Processor power dissipation

Modern personal computer CPUs (central processing units) consume a considerable amount of electric power. This must be considered when choosing a power supply unit, and when power consumption is constrained by production and cost.

CPUs in other electronics often use far less power. For example, the CPUs in mobile phones or pacemakers use just a few microwatts. CPUs in personal computers use a lot of power because, in this context, computing power is usually considered much more important than energy efficiency. Top-performance speed requires significantly more power in the cases of most (but not all) CPU architectures. One of the simplest way to reduce the power consumption of a PC's CPU is just to decrease its clock rate.

CPUs for desktop computers typically use more power than any other component inside the computer, except perhaps recent technology video cards which contain special purpose CPUs themselves. The steady trend in CPU power supplies over the past decade has been towards using lower voltages and having considerably higher currents. While energy-saving features have been instituted in PCs for when they are idle, the overall consumption of today's high-drain CPUs is considerable. This is in strong contrast with the much lower energy consumption of CPUs designed for low-power environments. One such CPU, the Intel XScale, can run at 600 MHz with only half a watt of power, whereas x86 PC processors from Intel in the same performance bracket consume roughly eighty times as much energy.

Processor manufacturers usually release two power consumption numbers for a CPU, the typical thermal power, which is measured under normal load, and the maximum thermal power, which is measured under a worst-case set of instructions. For example, the Pentium 4 2.8GHz has 68.4W typical thermal power and 85W maximum thermal power. When the CPU is idle, it will draw far less than the typical thermal power.

There are some engineering reasons for this pattern.

Performing at higher speed always requires higher power. Reducing processor speed when possible saves power.
New features generally require more transistors, each of which uses power. Turning unused areas off saves power.
As a processor model's design matures, smaller transistors, lower-voltage structures, and design experience reduce power consumption.