Jump to content

Guide: Picking a PSU capacity


When it comes to getting a new PSU, picking one that meets your needs without being overkill or barely running at capacity takes more than just using some website to arbitrarily estimate the watt usage of a build.


Most web sites, including pcpartpicker.com, tend to use CPUs and GPUs TDP measured in Watts, which is NOT the actual or maximum watt draw - It is simply the heat output of the given part.
Some GPU makers do provide a different number like TGP (Total Gaming Power) that is closer to actual power consumed while gaming, but even that doesn't reflect possible short transient power spikes.

You can try to use some arbitrary formula or recommended PSU calculator that tries to compensate for all this, but they dont know what your needs are, so their recommendation may still fall short or overshoot an ideal PSU watt capacity for your needs.


You should also be aware of some other nuanced realities about PSUs that exist but aren't covered here in-depth:

  • Not all PSUs handle transient spikes well or the same
  • Not all PSUs can handle peak or exceeding their watt capacity the same
  • A PSU's watt rating does not mean that it will handle near the same watt capacity for pcie-specific connectors, though some do. This is generally noted in PSU specs under the "12V+" max output shown in Amps and Watts. OEM (in pre-built PCs) or lower quality PSUs may have a 12V+ watt max output that is much lower than the overall capacity.


So here is a guide to picking the best Watt rating for your PSU that considers many different factors you may (or may not) care about:

1) Have a website add up the TDP/TGP/watt usage for all parts - This is the absolute minimum watt rating to consider for a PSU, but if there is max power draw or power spikes by parts that are longer or just as large as what the PSU can handle, then dont be surprised if it runs hot or there are unexpected shutdowns. Use this number at your own risk!



2) Research the actual gaming and peak power usage for your GPU, CPU and anything else that could have significant variations in power draw. Use the manufacturers website or reviews that measure average gaming power draw or peak power draw - Whichever one you are more comfortable with. Use this to update the total power usage of your build.



3) Do you care about PSU noise? - Then lookup the manufacturer's website or tech reviews for the PSU that has a chart showing the noise level based on watt load. Make sure that the expected power usage of your build won't result in noise higher than what you want. If the PSU & watt load you are looking at will be noisier than you want, then either find a quieter PSU or one that can handle more watts and thus likely to run the fan at lower RPM. Keep in mind that some PSUs can run passive (fan off) below certain wattage as well.



4) Do you see yourself getting a higher class of CPU or GPU during the PSU's warranty period? For example if you have a CPU that ends in 6XX but you might go for an 8XX in a few years, then you'll want to adjust your PSU capacity to support that. Just look at the difference in power consumption between the two parts in the latest gen, and add it to your PSU capacity. Same thing with GPU models. If you might upgrade to the same glass but are concerned that next gen parts are going to be higher power consumption, then make an educated guess based on historical on gen to gen increase for the same model. Mid range and budget class parts are less likely to have substantial generation to generation changes in power consumption.



5) Do you care about energy efficency? - Then lookup the manufacturer's website for power efficiency curve of system (PSU) load % versus efficency %. You can make sure that your average power usage while your PC is running is as close to the highest part of that curve - This peak may be between 30 to 50% of PSU load. I say "average" because your PC may not be maxing out your CPU and GPU the entire time it's running, especially if you use it for low usage productivity, web browsing, or watching videos. The only way to know your average system PSU power draw is to plug the PSU into some device between it and an outlet that will tell you the Watt rating, or can average the usage over time. I personally have device called Kill-a-Watt. UPS battery backups can do this too as long as you don't exceed their watt rating.


Keep in mind that the power draw of your system is different than power draw at the outlet - The formula is something like this: [Power draw from PSU to outlet] / [PSU % efficiency as a decimal] = Power draw of system to the PSU.


You have two main options if you want to improve power efficiency: Change the watt rating of the PSU, or get one with a higher efficiency rating. As for ratings, there's 80+ Bronze/Gold/Platinum that is basically self reported by PSU makers, or Cybernetics does their own independent testing of efficiency as well as noise.


Manufacturer chart examples: Here is an example of a PSU maker that has charts regarding power efficiency and noise level versus load: https://www.corsair.com/us/en/p/psu/cp-9020200-na/rmx-series-rm850x-850-watt-80-plus-gold-fully-modular-atx-psu-cp-9020200-na


Walking through the questions above, I thought this PSU's capacity was ideal for my system because:

  • I knew it would handle any potential transient spikes from my RTX 3080
  • It may handle a future generation GPU in the 70 or lower series and maybe the 80 series, but I never buy 90 series
    • I also tend to buy middle tier CPUs (like 5600X) and not higher end CPUs that require more power
  • Fan would be off/passive at idle, and whisper quiet under gaming load (~550W)
  • Efficiency would be good under gaming load and great for moderate usage
Edited by NobleGamer
Added nuanced realities, example manufacturer chart & example system parts
Link to comment
Share on other sites

Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now