How using Power over Ethernet can save users $300 a year in energy costs

June 3, 2011
An organization with 12 wireless access points can use midspan PoE three ways to achieve power savings.

by Daniel Feldman, Microsemi Corp.

Power over Ethernet (PoE) technology has become an increasingly important tool for simplifying network deployment while improving remote-management capabilities and overall energy efficiency. The latest advances have presented new opportunities to further reduce power consumption and associated energy costs.

Midspan PoE devices' ability to monitor, reduce and manage power use offers network administrators the biggest opportunity to significantly reduce energy consumption and associated costs.

Midspans can be used in three key ways to squeeze the most power efficiency possible out of the network. The first is to leverage midspans' ability to deliver power over all four pairs of structured cabling wires. Today's midspans feature two interfaces, each of which takes 25.5W into the same box -- one over the pairs using pins 1, 2, 3 and 6 and the other over the pairs using pins 4, 5, 7 and 8. Connecting the two doubles the standard power delivery to 51W while still fully complying with the 802.3at-2009 standard. Alternatively, four-pair powering can be used to cut power dissipation. Specifically, instead of delivering 51W over a twisted-pair cable, this same four-pair configuration can be used to power two-pair devices with 30W of power, which dissipates up to half the power while consuming almost 15 percent less energy compared to conventional two-pair solutions. That's a savings of more than 2.5W per port.

Using this approach with a 12-port midspan, power dissipation is reduced by approximately 30W, which yields an annual savings of 263 kWh (30W x 24 hours x 365 days). At $0.10/kWh, this is $26 in annual savings per 12 PoE-powered wireless LAN access points.

The next tactic for improving efficiency is to use midspans' remote powered device (PD) monitoring and configuration capabilities. Network administrators can monitor per-port and total power consumption, and configure PDs for instant and scheduled port on/off functions, as well as UPS status port on/off functions. An organization with 12 wireless LAN access points could cut its round-the-clock operation down to, say, 10 hours per day (8 a.m. to 6 p.m. Monday through Friday), which would reduce usage from 2,933 kWh (12 APs x 27.9W x 24 hours x 365 days) to 837 kWh (12 APs x 27.9W x 10 hours x 250 days), for a savings of 2,096 kWh per year. At $0.10/kWh, that is a savings of $210 annually.

The third way to improve power efficiency is to minimize the effects of idle power consumption. Many PoE midspans and switches use switching power supplies (SPSs) that are 90-percent efficient at full load, which means as much as 220W of AC power is consumed for 200W of PoE power, or up to 440W for 400W of PoE power. The problem is that SPS units have high switching power losses when idle -- as much as 20 to 40W with zero-W load for 200W-rated units, and 40 to 80W with zero-W load for 400W-rated units. The solution is to employ midspans that come with built-in power supplies that are capable of powering real needs, and only upgrade to full power per port in case there are loads requiring that, via an external power supply. With this improved efficiency, it is possible to use a small (450W) internal power supply to handle all real-time requirements, and then augment it with an external 450W or 1kW power supply when needed. This yields annual savings of 394 kWh (45W x 24 hours x 365 days), which at $0.10/kWh equates to $39 annually for every 12 wireless LAN access points.

Calculating the aggregate savings possible with each of these approaches, organizations can save more than $300 annually using the latest energy-efficient PoE midspan technology. This includes the following.

  1. Four-pair powering: Saves 2W per port or $53/year
  2. Power management: Saves $210/year by enabling ports to be turned off when not needed
  3. Smaller internal supplies (augmented with external supplies as needed): Saves $39/year by reducing idle power consumption.

Daniel Feldman is business unit manager for communications power in the analog mixed signal group of Microsemi Corp. This is an excerpt of an article that will appear in the July 2011 issue of Cabling Installation & Maintenance magazine.

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