How much power supply you actually need

A power supply is sized from what the machine draws, not from the largest round number the budget reaches. The draw is mostly two parts, the rest is small, and the right size leaves room above the total for reasons that have more to do with efficiency and quiet than with running out of power.

Add up what the build draws

Two components set the figure: the processor and the graphics card. Everything else — drives, fans, memory, the board itself — adds up to less than either of them, and can be treated as a modest fixed sum. Take the processor's rated power at full tilt, add the graphics card's, add roughly a hundred watts for the rest, and you have the peak the build pulls when both are working hard at once.

The graphics card maker publishes a recommended supply wattage for each card, and it is a sound floor to start from because it already carries the assumptions above. Where your processor is a hungrier one than the card maker assumed, or you run both flat out together as a game or a render does, work from the parts rather than the single recommendation. What you are sizing against is that simultaneous peak, not the wattage on a sticker or the machine sitting idle.

Why headroom, and how much

Leave room above the peak, for three reasons that are not about the supply failing. A supply is most efficient near the middle of its range, usually somewhere around half load, and least efficient at the extremes, so a unit sized to sit near half under your typical load wastes the least as heat. That efficient middle is also the quiet one: many supplies let the fan idle or spin slowly until load climbs, so a unit rarely pushed hard is a unit rarely heard. And a modern graphics card does not draw a steady figure — it spikes for milliseconds well above its average, and headroom is what absorbs those spikes without the supply's protection tripping.

Headroom has a ceiling, though. A supply runs off its efficient middle at light load just as it does near its limit, so a very large unit under a small build sits in the inefficient low end most of the time, and the extra capacity is money spent on watts the machine never asks for. The aim is a unit whose typical load lands near half and whose peak leaves comfortable room — not the largest one the budget allows.

What 80 Plus tells you, and what it does not

The 80 Plus badge and its tiers rate one thing: efficiency, the share of the power drawn from the wall that reaches the components instead of leaving as heat. A higher tier wastes less, which means a little less on the power bill and a little less heat to clear from the case. It says nothing directly about the quality of the protection circuitry, the capacitors, or the fan — a unit can rate well on efficiency and be built plainly elsewhere. The tiers tend to track build quality because the better-made units are usually the ones taken to certification, but the badge measures efficiency, and the rest of the unit is a separate read.

Modular cables, form factor, and fit

Two fit questions decide whether a supply belongs in your case at all, before efficiency enters. The form factor — ATX for most towers, the smaller SFX and its variants for compact builds — has to match what the case is built to hold, and a supply in the wrong form factor does not mount. Cabling is the softer choice: a fully modular unit lets you attach only the cables the build uses, a non-modular one has them all fixed on, and a semi-modular one splits the difference; it is a matter of tidiness and airflow, not power. One connector question is worth checking on a current build — whether the unit carries the native 12V-2x6 lead a recent graphics card wants, or leaves you running it from adapters.

How we track a unit's wattages

A supply sold at several wattages is one product with several sizes here, not several unrelated products. Each wattage is tracked on its own, with its own current price and its own price history. The wattages move independently — a run on 850 W units does not touch what the 650 W sells for — so folding them into one figure would misprice every one of them.

The unit's own page is where the wattages separate. The price that leads the page is the cheapest wattage currently tracked; below it, each wattage is listed with its own price, and the chart draws one line per wattage. Decide the wattage you are sizing for before you read the price.

Every power supply in our catalog is on one page.