The power supply is the least glamorous component in a build and the only one whose failure can take the rest of the machine with it. It is also the component most often chosen badly, because the specification that matters most — the quality of what is inside — is not printed on the box, while the numbers that are printed invite a calculation that produces the wrong answer.
This guide explains how to arrive at a wattage figure you can trust, what the efficiency certifications actually measure, and which specifications are worth reading past.
Why adding up TDPs does not work
The intuitive method is to add the processor's TDP to the graphics card's TDP, add something for the rest, and buy a supply above that number. Every term in that calculation is misleading in a different direction.
TDP is a thermal specification. It describes the heat a cooling solution must be able to remove, which is related to power consumption but is not a measurement of it. Modern processors routinely draw substantially more than their nominal TDP during sustained boost, and manufacturers define the term differently from one another and from one generation to the next.
The graphics card figure is closer to a real power number, but it describes typical board power under a sustained load rather than the peaks the card actually produces. And "something for the rest" is a guess for a quantity that is genuinely small — drives, fans, memory and the motherboard together usually account for less than a graphics card does on its own.
The result is a calculation that looks rigorous and lands too low, in a way that produces a machine which works fine until it does not.
Transient spikes: the failure nobody expects
This is the specific mechanism behind most "my PC restarts during games" reports on builds that looked correctly specified.
Modern graphics cards do not draw power smoothly. They produce brief excursions far above their rated draw — spikes lasting on the order of a millisecond, sometimes reaching double the card's nominal figure. Because these events are so short, they contribute almost nothing to average power consumption and do not appear in the numbers on a specification sheet or in most reviews.
A power supply, however, does not respond to averages. Its over-current and over-power protection circuits respond to instantaneous conditions. A supply running close to its rated capacity can see one of these spikes, correctly conclude that it is being asked for more current than it can deliver, and shut down to protect itself. From the user's side this presents as an abrupt reboot under load — no error, no warning, and nothing wrong with any individual component.
The defence is margin. A supply operating at 60–70% of its rating has enough absolute headroom to absorb these excursions without its protection engaging. This is the real reason for the recommendation, and it is a better reason than the vague appeals to "future-proofing" usually offered.
A calculation that works
- Take the graphics card's rated board power. This is the dominant term.
- Take the processor's maximum power draw rather than its base TDP, if the manufacturer publishes one. If not, use the TDP and treat it as an underestimate.
- Add roughly 100 W for everything else — motherboard, memory, drives, fans and peripherals. This is generous for most builds.
- Multiply the total by about 1.4. This is the step that covers both transient spikes and the efficiency band.
- Round up to the next standard capacity available on the market.
For a build with a 220 W graphics card and a 125 W processor, that gives roughly (220 + 125 + 100) × 1.4 ≈ 620 W, so a 650 W supply. The arithmetic is deliberately conservative, and the cost difference between adjacent capacities is usually small enough that erring upward is inexpensive insurance.
Our PC Builder performs this calculation automatically from the components you select and displays the recommended minimum wattage with the margin already applied.
What 80 Plus certification actually means
The 80 Plus tiers — Bronze, Silver, Gold, Platinum, Titanium — certify one thing: efficiency, meaning the proportion of power drawn from the wall that reaches your components rather than being lost as heat. A supply at 90% efficiency delivering 500 W to the machine is pulling around 555 W from the socket; the missing 55 W leaves as heat.
| Tier | Efficiency at 50% load | What it changes in practice |
|---|---|---|
| 80 Plus | ~82% | Baseline. Adequate but increasingly uncommon |
| Bronze | ~85% | The sensible floor for a gaming build |
| Silver | ~88% | Uncommon; usually skipped over for Gold |
| Gold | ~90% | The value sweet spot for most builds |
| Platinum | ~92% | Marginal efficiency gain; often better-built units |
| Titanium | ~94% | Diminishing returns for typical use |
Two points are routinely misunderstood. First, higher efficiency does not mean more power available — a 650 W Bronze unit and a 650 W Gold unit both deliver 650 W. The Gold unit wastes less doing it, runs cooler and costs marginally less to run.
Second, and more importantly, 80 Plus certifies efficiency and nothing else. It says nothing about the quality of the capacitors, the design of the protection circuitry, the accuracy of voltage regulation under load, or whether the unit will still hold its rating after three years. A Gold-rated supply from an unknown brand is not automatically better than a Bronze-rated unit from a manufacturer with a track record. Efficiency is a useful signal — it is not a proxy for quality, and treating it as one is the most common error in choosing a supply.
Modularity
Modular cabling is a convenience feature and has no bearing on electrical performance.
- Non-modular: every cable is permanently attached. Cheapest, and unused cables have to be stowed somewhere inside the case, where they obstruct airflow.
- Semi-modular: the essential cables (motherboard, CPU) are fixed; peripheral cables detach. A reasonable compromise and common at mid-range prices.
- Fully modular: every cable detaches. Easiest to build with and to keep tidy, at a small price premium.
One safety note that matters: modular cables are not interchangeable between manufacturers, or even reliably between different models from the same manufacturer. The pinout on the supply side is not standardised. Using a cable from one unit with another can short components and destroy hardware. Always use the cables that came in the box with that specific supply.
Connectors: check these before wattage
A supply can have ample capacity and still be unusable with your graphics card if it lacks the right connectors. This catches people out more often than wattage does.
- PCIe power connectors: count how many your graphics card requires and confirm the supply provides that many as separate cables. Daisy-chained connectors on a single cable are acceptable for lower-draw cards and inadvisable for high-draw ones.
- 12V-2x6 / 12VHPWR: the connector used by recent high-end cards. If your card uses it, confirm the supply has a native cable rather than relying on an adapter.
- EPS/CPU power: most boards need one 8-pin; higher-end boards may want an additional 4- or 8-pin. Check the board, not the processor.
- SATA and Molex: rarely a constraint, but count them if you are fitting several drives.
Signals of a supply worth buying
Since the important qualities are not on the box, these are the proxies worth using.
- Warranty length. A manufacturer offering seven or ten years is making a costly statement about expected lifespan. Two years is a statement too.
- A named manufacturer with a reputation to protect, rather than a brand that appears only on budget listings.
- A single strong +12V rail rather than several weak ones, which simplifies distributing load to a modern graphics card.
- An explicit list of protections — over-voltage, over-current, over-power, over-temperature, short-circuit. Their absence from the specification is informative.
- Independent review coverage. Power supplies are among the most thoroughly tested components, and a unit nobody has measured is a unit nobody can vouch for.
What not to do
- Do not carry over a supply from a build that is more than about seven years old. Capacitors age, capacity declines, and the money saved is trivial against the risk.
- Do not buy an unbranded unit whose rating cannot be verified. Wattage claims on the cheapest units are frequently aspirational.
- Do not size the supply exactly to your calculated draw. That is precisely the condition under which transient spikes cause shutdowns.
- Do not use adapters to create graphics card connectors your supply does not natively have, if it can be avoided.
- Do not treat 80 Plus certification as evidence of build quality.
In short
Calculate your draw honestly, multiply by about 1.4, check that the connectors match your graphics card, and buy from a manufacturer whose warranty suggests they expect the unit to last. The power supply is the one place in a build where buying slightly more than you need is straightforwardly the correct decision.
You can browse tracked power supply prices across Tunisian retailers in our catalogue, or let the PC Builder calculate the figure for your specific parts list.