Measuring server energy efficiency

Increasing power densities and soaring energy costs make measuring the energy efficiency of servers more important than ever. Obtaining these metrics requires the proper tools and methods to achieve accurate results.

Many people attempt to measure power using an amp meter. There are two problems with this approach.

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First, power is measured in watts not amps. Watts = amps x volts x power factor. Attempting to use an amp meter to measure power forces to you guess at the voltage and power factor of the circuit creating the potential for considerable error.

The second problem is both amps and watts are instantaneous measurements. Even an accurate power measurement still only tells you about a servers "performance" at that particular moment.

To accurately measure a server's efficiency while running benchmark tests you need to measure the cumulative power consumption over the entire test. Cumulative power consumption over time is an energy metric measured in watt-hours. For this you need an electric watt-hour meter. Watt-hour meters are designed to continuously monitor the amperage, voltage and power factor of a circuit to accurately determine the true energy usage. Once you have energy measurements for various servers running the same benchmark tests you have way to compare the workload achieved for the amount of energy consumed.

There are a few important things to consider when choosing an appropriate watt-hour meter. One is the resolution of the meter. Many watt-hour meters are intended to monitor large loads over long periods of time and accumulate their readings in kWh (kilowatt-hours). This means the accumulator only increments each time there is 1,000 watt-hours of energy usage. Attempting to use such a low-resolution meter on smaller circuits over short periods is like using a calendar to measure a 100-yard dash. A high-resolution meter that measures in watt-hour resolution or better is needed for this type of application.

Watt-hour meters come in many form factors. Some meters can simply be plugged inline between the load and source using standard plugs and receptacles. Other meters require you tap into the circuit to obtain the voltage measurements and clamp a current transformer around a conductor of the circuit to obtain the amperage measurements. There are also branch circuit watt-hour metering systems available that can measure the energy usage of each individual circuit within a power distribution unit.

A functional consideration is that some watt-hour meters provide network connectivity for automated reading while others require reading them visually from a digital display. A big advantage of meters with network connectivity is you can easily log a series of readings for trending and analysis.

Power and energy measurements are important metrics for data center operators today. In addition to determining equipment efficiency these metrics also indicate heat loads to assist with cooling management. Energy measurements can also be used to allocate energy costs to those responsible for their consumption creating an ongoing incentive to deploy energy efficient equipment.

About the author:Diesso has been involved in the development and deployment of automated metering solutions for over 10 years and is the founder of Smart Works, Inc which provides power and environmental metering solutions to data centers.

This was first published in May 2006

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