PC Power Supply Load Calculator

About this calculator

The question behind "will my supply cope" is a division. Enter what the system draws and what the supply is rated at, and this gives the percentage, the capacity left, and whether there is room for whatever you are thinking of adding.

How to use this calculator

1. Enter the total system load in watts. The PC power calculator works it out from a component list if you do not have it.
2. Enter the supply's rating from the label on the side of the unit.

The formula

Load %        = System load / Rated capacity x 100
Capacity left = Rated capacity - System load

A rating of zero is refused rather than divided by, and a load above the rating is reported as what it is — over capacity, and by how much.

Worked example

470 W of components on a 650 W supply:

Load %        = 470 / 650 x 100 = 72.3%
Capacity left = 650 - 470        = 180 W

Which leaves room for a card drawing up to about 180 W more, before any margin.

Notes and limits

• A supply is most efficient somewhere near the middle of its range and least efficient at very low load. Running at 70% is entirely normal and slightly better for efficiency than running at 15%.
• The rating on the label is a sustained figure at a stated temperature. A supply in a hot case in an Indian summer has a harder time than the same unit on a test bench.
• If the load exceeds the rating, a good supply shuts down under protection. A poor one fails less gracefully, and can take other parts with it.
• Capacity left is not the whole story for an upgrade: a new graphics card may also need connectors the supply does not have.

What this calculation assumes

• Both figures are ones you entered. No supply or component data is stored in this page.
• The rating is treated as the supply's continuous capacity, which is what a reputable label states.