PSU stands for Power Supply Unit. It’s the part of your build that converts the wall voltage into the various lower-voltage rails your computer’s components run on. It’s the most boring component to shop for, and also the one that, if you cheap out, can literally light other components on fire. Spend the time to get this right.

What the PSU does

Your wall outlet delivers AC power at 110-240V. Your computer’s components run on DC power at much lower voltages — 12V, 5V, and 3.3V. The PSU’s job is to do that conversion cleanly, reliably, and at the wattages each component demands.

A good PSU does this with stable output even under heavy load, recovers gracefully from spikes, and protects connected components if something goes wrong. A bad PSU does the opposite: noisy output, sagging voltage under load, and failure modes that can kill your motherboard or GPU.

How to calculate the wattage you need

Add up the power draw of your major components, add a buffer for the rest, and add headroom for spikes and future upgrades.

Rough numbers for typical components:

  • CPU: 65W (low-end) to 170W (high-end). Check the TDP, and roughly double it for transient spikes.
  • GPU: 100W (entry) to 450W (flagship). Look up the TDP for your exact card.
  • Motherboard, RAM, drives, fans: 50-80W total for a normal system.

So a build with a 105W CPU and a 300W GPU realistically draws about 500W under heavy gaming. Buy a PSU that delivers around 750W to 850W — that gives you a comfortable buffer for spikes, future upgrades, and so the PSU never runs at 100% (which is loud and inefficient).

If you’re using a top-tier GPU like a 4090-class card, look at 1000W or higher.

80 Plus efficiency ratings

80 Plus is a certification for how efficient the PSU is at converting AC to DC. The ratings go:

  • 80 Plus (white) — at least 80% efficient at typical loads.
  • Bronze — slightly better.
  • Silver / Gold — significantly better. Gold is the sweet spot for most builds.
  • Platinum / Titanium — top of the line. Worth it for very high-wattage systems or if you care about quiet operation, otherwise overkill.

Higher efficiency means less heat, lower fan noise, and lower electricity bills. The price difference between Gold and Platinum is usually not worth it for most users.

Modular, semi-modular, non-modular

This is about cables.

  • Non-modular: all cables permanently attached to the PSU. Cheaper, but you have a lot of unused cables to hide.
  • Semi-modular: the essential cables (24-pin and CPU power) are fixed; everything else is detachable.
  • Fully modular: every cable detaches. Best for clean cable management.

For a clean build, fully modular is worth the small premium. Semi-modular is a fine compromise.

ATX 3.x and the new GPU connector

Newer GPUs from Nvidia (and increasingly others) use the 12V-2×6 connector (also called 12VHPWR), which can deliver up to 600W through a single cable. PSUs that support this natively are labeled ATX 3.0 or ATX 3.1. If you’re buying a new high-end GPU, getting an ATX 3.1 PSU with a native cable is a much better experience than using the adapter that ships with the card.

Form factor

Most builds use standard ATX PSUs (150 x 86 x 140-180mm). Compact builds may need SFX or SFX-L PSUs. Always check that your case supports the size you’re buying — some cases have weird PSU mounts.

Brands to trust (and avoid)

PSUs are almost all built by a handful of original equipment manufacturers, then rebadged. Two PSUs from different brands can have identical internals. The reliable rule is: look up the model on independent reviewer sites that test PSUs (search the model name + “review”). They’ll tell you the actual quality.

What you don’t want: any no-name brand with vague marketing, no efficiency rating, and a suspiciously low price. A bad PSU can take out everything else when it fails. The savings aren’t worth it.

Cable management for the PSU

Most modern cases have a separate basement compartment for the PSU and excess cables. Run cables behind the motherboard tray and through grommets on the front side. Don’t cram everything into the same space as the components — airflow matters.

Final checklist

  1. Calculate your wattage needs, add a 25-30% buffer.
  2. Go for 80 Plus Gold minimum.
  3. Prefer fully modular if budget allows.
  4. Get an ATX 3.1 unit if your GPU uses the 12V-2×6 connector.
  5. Stick to brands and models with positive independent reviews.
  6. Don’t buy used PSUs. Ever.

A good PSU should outlast multiple builds. Budget for it accordingly.