Power supply calculator

Estimate how much power a system will draw and what capacity of power supply suits it. The calculator shows its full arithmetic, so you can check the assumptions against your own parts rather than taking a number on trust.

How this calculator works

It adds a typical sustained draw for each component class you select, plus allowances for the motherboard, memory, drives, fans and any pump. That gives an estimated total draw under load.

The total is then multiplied by 1.35 and rounded up to the next 50 W. That multiplier is doing three jobs at once, and each one is worth understanding.

Why the headroom multiplier exists

Transient spikes. A graphics card does not draw a steady figure. It draws in bursts that can briefly reach well above its rated board power, for periods measured in microseconds. These are too short to heat anything, but they are long enough to trip a power supply protection circuit, which shuts the machine off instantly. A system that restarts under load when it seemed to have enough capacity on paper is usually running into this.

Efficiency at part load. Power supplies are most efficient at roughly half their rated capacity, and become less so at both extremes. A supply running continuously near its limit converts more of what it draws into heat, which means more fan noise and more thermal stress on its own components.

Ageing. The capacitors inside a power supply degrade over their service life. A unit that comfortably delivered its rating when new will deliver less after several years of use, particularly if it has run hot.

Where manufacturer recommendations override our arithmetic

When a graphics card manufacturer publishes a recommended power supply capacity and that figure is higher than our calculation produces, we use theirs.

This is not deference for its own sake. The card manufacturer has measured the transient behaviour of that specific design on a test bench with instruments. Our calculation adds published sustained figures together. Their number accounts for something ours cannot see, so when the two disagree, theirs is the one to follow.

What this calculator cannot tell you

It cannot account for your specific parts. It works from class averages, so a particularly power-hungry model within a class will draw more than the figure shown. If you know your exact components, the PC Builder uses their published figures instead.

It says nothing about the quality of a supply. Capacity is one number among several that matter: ripple, voltage regulation, protection circuitry and build quality all affect whether a unit delivers its rating cleanly and what happens when something goes wrong. A poor 850 W unit is a worse choice than a good 650 W one for a system that needs 600 W.

It cannot include peripherals drawing power from the board, additional expansion cards, or anything you add later. If you expect to add a second drive or a more demanding card within the life of the machine, size for that now.

Reading 80 PLUS ratings

The 80 PLUS certifications, from White through Bronze, Silver, Gold, Platinum and Titanium, describe efficiency: how much of the power drawn from the wall reaches your components rather than becoming heat. They do not describe reliability, ripple or build quality, although in practice manufacturers rarely put a Titanium rating on a poorly built unit.

The practical difference between Bronze and Gold on a typical desktop is a few watts of waste heat and a modest difference on an electricity bill. It is a reasonable tiebreaker between two otherwise similar units, not a reason to choose a worse supply from a weaker manufacturer.

A safety note

Never open a power supply housing, even one that has been unplugged for a long time. The capacitors inside can retain a dangerous charge, and there is nothing in there intended to be serviced. A faulty supply is replaced, not repaired.

Related reading: How to choose a power supply and What 80 PLUS ratings actually mean.