1. What the power supply has to do
A power supply has three separate jobs: deliver enough power for the whole system at once, provide the connectors each part expects, and physically fit the case. A failure in any one of the three leaves the build dead or incomplete, and each has its own documentation to check against, so treat them as three separate checks rather than one.
- It must supply enough power for all parts together, including short peaks; the figures to collect are in the next section.
- It must have the right connectors for the processor, motherboard and graphics card.
- It must fit the case; Case and cooler clearance covers the space it needs in there.
2. Collect the manufacturers' power figures
You cannot measure your own build before you have bought its power supply, so the starting point is the figures the manufacturers publish for each part. These are guidance values: they exist to let you size a supply, not to describe what your particular system will draw in use. Collect them before you look at any supply, so the choice is driven by the parts rather than by the shelves.
- The graphics card’s product page gives a recommended power supply and often a board power figure; record both.
- The processor’s product page gives a power figure; read what the manufacturer says that figure means before you use it.
- Treat these as guidance figures, not measurements of your build; the supply you choose must cover them together, not one at a time.
3. Add the figures up and leave headroom
Add the documented figures for the parts that draw the most, then allow for the rest of the system: the motherboard, memory, storage and fans all take a share of the total. The result is a figure to compare against the supply’s rated output, and the graphics card maker’s recommendation is a floor to sit above, not a target to land on.
- Add the documented figures for the parts that draw the most, then allow for the rest of the system.
- Choose a rated output comfortably above that total; the graphics card maker’s recommendation is a minimum.
- Do not assume a higher rating is needed, or a lower one wasted, without checking the documents for your parts.
4. Match the connectors to your parts
Connector names sound interchangeable and they are not: each part expects specific plugs, and each power supply model ships with a specific set of cables. The count of connectors on the supply’s own cables must cover what the build needs, and cables from one supply model should not be moved to another, even where the plugs look alike.
- Processor power usually uses one or two 8-pin (or 4+4-pin) connectors; check your motherboard’s manual for exactly which it wants.
- Graphics cards use 8-pin PCIe connectors or the 16-pin 12V-2x6 type; check your card’s product page for the connectors it needs.
- Count the connectors the power supply provides on its own cables and compare the count against what the parts require.
- Use only the cables supplied for that power supply model.
5. Check the standard and the size
Power supplies follow form factors, and cases support particular ones. Newer standards add features such as the 16-pin 12V-2x6 connector, and the product page states which standard a supply supports. The builder can match the supply’s form factor against the case’s supported form factors; the remaining fit questions are clearance checks, not standard questions.
- The product page states the form factor (for example ATX) and whether it supports newer standards such as ATX 3.1.
- Use the builder to compare the power supply form factor with the case’s supported form factors.
- Length and cable room are separate checks; Case and cooler clearance covers both.
- For the full list of what is verified and what is not, see the compatibility page.
6. Buy the exact model, including the UK variant
Power supplies are regionalised: the same model name can be sold with different power cords and different variant suffixes depending on the market. The variant you buy must be the one intended for UK mains, and the full manufacturer part number is how you confirm it, because the short model name on a listing may not say which region it is for.
- Some models are sold in regional versions with different power cords; check that the one you buy is for UK mains.
- Match the full manufacturer part number and compare delivered totals in the power supply catalogue.
Sources and scope
This guide gives a checklist, not a product recommendation or a hands-on test. It relies on the specifications published by manufacturers, so always confirm details on the manufacturer’s product page for the exact model you are buying. Read how our guides are researched for the standard this site applies to all of its buying advice.