When building a PC or upgrading components, you may be wondering what size PSU to buy and if there is such a thing as an oversized PSU.
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By default, power supplies are not set to maximum power
The rating for a given power supply is its maximum load rating, not its standard load as it is currently plugged in and powered on.
Your computer might feel like a space heater at times – and it certainly brings warmth into your home – but it’s not quite like a space heater in the sense that a 500W space heater will either be on full blast or it won’t be on.
Your power supply is like a motor that can run harder when needed, but idles when there is no need. An 850W PSU doesn’t run at 850W every second, it runs exactly as the hardware demands. This requirement varies between hardware configurations and even what you do with a specific hardware configuration.
Your GPU, to highlight a particularly power-hungry component, might draw as little as 50W while fiddling with work documents (or watching YouTube to avoid working on those docs) but 300W under load while playing a demanding game.
We emphasize that you are not automatically signing up for higher electricity bills by installing a beefier PSU. You also don’t risk damaging your computer by adding a “motor” that is too powerful.
Other than small differences in efficiency between different sized power supplies at different loads, there isn’t really a difference and your components will only draw as much power as they need.
Oversizing is probably wise
The difference between what your hardware needs and how much extra wattage is still available is called “headroom”. If you have a build that only draws 450W at peak load but has an 850W PSU, then you have 400W of headroom.
PC enthusiasts can have pretty strong opinions about how much or how little headroom you need. Opinions generally gravitate toward more, and it’s almost always better to have more headroom than you think you need.
For example, the price difference between a high-end 500W PSU and a high-end 700-850W PSU is often only around $30-40. When you consider how expensive a PC build is in the first place and how long you’re using a good PSU, the difference is pretty trivial.
Especially trivial considering future upgrades might require buying a new power supply anyway. It would be both a waste of money and a nuisance to skimp on the PSU today only to turn around and have to buy and install a new one next year because your new GPU is more demanding.
Corsair RM1000X 1000W PSU
It’s not a $50 special, but it’s one of the top-rated power supplies with high efficiency, quiet operation, and performance to run next-gen GPUs.
$189.99
And in the age of $1000 GPUs, it’s not a bad idea to spend a little more to support and protect your hardware investment. Continue, a quality power supply can survive a PC build. You might refresh your entire build every few years to keep up with gaming trends, but unlike the GPU, a good PSU can ride along.
But don’t oversize to the max
Oversizing with a quality PSU is a great way to semi-future-proof your build and skip buying a new PSU if you get a bad GPU or otherwise upgrade components.
But there’s a point where returns begin to diminish, both in terms of efficiency and cost. You can read all about the details of the 80 Plus efficiency certification here, but suffice it to say that running a PSU at a very low or very high load for its rated limit is inefficient.
Suppose you buy a super premium 1200W power supply. This PSU is most efficient at around 50% load, or 600W, and loses a few percentage points in efficiency at very light loads (20% or less) or very high loads (at or near 100%). So if your build is idling at around 150-200W and only has a peak demand of say 400W, not only are you rocking ~66% headroom, but your idle load is at or below 20%.
Wasting a little money on your electric bill because of this 3-5% inefficiency is not the end of the world. But good 1200W (and larger) power supplies don’t come cheap.
You could have gone for a top-tier power supply with a lower wattage and saved money upfront and over time with that small gain in efficiency. At this point, putting the extra $100 into a better CPU or GPU is probably a much better use of your funds.
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