus means the PSU has an 80 percent efficiency rating. 80 Plus Bronze has about an 82 percent efficiency rating, 80 Plus Silver comes in around 85 percent, 80 Plus Gold has a rating around 87 percent , while 80 Plus Platinum is about 90 percent efficient. The highest level, 80 Plus Titanium is about 92-94 percent efficient. Higher levels are usually more expensive, so pick a PSU with the highest efficiency rating you can afford.
Connections and Modular Designs
A power supply has a number of connections that go from it to the different components in a computer, including the motherboard, hard drives, video card, and case fans. You need to know what kinds of connections you need and how many of them. That includes 6+2-Pin PCIe connectors, SATA power connectors, and the main motherboard connector.
Motherboards typically use 20Pin or 24Pin connections, and some power supplies use a 20Pin with an optional 4Pin connection for compatibility with motherboards with both 20Pin and 24Pin connectors. A modular design means the power supply has detachable cables and connectors. PSUs can have half a dozen or more cables, and unused connections easily get in the way and are hard to work around inside a computer case. So consider a modular PSU to help you keep the inside of your case streamlined.
+12V Rails
The +12V rail on a power supply is what provides power specifically to highly demanding components, primarily the CPU and graphics card. You want to look at how many rails the power supply has, as well as the amperage on each of them. Most power supplies have a single +12V rail that distributes power to the CPU and graphics chip. With just one rail, make sure the amperage on that rail is high, as this allows a lot of power to go to your CPU and GPU.
Multiple Rails
Some power supplies have multiple +12V rails, which splits the power and amperage between each rail, rather than having all the power go through a single one. Multiple rails are somewhat safer, since all the power on a single rail can potentially overload it more easily. Ultimately, one rail or two +12V rails can give you any level of performance you need, but multi-rail PSUs are somewhat less likely to damage components if there is a short. Maximum power and enough connections for your system are more important than the number of rails.
CrossFire and SLI Support
Although related to the number of connections in general, make sure you specifically look at how well a PSU is suited to a CrossFire or SLI setup. If you plan on using multiple video cards together, then make sure it has plenty of 6+2-Pin connectors for PCIe components like video cards. Beyond that, many power supplies clearly indicate that they are ready for CrossFire or SLI, which means they have powerful +12V rails with plenty of amperage to support a demanding CPU and multiple video cards.
Cooling and Noise Levels
Power supplies often generate a lot of heat, so make sure you pick a model with enough fans to keep it running as cool as possible. The more powerful the PSU, the more you should consider cooling, with larger, faster fans offering the best cooling. If you’re picking a high-power PSU, then look for a model with multiple fans to really reduce heat. This can also generate a lot of noise, so look at the noise level of the fan or fans on a power supply to make sure it won’t be excessively loud.