How to Pick a PSU/Power Supply


So pilgrim, word on the street is that you’re looking for a comprehensive guide on how to choose a power supply unit, or PSU. Well, you’ve come to the right place. I WILL HELP YOU. JUST HOLD STILL.

SO, there are 3 important things to look for when shopping for a power supply PSU and 4 less important things.


Wattage is the most obvious of the criteria we’ll talk about, but there are some wattage misunderstandings that need to be cleared up. The first is that most people think they need WAY more than they actually do, and second is that, when talking about wattage, more is not always better.

Suppose you’ve got a standard, middle-range gaming system with a decent CPU, a decent video card, a couple of hard drives/SSDs, etc. but nothing crazy. A system at this level only needs about 400-Watts of power. Many at-home system builders will go nuts and buy something in the 600W to 700W range, which is way too much. While it’s definitely better to have too much power vs. too little, there is a significant issue associated with overdoing it.

This is only here because I started this article with “pilgram.” I may have ADHD.

The problem with too much power is inefficiency. We’ll obviously talk about this more when we get to efficiency, but when thinking about PSU power supply wattage, it is important to understand. Power supplies function best when they’re run at around 50% of their max capacity and then become less efficient when they move above or below that mark. So if your system only needs around 200-Watts and you install an 800W PSU supply, you’re going to lose a lot more energy than you would with a 400W PSU supply.

Lesson Learned: Think about PSU wattage like pants—get the pair that fits, nothing too big or two small. You’ll end up far more comfortable.


Amperage is an oft-overlooked issue that is actually one of the most important factors to consider when selecting a power supply. When your power supply pulls power from the wall, it not only converts the energy from AC to DC, it divides the stream of power and converts it to several different voltage standards so that it can properly power the different elements within your system.

On a manufacturer page for a power supply you’ll typically see a graph that looks like this:

This graph is an expression of how much power the PSU power supply will be able to funnel down a certain channel (called rails)

The most important of these rails is the +12V, because it will be responsible for providing power to your graphics card and your CPU. A low-quality power supply will often have a high total wattage, but will funnel that power down the wrong rails, leaving you with amp output on the primary rail. Another common issue is that your power supply PSU will have dual +12V, which means that it will be splitting the +12V amperage along two paths, neither of which will typically be able output enough power to support a high-end video card.

Lesson Learned: The number of watts won’t count for much if the power supply doesn’t put them where it counts. Buy a power supply with a high number of amps on the +12V rail.


Power supply purchasing is made more difficult by the fact that it’s a mostly un-regulated industry. Manufacturers can make just about any claim concerning their product they want. One of the biggest ways that shady manufacturers will fudge the value of their product is by not disclosing the efficiency of their power supply PSU.

As a PSU power supply converts the AC wall current into DC power, a certain percentage of the energy will be lost in the form of heat. It is this percentage of energy lost that determines efficiency. Higher quality power supplies will lose around 20% or less of their intake, poor quality power supply may loose as much as 40-50%.

The most reliable way to find an efficient power supply is by looking for 80+ certification. 80+ is a requirement from a 3rd party that the power supply meet a minimum of 80% efficiency under certain loads and provides so assurance of the power supply’s quality.

Lesson Learned: Buy a power supply that’s 80+ certified. It will cost a little bit more than a non-certified model, but it’s worth it.

Other Stuff

Connectors: A PSU power supply’s connectors are the plugs that you plug into your components. You’ll want to make sure that your power supply comes with all the plugs to power the components you want to power.

Form Factor: A hefty majority of cases are compatible with the standard ATX power supply form factor. Smaller HTPC cases may require a smaller flex-ATX form factor power supply.

Modular: Modular power supplies have connectors that can be removed if they’re not needed for your build. This will reduce the number of cables running through your system, making your build look much neater and cleaner.

Cooling: A better fan is better than a worse fan in just about every circumstance. Larger fans are typically quieter, so look for a 120mm fan over a smaller one.

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