CPU electrical consumption

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Modern personal computer CPUs (central processing units) consume a considerable amount of electrical 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 the manufacturers are 'rewarded' (by sales) for raw speed instead of energy efficiency. This 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 slow 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 are essentially special purpose CPUs (+memory) 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, first is the 'typical' thermal power, which is how much power the CPU draws under normal load, and the other is the maximum thermal power, which is how much power it can draw if you give it a worst-case set of instructions. As an example, the Pentium 4 2.8GHz has a typical thermal power of 68.4 W, but a maximum thermal power of 85 W. When the CPU is idle, it will draw far less than the typical thermal power. Manufacturers generally don't release the idle thermal-power statistics for desktop chips.

There are some engineering reasons for this pattern.

First, higher speed always requires more power for equivalent operations than lower speed, even in the same chip. So, if I write an image file to a display whilst running at maximum speed, the power required will be higher than writing the same image file to the same display whilst running at a slower speed.
Second, new features (more CPU instructions, increased pipeline size, increased on-chip cache size, ...) all require more transistors. And each transistor requires power to work. More transistors working, more power needed. One trick used in recent CPU designs is to selectively shut down (or slow down) unused or secondary parts of the CPU.
Third, for any given CPU design, second and later generation implementations will usually use smaller transistors (less power draw), or structures built for lower voltage (typically using reduced power). Experience with a given design usually allows reduced power requirements.

There is an additional trend, not related to CPU power draw, which has increased power supply size requirements in recent years. New interface arrangements (eg, USB and Firewire) include a provision for powering external peripheral devices like disk drives or scanners on the interface cable. The amount of power the standard requires to be available is limited, but it must come from the computer's power supply. If you have several such peripherals, and they are often active, you will need a larger power supply than otherwise, and you may need special arrangements on your motherboard to support the additional current (the ATX 12V power supply connector).

Table of contents

1 ARM/StrongARM/XScale CPUs

1.1 Intel XScale

2 PC (x86) CPUs

2.2 AMD Athlon
2.3 AMD Athlon XP
2.4 AMD Athlon 64
2.5 Intel Pentium 4
2.6 VIA C3
2.7 VIA Eden-N

3 External links