Think of a power supply as the water main for your PC. Wattage is how much water it can deliver steadily. The 12V rail is the main pipe that feeds the thirstiest parts, the CPU and graphics card. Efficiency is how little leaks away as heat. Connectors are the taps: you need the right number and type. Protections are the shutoff valves when something bursts. Quality matters more than a big number, because a failing unit can take other parts with it.
The continuous power the PSU can deliver, in watts, around the clock.
When it matters: Add up your CPU and GPU power plus the rest of the system, then leave headroom so the unit runs at roughly 50 to 80 percent load. Beyond that, bigger adds nothing.
The short power spike the PSU can survive, for a fraction of a second, without shutting down.
When it matters: Modern graphics cards spike far above their rated power for brief moments. Units that handle this avoid random shutdowns under heavy load.
Skip it if: Your graphics card draws little power.
How many amps the 12-volt output can supply. The CPU and graphics card draw almost all their power from it.
When it matters: Multiply by 12 to get watts. A good unit delivers nearly its full rating on 12V. A much lower figure means the label wattage is padded.
Whether all 12V power comes from one rail or is split into several rails with separate current limits.
When it matters: Single rail is simpler for high-power graphics cards. Multi-rail can trip protection sooner if one rail overloads. Both are safe in quality units.
Skip it if: You are buying a quality unit from a well-reviewed platform.
The ATX spec version the PSU follows, such as ATX 2.x, 3.0 or 3.1.
When it matters: ATX 3.0 and 3.1 add power spike handling and the 16-pin 12V-2x6 graphics card connector. Choose 3.1 for a modern high-end GPU.
The PCI Express graphics power spec the PSU meets, such as 5.0 or 5.1. It defines the 16-pin connector.
When it matters: PCIe CEM 5.1 brings the revised 12V-2x6 connector with safer sensing. It usually pairs with ATX 3.1.
Skip it if: Your graphics card uses 8-pin plugs.
A design where the PSU outputs only 12V and the motherboard makes the other voltages.
When it matters: It needs a matching ATX12VO motherboard, mostly found in prebuilt office PCs. It will not work with a standard ATX board.
Skip it if: You are building with a standard ATX motherboard.
How long the PSU keeps output steady after the wall power drops out, in milliseconds.
When it matters: Longer hold-up rides through brief flickers and gives a UPS time to switch over. Very short hold-up is a sign of cost cutting.
Skip it if: Your power is stable or you use a line-interactive or online UPS.
An efficiency certificate: White, Bronze, Silver, Gold, Platinum or Titanium. Higher levels waste less power as heat.
When it matters: Efficiency is not quality: a Gold badge says nothing about protections or ripple. Gold is the common sweet spot; Titanium rarely pays back at home.
Cybenetics ETA rating: an efficiency grade from tests at many load levels, from Bronze up to Diamond.
When it matters: More detailed than 80 PLUS because it tests more loads. Useful for telling apart two units with the same 80 PLUS badge.
Skip it if: Only one unit you are considering has a rating.
Cybenetics LAMBDA rating: a noise grade from lab tests, from Standard up to A++.
When it matters: The most dependable noise figure, since maker dBA claims are not comparable. Aim for A or better if you want a quiet PC.
Skip it if: Your PC sits in a noisy room or you game on headphones.
Average efficiency and fan noise measured in lab tests across loads, not taken from the maker's box.
When it matters: Better than labels for comparing units. Noise figures only compare when measured the same way at the same distance.
The size of the PSU's cooling fan and its bearing type, such as fluid dynamic or ball bearing.
When it matters: Bigger fans can spin slower for the same airflow. Fluid dynamic bearings run quieter; sleeve bearings wear faster. Measured noise ratings tell you more.
The fan stays off at low load and spins up only when the unit warms.
When it matters: Silent at idle and light use. It is not required for a quiet unit; a good fan curve can match it. A switch lets you keep the fan always on.
Skip it if: You run heavy loads most of the time.
Native 16-pin graphics card power cables. 12V-2x6 is the revised version of 12VHPWR. Native means no adapter.
When it matters: Most new high-end NVIDIA cards need one. A native cable beats adapters. Push the plug fully in and avoid sharp bends near it; poor seating causes melted plugs.
Skip it if: Your graphics card uses 8-pin plugs.
The most power each 16-pin graphics card cable can deliver. The connector is rated up to 600W.
When it matters: Smaller PSUs may limit a cable to 300W or 450W. Make sure it covers your card's board power.
Skip it if: Your graphics card uses 8-pin plugs.
The number of 8-pin (6+2) graphics card power plugs.
When it matters: Many AMD and older NVIDIA cards need two or three. On power-hungry cards, use a separate cable for each socket rather than one cable's daisy-chained pair.
Skip it if: Your card uses a 16-pin connector and you have a native cable.
CPU power plugs for the motherboard (EPS 8-pin, sometimes split 4+4).
When it matters: High-end boards may have two 8-pin CPU sockets. Most CPUs run fine with one, but check your board manual.
Skip it if: Your motherboard has a single CPU power socket.
Other power plugs: SATA for drives, fan hubs and lighting controllers, Molex for older devices.
When it matters: Count your drives, hubs and controllers. Check cable lengths too, since far drive bays may be out of reach.
Skip it if: Your build uses only M.2 drives and no hubs.
The PSU's size standard: ATX for most towers, SFX or the longer SFX-L for small cases.
When it matters: ATX units do not fit most small cases. SFX-L may not fit every SFX case. An SFX unit can go in an ATX case with a bracket.
The PSU's depth in millimeters, front to back.
When it matters: ATX units vary in length. Compare with your case's PSU max length and leave room for modular cables behind the unit.
Skip it if: You have a large case with a spacious PSU area.
Which cables detach: non-modular (all fixed), semi-modular (main cables fixed) or fully modular.
When it matters: Modular units make building and cable management easier, most of all in small cases. Never mix modular cables between brands or models; pinouts differ and can destroy parts.
The cable style (flat, round or sleeved) and the length of each cable.
When it matters: Flat cables route easily behind a motherboard tray. Big towers need longer cables, especially for the CPU power plug at the top of the board.
Skip it if: You are building in a standard mid tower.
The factory that designed and built the unit, and the internal design it uses. It is often not the brand on the box.
When it matters: One brand can sell strong and weak lines under the same logo. Knowing the platform shows which units share the same internals.
Skip it if: You rely on expert reviews of the exact model.
The circuit design inside, such as LLC resonant with DC-to-DC conversion for the smaller voltages.
When it matters: Modern designs with DC-to-DC conversion give tighter voltage control. Older group-regulated designs are a sign of a budget unit.
Skip it if: You are buying a recent, well-reviewed unit.
Whether the main capacitors come from Japanese makers, known for long life at high heat.
When it matters: A decent quality signal, but the whole design matters more. Good capacitors cannot fix a weak platform.
Skip it if: As a deciding factor; warranty and reviews tell you more.
Safety circuits that shut the unit off on faults: OCP (overcurrent), OVP (overvoltage), OPP (overpower), SCP (short circuit), OTP (overtemperature).
When it matters: They protect your other parts when something fails. A unit missing OCP or OTP is a red flag.
Mean time between failures: a statistical reliability estimate in hours.
When it matters: Test conditions differ between makers, so the figures are hard to compare. Warranty length says more about the maker's confidence.
Skip it if: Almost always.
How long the maker covers the unit, in years. Premium units often carry 10 years or more.
When it matters: A long warranty signals the maker's confidence in the design. Short warranties often mark budget lines.