Desktop CPU Power Survey, April 2006

CPUs|Motherboards | Power
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Let's review the question we're trying to answer here:

"What is the best power efficiency achievable with currently available AMD and Intel processors that can be used on a desktop PC?"

In keeping with the spirit of the question and its focus on minimizing power consumption, the approach we took was to optimize each CPU for the lowest stable Vcore setting. This gave us the lowest CPU power consumption. We then took power measurements. The procedure was as follows:

Step One: The CPU was installed and tested at totally stock setting on the test platform. The CPU heatsink/fan was set to ensure that overheating could not be a source of instability; we generally disregarded noise for this project.

Step Two: We then used CrystalCPUID to reduce the core voltage of the CPU (Vcore) step by step while running the CPUBurn stress utility (one or two instances to match the number of cores or hyperthreading) at medium priority. With each Vcore setting, the system was run for a minimum of 10 minutes to watch for instability. We'd run other programs simultaneously to check for instability; if there was any, it showed up fairly quickly.

Step Three: When instability was encountered, the system would often need to be rebooted. We then moved the Vcore incrementally above the setting where instability occurred, seeking the lowest Vcore at which stability under load could be reached.

Step Four: Once the minimum stable Vcore was found, we confirmed it by running CPUBurn for 20 minutes before taking measurements.

NOTE: You might criticize that CPUBurn for 20 minutes is not a great test of system stability, and we would agree, but we think it's good enough. A system that goes 20 minutes of CPUBurn without instability is reasonably stable. In real world conditions, it's rare for a desktop system to undergo this high a load for this long. It may be that for these processors to survive a 24 hour test with Prime95 — an often used standard for system stability — the Vcore would have to raised slightly higher. But that would have taken way too long, and this project was tedious enough already.

While we took care to obtain accurate details about CPU power consumption, in the real world, power efficiency is best measured at the AC socket. Drops in efficiency of the PSU and the VRM at low and high power levels tends to compress CPU power differences. The concept of Average Power as a measure of real world computer efficiency is also introduced and discussed.


All processors were measured in three states to establish the full range of power consumption:

  • Idle, with Cool'n'Quiet, PowerNow!, or Enhanced Intel Speed Step (EIST) enabled when available.
  • Under heavy load using CPUBurn to stress the processor at the default Vcore for the CPU.
  • Under heavy load using CPUBurn at the minimum stable Vcore for the CPU.

Two power measurements were taken in each state:

  • DC power at the 2x12V (AUX12V) connector on the motherboard.
  • Total AC power consumed by the system as a whole.

DC power measurements involved a high precision current sensor plugged directly into the 2x12V connector on the motherboard, so that all power through this connection passed through the power meter. The line voltage (nominally +12V) and the current are measured with multimeters, and multiplied together to get the total power running through the connection. Because the CPU only draws power through the 2x12V connection and nothing else does, this tells us the amount of power consumed by the CPU and the voltage regulators on the motherboard.

The DC power measurements do not take the efficiency of the voltage regular module (VRM) on the motherboard into account. VRM efficiency does vary somewhat from board to board, and also with power level. The average VRM efficiency is not much higher than 80%, but not likely to be lower than 75%. So the actual CPU power draw is probably around 20% lower than the 2x12V current we're reporting. Little is known about VRM efficiency at very low power levels, like <10W in idle. We suspect VRM efficiency could drop substantially below 75%..

FYI, if we could measure power right at the CPU socket, we could characterize not only CPU power demand but also the VRM efficiency of motherboards at different power levels. However, taking voltage / power measurements directly at the CPU socket requires an investment we cannot justify. We know of a system developed by a power engineering team at Intel; the components would cost US $3,000 for basic equipment plus ~$3,000 per CPU socket type.

An custom-built shunt featuring a LTS 25-NP current sensor allowed us... measure voltage and current on the +12V AUX connector that powers the CPU.
The final accuracy for this power calculation is better than ±1W, maybe as good as ±0.1W.

AC power was measured to obtain a power profile of each system as a whole. By design, this includes power lost in the power supply itself during conversion from AC to DC. Most power supplies become less efficient as they approach zero output. At the low power loads of these systems, the power conversion loss may account for as much as 50% of the total system power. Measurements for AC power were read off of the digital display on the Extech power meter.

The Extech Power Analyzer / Data Logger 380803 power meter kept track of AC power.

Complete List of Test Tools:

  • AC power was measured with an Extech Power Analyzer / Data Logger 380803 power meter.
  • High accuracy Extech MM560 True RMS multimeter.
  • Two other multimeters of good precision.
  • High precision LTS 25-NP Current Sensor (to read the AUX12V current), courtesy of Intel.
  • A Fluke 36 Clamp Meter
  • Processor voltage was monitored using SpeedFan 4.27 or alternative motherboard utilities where SpeedFan was not supported.
  • Athlon 64 and Turion 64 specifics were documented using the utility A64 TCaseMax v1.18
  • Other processor details were checked with CPU-Z.
  • CrystalCPUID, a utility for setting and modifying clock speed and voltage.

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