PSU specs: what to look for before you buy

Wattage is a budget, not a performance figure. Add up the processor and graphics card first, since between them they account for most of the draw, then allow headroom for transient spikes. Modern graphics cards can spike well above their rated power for microseconds, and a supply running at 95 percent of its rating has nothing left for that.

Sizing roughly: a light build with integrated graphics is comfortable at 450 to 550W, a single mid-range card wants 650 to 750W, and a high-end card wants 850W or more. Oversizing costs little in efficiency and buys quiet operation, because a supply loafing at half load runs its fan slower. Undersizing is the one mistake that can damage other components.

80 PLUS ratings run Standard, Bronze, Silver, Gold, Platinum and Titanium, and they measure how much mains power is wasted as heat at 20, 50 and 100 percent load. Gold is the mainstream choice and is where the price stops rising steeply; Platinum and Titanium add a few percent and a lot of cost.

Efficiency also decides noise, because every wasted watt becomes heat that the fan has to move. A Titanium unit at 50 percent load converts around 94 percent of the input, so it runs cooler and quieter than a Bronze unit doing the same job. Cybenetics is the more useful modern rating, since it tests the whole efficiency curve and the noise level separately rather than giving a single badge.

ATX is the standard at 150mm wide and 86mm tall, with depth varying from about 140mm to 200mm. Depth is the number that catches people out, because a long 1600W unit will not fit a case designed around a 140mm one. Check the case's power supply clearance before buying a deep unit.

SFX and SFX-L are for small form factor builds, at 125mm wide instead of 150mm. SFX units cost more for the same wattage because everything inside has to be packed tighter, and they use smaller fans, which is why they are louder at high load. If the case takes ATX, buy ATX: it is cheaper, cooler and quieter for the same money.

A non-modular supply has every cable soldered in, so the unused ones live in the case and block airflow. A semi-modular unit fixes only the essential 24-pin and CPU cables and lets you attach the rest. Fully modular means everything detaches, which makes installation easier and lets you buy custom cables later.

Modularity does not change how well a supply performs, and a good non-modular unit beats a mediocre modular one. It is most valuable in a small case where there is nowhere to hide a bundle of spare cables. On a tight budget, spend the difference on efficiency or warranty instead.

ATX 3.0 and 3.1 are the standards written for graphics cards that spike far above their rated draw. They require the supply to tolerate short excursions well beyond its rating, which is exactly the situation that used to trip a cheap unit's protection and shut the machine down mid-game.

The other half is the 12V-2x6 connector, the revised version of the 12VHPWR plug that can carry up to 600W over a single cable. If you are buying a current high-end card, buy a supply that has a native 12V-2x6 cable rather than one that needs a three-way adapter, because the adapter is an extra failure point. ATX 3.1 also fixes the sense-pin behaviour that caused the earlier connector's problems.

Fan size and bearing type decide the noise more than any other part of the specification. A 135mm or 140mm fan moves the same air as a 120mm one at lower speed, and a fluid dynamic bearing runs quieter and lasts longer than a sleeve bearing, which is the cheapest option and the first thing to wear out.

Look for a zero RPM mode, which stops the fan entirely below roughly 30 to 40 percent load. For an office machine or a media build that means silence almost all the time. Cybenetics grades noise from A++ down, and a unit rated A++ is a genuinely quiet supply rather than one that is merely tolerable.

Two supplies with identical wattage and efficiency badges can be built very differently inside. The topology describes how the unit converts and regulates power, and modern designs combine active PFC with an LLC resonant stage, synchronous rectification and DC to DC converters for the minor rails. Those terms are not marketing: they are what keeps the voltages stable when the graphics card suddenly demands 400W.

Capacitors are the other tell. Japanese electrolytic capacitors rated to 105 degrees, from makers such as Rubycon and Nippon Chemi-Con, are the quality benchmark; cheaper units use lower-temperature or unbranded parts that age faster. A supply that does not name its capacitor brand is usually telling you something.

Everything that draws serious power in a PC runs on the +12V rail, so its current rating is the number that matters. A single rail puts all of it behind one over-current limit, which is simple and works well for most builds. Multiple rails split it into separate limits, which can be safer because a fault in one cable is caught faster, but they can also trip unexpectedly if one rail is loaded heavily.

Neither is better in the abstract. Single rail is easier to reason about and is what most mainstream supplies use; multi rail is common on high-end units and can often be switched between modes in software. What matters much more is the total +12V wattage, which should be close to the unit's overall rating.

Count the connectors you actually need: one 24-pin for the board, one or two 4+4 pin CPU cables, one PCIe lead per graphics card connector, and one SATA per drive plus a spare. A supply that covers the build but leaves no spare SATA or PCIe lead is a supply you will outgrow.

Check the cable lengths and types too, since short CPU cables are a common problem in tall cases and stiff flat ribbons are harder to route than individually sleeved ones. If the unit includes a 12V-2x6 cable, make sure it is a native cable from the power supply rather than an adapter that joins three 8-pin leads.

Warranty length is the manufacturer's own bet on the unit, and it correlates with quality more reliably than any other single figure. Three years is a budget supply, seven is mid-range, ten is a serious unit, and Seasonic's PRIME line goes to twelve. If a maker will not stand behind a supply for a decade, that is information.

MTBF is the mean time between failures, usually quoted in hours at a stated temperature. Figures above 100,000 hours are common on good units and mean little on their own, since the number depends on how it was measured, but an unusually low figure, say 60,000 hours, is worth noticing. Read the two together with the warranty rather than instead of it.