Performance per watt is the number that matters

For a long time the question asked of a processor was how fast it was. The more useful question now is how much work it does for each unit of energy. That figure is called performance per watt, and it shapes nearly every device you own.

Every watt becomes heat

Almost all the electricity a chip draws ends up as heat. A processor that uses more power needs a larger heatsink, a louder fan or a thicker case to get rid of it.

In a desktop computer this is an inconvenience. In a phone there is no fan at all, so the limit is how warm the device can get in your hand. When a chip reaches that limit it slows itself down, which is why a phone can feel sluggish after a long game.

The battery sets the budget

A portable device carries a fixed amount of energy. An efficient chip can spend that budget in two ways:

• do the same work and last longer, or
• do more work in the same number of hours.

Either way the gain comes from efficiency. A faster chip that draws proportionally more power gives the user nothing new.

Two kinds of core

Many modern processors combine two kinds of core. Large cores handle demanding work quickly and use more power doing it. Small cores handle background work slowly and frugally. The system moves tasks between them, so checking mail does not cost the same energy as editing video.

The same rule at large scale

Data centres face the same constraint with larger numbers. The electricity a building can draw and the heat it can remove are fixed, so the amount of computing it can house depends on how efficient the chips inside are.

Speed still matters. It is simply no longer the quantity in short supply. Energy is.