How To Tell What Size Power Supply a Computer Uses

Building a computer yourself, or even upgrading one, isn’t difficult, but it does require that you at least have a basic understanding of how all of the pieces go together. To build or upgrade one, you need to understand what video cards are compatible with your motherboard, what processor socket types are compatible with your motherboard, and arguably most importantly, how much power it’s going to take to keep all of that running. After all, if you don’t buy the right power supply, your computer won’t stay running at all. With the wrong power supply installed, turn on your PC, and it’ll immediately turn off.

How To Tell What Size Power Supply a Computer Uses

So how do you tell what power supply your currently have in your system? If you’re building a PC, how do you tell how much wattage you will need to keep it running? Or, if you’re upgrading a PC component, do you also need to upgrade the power supply to account for the added power draw? These are all questions we’ll cover below. Let’s dive right in!

Determining the Output of Your Current Power Supply

In most cases, to tell what size power supply you have, you’ll have to open up your PC case. That’s usually just a few screws around the back of the system, and then a side easily slides off. Then, you just need to look and see what wattage your power supply is. The power supply itself usually tells you with a label on one of its sides that gives you some general specs. Usually you’ll on the label a column that says MAX LOAD: 500W, or whatever your model of power supply is capable of. If you don’t see that, the model number is always on that label, which makes it easy to lookup online and find out with a simple Google search.

If you don’t see the label, then it’s probably on a side of the power supply that isn’t visible. All power supplies have an identification label, as required by UL — formerly referred to as Underwriters Laboratories. That said, to find the label, you’ll need to carefully remove the power supply from your system. Before proceeding with removing it from your PC, make sure all power is cut off from the system — you don’t even want it plugged into the wall outlet or power strip. As a safety measure, make sure to turn the power supply to the OFF position, too. This is usually represented with an O icon, either on the back of the case, or on the power supply itself inside of the case.

Once you pull the power supply out, you should see a label on the non-visible side. If you don’t, we wouldn’t recommend putting that power supply back in your computer — power supplies without labels are dangerous to use, and is a sign of a low quality component that could potentially fry all of your computer parts.

Unfortunately, you generally aren’t able to tell what type of power supply you have by way of software. This is because most power supplies aren’t intelligent, which means you cannot use software to pull up its specs.

Do You Need to Upgrade the Power Supply if You Upgrade a Part?

If you upgrade a component on your computer to something more powerful, you may or may not need a new power supply. If you already have a power supply that has way more capacity than you already need, you’re good. However, you’ll always want to make sure that you’re not exceeding your power supply’s recommended output. So, it’s recommended that you double check how much your power supply’s maximum load is capable of — just follow the steps above — and then make that, say, your video card doesn’t put you over the top.

That’s why, as a general rule when building a PC or upgrading a power supply, PSU for short, it’s recommended to get one that’s 150W above your demand.

How do I Know How Much Wattage I Need When Buying a Power Supply?

And now we come to the most difficult part about buying a power supply. How much wattage — or Max Load — does your power supply need? That’s not something that we can answer because it’s going to be a different case for every PC out there. Luckily, there are a handful of free online tools and an equation that will help you find the wattage that you need.

Using the Equation

The basic equation for calculating the amount of power, or watts, is P = I x V. Put simply, this equation is Power = Amps x Voltage. So, the amount of watts needed will be equal to the amount amps times the volts of the parts used, i.e. the GPU, HDD/SSD, Optical Drive, etc.

Using an Online Tool to Calculate the Wattage for a Power Supply

Both OuterVision’s Power Supply Calculator and PCPartsPicker will help you determine the power supply wattage that you need. The way these work is that you enter in the PC components that you have in your PC — or the PC parts that you plan on buying — and it will then calculate the power draw of all of those components. Then, it’ll tell you how much wattage you need in your power supply, based on the power draw of those components. As an added bonus, PCPartsPicker is able to show you if you’re building a machine with all compatible components so you don’t purchase the wrong hardware during your PC build.

Now that you know how much wattage you have to support, you’re ready to go out and purchase a new power supply (or stay with your old one, depending on your results)! However, there’s one more thing to keep in mind….

Stay Away From Some Manufacturers

Buying a power supply without knowing anything about them and the manufacturers that build them is like playing Russian Roulette. It’s not a game you want to play, especially if you have some really expensive components in your machine. There are honestly power supplies out there that are deadly, and should be avoided at all costs. The power supply can literally be the life or death of your machine.

So how do you know what power supply brand or manufacturer to buy from? We’ve done some of the legwork for you, and have compiled a list of all of the brands to stay away from, as well as show you some of the top brands that you can trust. As usual with something like this, the rule of “you get what you pay for” can be followed here.

Suppliers to Stay FAR Away From

Just like with the rampant issue of sub-standard and counterfeit batteries, you also need to be wary of power supplies and certain suppliers.

  • Diablote
  • Apevia
  • Coolmax
  • Logisys
  • Sparkle
  • Raidmax
  • NZXT
  • Enermax
  • Cougar
  • Bitfenix
  • FSP

Although not a comprehensive list, this should help steer you in the right direction. Remember, it’s worth investigating the reviews, sites, and datasheets of any PSUs you decide to purchase, after all, you have critical data attached to these devices.

Top Suppliers You Can Trust (in Order)

Amidst the sea of questionable parts and counterfeit production, it’s hard to know who’s reliable and what’s authentic. Here’s a list that will hopefully serve you will, it’s definitely done us right in the past.

  • Seasonic
  • XFX
  • Superflower
  • EVGA
  • Corsair
  • Cooler Master
  • Antec

And as per the norm, if you don’t see a label or some sort of identification on your power supply, don’t put it in your PC! If you get one without identification from one of the top brands — ship it back and they’ll be happy to send you over a new one.


As you can see, finding out what power supply you have — as well as how much wattage you need for your newly built PC or upgraded parts — can be quite a task. Thankfully finding out how much you need isn’t nearly as tough as it used to be. Now, we have large databases of computer parts where we can easily add up their power load through the magic of software.

Do you have a power supply that you stand by for all your power needs? What is it? Get the conversation started in the comments section below — we’d love to hear from you!

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