- Power usage effectiveness is the ratio of total energy use to that of IT equipment.
- There are three levels of PUE measurement: basic, intermediate and advanced.
- Metering is necessary to accurately determine PUE value; it can also help improve energy efficiency.
Power usage effectiveness (PUE) is an important tool for measuring data center energy efficiency. What's PUE? It's the ratio of total energy use to that of IT equipment. PUE, along with metering, is now a key component in many standards, including Executive Order 13693, which mandates that advanced meters must be installed in all federal data centers by 2018. Existing facilities must achieve a PUE below 1.5 and new data centers must have a PUE from 1.2 to 1.4.
The U.S. Department of Energy's Better Buildings Challenge (BBC) and short-term Accelerator programs require participants to improve energy efficiency by 25 percent within five years in one or more data centers, as well as the installation of meters tracking data.
Green Grid, which originally developed the PUE concept, separates PUE measurement into three levels:
Basic. The IT load, which represents all power delivery and cooling components, is measured through a meter at the output of the UPS equipment. For stand-alone data centers, PUE1 = M1/M2, where M1 is the utility readings and M2 is UPS output. This data is collected by manually reading existing meters and can therefore be less accurate.
Intermediate combines automated metering with manual measurements. In addition to utility and switch gear meters, there are meters installed on other equipment, such as chilled water plants and HVAC equipment, to measure outdoor air temperatures.
Advanced has all points automatically metered with measurements taken over time. Meters are used to determine energy end use and electrical distribution is metered at each step. There are also temperature readings for chilled water, supply air and rack intake air temperature.
An embedded data center is located within a building and also measures the power to the IT equipment at the UPS output. However, cooling energy is measured using thermal meters installed on the chilled water pipes serving the data center HVAC system or is calculated if measurement isn't possible. The PUE calculation becomes more complicated when additional metering beyond the UPS output is installed.
Whether you join the Challenge or not, installing metering can help you improve your data center's efficiency. Those who've joined the Accelerator program have achieved energy reductions ranging from 20 to 40 percent with a short payback. To take full advantage of metering, the U.S. Department of Energy's Lawrence Berkeley National Laboratory (LBNL) recommends the following:
- Determine what type of metering you currently have
- Review drawings of the electrical, mechanical and cooling systems; verify actual conditions.
- Interview staff and do a walk-through of the site
- Define needs based on input from facility and IT managers to obtain buy-in
- Decide what approach to use for the PUE calculation
- Review design and develop a metering plan before implementing
- Commission all installed systems to ensure that they meet design requirements using portable meters
- Train staff on how to monitor, measure and use meter data
To avoid major interruptions, schedule meter installation during annual system maintenance. If you have dual-fed power systems, shut down one system at a time. For thermal meters, use hot-taps (where ports for sensors are installed on fully operational systems) or ultrasonic sensors, which can be simply clamped onto chilled water pipes.
For further information on metering, see:
Data Center Energy Efficiency Measurement Assessment Kit from the LBNL
Metering Best Practices. A Guide to Achieving Utility Resources Efficiency from the U.S. Department of Energy
Image source: U.S. Department of Energy