Power Distribution Units (PDUs) deliver conditioned power from the uninterruptable power supply (UPS) system to servers, networking equipment and other electronic devices in the data center. PDUs are part of a data center’s electrical distribution system, which includes utility or generator-supplied power, building switchgear and transformers, and UPS systems. Although they are often overlooked, electrical distribution system losses can account for 10% to 12% of the total energy consumed by the data center, on average.1
Data centers may deploy one or two types of PDUs:
- PDUs may simply be well-constructed power strips. However, today’s “smart,” or “intelligent”, PDUs allow data center managers to remotely monitor real-time power use, data and event logs, current drawn by each PDU, and current drawn by each outlet. Some can remotely perform on/off switching for each outlet as well. These units improve our understanding of energy-use, can help identify comatose servers, can inform capacity planning, and may alert us to equipment failure.
- Floor-mounted PDUs traditionally contain isolation transformers that reduce high voltage and current to more common and useful levels, which they feed downstream to rack-based PDUs. (For example, they might reduce electricity entering the data center at 240 volts and 30 amps in a single phase to multiple 120 volt 15 amp or 120 volt 20 amp plugs. Floor-mounted PDUs may be integrated with monitoring software to trend power consumption and help plan for future power needs.
Use PDUs with high-efficiency transformers
If your data center uses floor-mounted PDUs, know that PDUs containing high-efficiency transformers are 2% to 3% more efficient overall compared to a PDUs with generic lower-efficiency transformers.
Monitor individual devices to identify energy-efficiency problems and opportunities
Because “smart” PDUs can tell you how much power each device uses, you can easily identify the energy hogs. PDUs can also identify equipment that is no longer needed. For example, when a PDU indicates that a certain server is running at an average of 35% of peak power, you’ll be able to earmark that server as a candidate for decommissioning.2
A failed power supply, a significant temperature increase, a sudden surge in power usage -- these changes can all be identified by an intelligent PDU. Some PDUs can send you an alert when something is perceived to be outside normal operating conditions. For example, when a PDU is reaching its total power capacity, it can alert the data center manager before breaker capacity is reached, letting him/her know that it’s time to bring in more PDUs to prevent outages.
Monitor power consumption to improve capacity planning
PDUs allow you to monitor rack-level power consumption, store the data, trend it, and use it to make decisions about your data center.
For example, suppose you have concerns about reaching the upper limits of power capacity in your facility. “Smart” PDUs perform outlet-level monitoring, which allows you to pinpoint areas within the data center where a simple equipment rearrangement may free up power capacity.
Turn off unloaded PDUs
Like other equipment in the data center, PDUs should be turned off if they are not in use. In some facilities, the array of UPS modules and/or PDUs has more than enough capacity to serve the IT load. It may be possible to shut down some modules and still retain the required level of redundancy.3 This will allow the remaining units to operate at a higher load factor, which usually translates to higher efficiency.4 Learn more about reducing energy losses from uninterruptable power supply (UPS) systems.
Maintain balanced PDU loads
Unbalanced electric loads in 3-phase systems cause higher current flows between the legs of the transformer, resulting in waste heat and less efficient transformer operation.5 Redistribute the loads to improve balance. If the transformer is in an air-conditioned space, the reduced waste heat will also reduce the load on the cooling system.
Benchmark your efficiency by calculating PUE
Power data obtained from PDUs can help you calculate Power Usage Effectiveness for your data center. PUE is a widely accepted benchmarking standard that helps managers determine how effectively data centers use electricity. It provides insights into efficiency efforts, and can also help determine when something has gone awry.
PUE is the ratio of total energy used by a data center facility to the energy delivered to IT equipment – see Figure 1, below. Total power used in each rack, as reported by PDUs, can be compared to the building’s overall power usage to calculate PUE. PUE approaches 1 as the data center becomes more efficient. A 2014 survey6 by the Uptime Institute indicated that the average reported PUE is 1.7. Learn more about benchmarking your data center’s energy efficiency.
Savings and Costs
PDUs containing high-efficiency transformers are 2% to 3% more efficient overall compared to a PDUs with generic lower-efficiency transformers.
Target, in one of their two ENERGY STAR certified data centers, powered down two unloaded 300 kVA PDUs to save 261,000 kWh annually. Read their case study (PDF, 609 KB) to learn more.
Tips and Considerations
Data center infrastructure management (DCIM) systems help make the most of smart PDUs. For example, a single DCIM dashboard can offer real-time power loads, trends, and capacity forecasts. They can also help drive efficiency initiatives with real-time PUE gauges.
1 Evaluating UPS System Efficiency, by K. Kutsmeda, Consulting-Specifying Engineer, June 11, 2015. http://www.csemag.com/single-article/evaluating-ups-system-efficiency/73497ade33f9a66ead63a77f33e94d4e.html
2 5 Ways to Improve Data Center Energy Efficiency with PDUs, The Right Signals Blog, Belden.com, March 10, 2016. http://www.belden.com/blog/datacenters/5-Ways-to-Improve-Data-Center-Energy-Efficiency-with-PDUs.cfm
3 Redundancy refers to the duplication of critical components or functions of a system with the intention of increasing reliability.
4 Data Center Master List of Energy Efficiency Actions, Center of Expertise for Energy Efficiency in Data Centers, Lawrence Berkley National Laboratory, US Department of Energy, Feb. 11, 2016. https://datacenters.lbl.gov/resources/data-center-master-list-energy
5 Data Center Master List of Energy Efficiency Actions, Center of Expertise for Energy Efficiency in Data Centers, Lawrence Berkley National Laboratory, US Department of Energy, Feb. 11, 2016. https://datacenters.lbl.gov/resources/data-center-master-list-energy
6 2014 Data Center Industry Survey, Uptime Institute. 2014. Available online at: https://journal.uptimeinstitute.com/2014-data-center-industry-survey/