Ask LH: How Do I Work Out Which Power Supply Unit I Need?

Hello Lifehacker, I have been wondering, how can I calculate how much power my computer needs? I'm really confused about this, so please explain how I can calculate how many watts my power supply unit (PSU) will need if I upgrade. And any other tips on choosing one? Thanks, Power Receiver

Desktop PSU picture from Shutterstock

Dear PR,

Good question! Unless you're building a ridiculously powerful gaming rig, you don't need an especially beefy PSU. Unlike storage space, it's not a matter of getting the biggest you can afford — having too much power will just be a waste of money and can also cause fluctuations in performance.

With that said, you don't want the PSU to under deliver either — this can cause your system to crap out when it reaches its maximum power output. In other words, you need a PSU with ample wattage for the job (as well as some extra for future-proofing) but without overdoing it.

As a general rule of thumb, you want around double the wattage your system draws on at full load for maximum efficiency. Most desktop PCs with a single video card will be able to get by on a 550 watt PSU.

However, that's just a ballpark number — it really depends on the PC components you're planning to use and their individual power demands. Some component models are specially designed to have a low energy rating, while others, such as enthusiast-level GPUs, require more power than usual. So it really depends on your particular build (which means you need to choose your PSU last!)

The easiest way to get a tailor-made answer is to use an online PSU calculator. As their name implies, these apps calculate the wattage needed to power your system based on the components and system configuration you've chosen. You can check out two examples here and here. Naturally, make sure to include your USB peripherals in the final tally.

Other things to look out for include the physical dimensions (will it actually fit inside your desktop chassis?), the PSU connectors/pins (are they supported by your motherboard/CPU?) and whether it has an "80 Plus" rating (this means that the power supply is certified to provide at least 80 per cent energy to the system when at 100 per cent load.)

In terms of specific brands, the quality-end of the market is stitched up by a handful of manufacturers, all of which provide similar bang-for-buck. Some brands worth considering include Corsair, Antec, XFX and Cooler Master. If you care about PC noise, keep an eye out for models with larger, thermally controlled fans for quieter operation.

Whatever you do, try to avoid cheap no-name offerings on eBay — in addition to increasing the odds of an untimely PSU failure, you will also be putting your other components at risk. This is one occasion where it definitely pays to spend a little bit extra.

If any readers have additional tips of recommendations of their own, let PR know in the comments section below.

Cheers Lifehacker

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    Go get a 400W power supply if you don't have a graphics card, 600W if you do. If you know that 600W isn't enough then you know how to work it out for yourself.

    I just calculated mine for the first time, recommended 750 watts. Feel like I nailed it by buying an 850 then!

    Just read the recommendation on the video card you're buying. If it suggests a 700w power supply, get one of those. The only reason you'd add more to it is if you're running large numbers of HDDs as well, then maybe add another 100-200w to cover off all that.

    Dang, confused again! I was told 550W would be enough for my system (R9 290) despite the manufacturer saying more because actual draw is significantly less. I was told if I buy a good quality PSU then 550W would be fine - but this article suggests buying twice what the system will draw which aligns more with what the online calculators come up with. Does 80plus Gold mean I don't have to?

      My ideas, for what they're worth:

      Use a few calculators from overclocker sites to get a consensus (not calculators on PSU manufacturer websites).

      There isn't an important difference between 80+ bronze and 80+ platinum, but there's a huge difference between an 80+ bronze and a non 80+ PSU. Basically, if they don't have an 80+ rating, they're lying to you about their specs, and they can (and will) change component suppliers and specs on the fly, buying the cheapest crap possible. If the PSU goes, much may go with it, and this isn't a smart risk.

      Based on research and experience; I trust Seasonic and Zalman. I'm open-minded about Corsair. I actively distrust all the other major manufacturers.

      Be suspicious about silent PSUs.

      Always check newegg for comments, but be skeptical about both the glowing reviews and the damning ones.

      You may be tempted to accept a high wattage claim or a tempting price at the expense of the manufacturer or the 80+ rating. Counter that temptation by remembering that trusting the biggest liar or the cheapest components is foolhardy.

    How do I know how much power my whole system is drawing from the PSU at any one time?

      Get an energy power meter like this one:

    I tried the two online calculators mentioned in the article.
    For the same components, the ASUS page recommended a minimum of 600W and the eXtreme page recommended 359W.

    I'd say the first sounds about right for my PC and the second is way too low. :/

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