Power Supply Calculator

What is this?

A Power Supply Calculator works out what PSU wattage a PC build needs. It adds up what each component draws under load, adds headroom for transient spikes and ageing, and rounds up to a wattage that is actually sold. The CPU and graphics card typically account for 80 to 90 percent of the total.

How to Use the Power Supply Calculator

A PSU calculator works out what power supply a PC build needs. It adds up what each component draws under load, adds headroom, and rounds up to a wattage you can actually buy.

Two components dominate: the processor and the graphics card usually account for 80 to 90 percent of the total. Everything else - memory, drives, fans, the motherboard itself - adds up to less than a single mid-range graphics card.

Headroom of 20 to 30 percent is the usual advice. It covers transient spikes, the way capacitors age, and the fact that a power supply runs most efficiently at around half load rather than flat out.

1. Pick Your CPU Class

Choose the class that matches your processor and the rated wattage fills in. If you know the exact figure for your chip, type it over the top - the published number for your part always beats a class estimate.

2. Pick Your Graphics Card

Same idea. Choose integrated graphics if you have no separate card. Set the quantity if you run more than one, and tick overclocked if applicable.

3. Add the Rest

Motherboard size, memory sticks, NVMe and SATA SSDs, hard drives, fans, optical drives, USB fast-charging ports and liquid cooling. Each has a typical per-unit figure shown beside it.

4. Set Headroom

25 percent is a sensible default. Lower it if you want the smallest unit that works; raise it if you plan to upgrade or want the PSU running cooler and quieter.

5. Read the Recommendation

You get the recommended wattage, the load your build puts on it, a breakdown of where the power goes, and how much the machine draws from the wall at your chosen efficiency rating.

Key Formulas Used in the Calculator

Total Draw

$\text{Total} = \text{CPU} + (\text{GPU} \times \text{Cards}) + \text{Motherboard} + \text{Memory} + \text{Drives} + \text{Fans} + \text{Cooling}$

Every component's rated power added together. This is what the machine pulls under a realistic load, not idle.

With Headroom

$\text{Required} = \text{Total} \times \left(1 + \frac{\text{Headroom}}{100}\right)$

At 25 percent, a 400 W build needs 500 W. The margin covers transient spikes, ageing, and keeps the PSU in its efficient range.

Overclocking Allowance

$\text{CPU} + 30\% \qquad \text{GPU} + 15\%$

Overclocking raises power draw faster than it raises performance. These are rough allowances - a heavily tuned chip can exceed them considerably.

Draw at the Wall

$\text{Wall Watts} = \frac{\text{Delivered Watts}}{\text{Efficiency}}$

An 88 percent efficient supply delivering 445 W pulls 506 W from the socket and sheds the other 61 W as heat. Efficiency changes your electricity bill, not what your components need.

Benefits

• Every component wattage is editable, so the estimate can be made exact

• Rounds up to sizes power supplies are actually sold in

• Shows the load percentage, not just the wattage

• Breaks down where the power goes and what share each part takes

• Warns when the recommended unit would be running above 80 percent

• Warns when a build exceeds what one consumer PSU can deliver

• Converts to draw at the wall so you can see the running cost

When & Where to Use

• Sizing a power supply for a new build

• Checking whether an existing PSU can take a graphics card upgrade

• Seeing how much headroom a build has left

• Comparing the wall draw of different efficiency ratings

• Working out the cost of running a machine

• Deciding whether a smaller unit would be adequate

• Sizing for a multi-GPU or workstation build

Who Should Use This Calculator

The Power Supply Calculator is for anyone building or upgrading a PC - first-time builders sizing a unit, upgraders checking whether an existing supply is sufficient, and system builders specifying parts.

Tips to Get the Best Deal

The CPU and graphics card are almost the whole total - the rest barely moves it

Use the published figure for your exact parts rather than the class estimate

A PSU is most efficient around half load, which is why headroom helps twice

Modern graphics cards draw brief spikes well above their rated figure

Buying far more wattage than you need wastes money without helping

Check the connectors as well as the wattage - newer cards need 12V-2x6

A higher efficiency rating pays for itself only on a machine that runs a lot

Pro Tips

• The CPU and graphics card are almost the whole total.

• Use the published figure for your exact parts rather than the class estimate.

• A PSU is most efficient around half load, so headroom helps twice.

• Modern graphics cards draw brief spikes well above their rated figure.

• A bigger PSU does not use more electricity - it delivers what is asked of it.

• Check connectors as well as wattage; newer cards need 12V-2x6.