Pull the plug on excess energy use

Image courtesy flickr user Samuel M. Livingston

Big savings are available to those who estimate, benchmark, and take action on their plug load.

When Gridium customers review their Snapmeter reports they are often surprised by the high level of energy use when the building is closed. In addition to off-hours lighting and ventilation, one growing culprit is “plug load” from devices that connect into wall outlets.

A quick look around your own desk probably shows how small plug loads can add up across a building. Your LCD Monitor (50W), laptop (40W), digital picture frame (10W), and task light (30W) alone bring you up to 130W. This doesn’t count any laser printers (130W on, 900W while printing) and projectors (500W) you share with others. Now consider what happens at 2am on a Saturday. Maybe you’re the office treehugger, but is everyone in the building as diligent as you are about shutting off your equipment? Or maybe IT mandates that you leave your PC on all night for updates.

How to measure baseload and estimate plug load

Let’s first establish the magnitude of the problem. The easiest way to do this is take a look at your interval data and estimate the baseload, the minimum energy use achieved during a given time period. In the fancy version below, baseload is shown in red.

Measuring electric energy use baseload with plugload energy use data
Gridium baseload decomposition

Determine baseload by finding the minimum energy used each night.

Second, you’ll want to estimate what baseload could be in your building. According to the AHSRAE 90.1 standard, typical weekend usage should be 5% of weekday levels. In this example, baseload is about 100kW and peak loads are 500kW, indicating that baseload is about 20% of building load. In our experience, the AHSRAE standard is very difficult to meet in practice. Most office buildings in our data set have a peak-to-baseload ratio of about 3, with high performers above five.

ASHRAE example of electric energy use peak-to-baseload ratio
Gridium Interval Database

Most Gridium customers have a peak to baseload ratio of about 3. The best-performing quartile has a ratio of 4.5.

To estimate your potential savings, figure out what your baseload would be if you achieved a peak-to-baseload ratio of 4.5, matching the top quartile in our data set. For example, if your peak is 900kW, then your target baseload is 200kW (900kW/4.5). Subtract 200 from your actual baseload and, voila, you’ve got a first order estimate of your baseload reduction target.

A few more calculations will turn your baseload reduction target into a potential cost savings. The details depend on how many hours per day your building spends at baseload and your utility rates, but as a rough guide, a 100kW reduction is worth about $29,000 per year. Baseload is low but steady, and those kilowatt-hours really add up.

These numbers are imprecise but directionally correct. More detailed analysis takes into account HVAC as well as plug load, and also adjusts for confounding factors such as data centers. You might also check your results against a normalized basis. Recent PIER research showed plug loads ranging from 0.7- 0.9 kWH per square foot per year or about 5-7% of the building load.

Ok, Houston. Now What?

The PIER study above showed total savings of over 40% at the studied office.  If you’ve benchmarked, you know where you are and what total potential savings could be.

If you are a multi-tenant facility, your options are limited. If you have large tenants with dedicated facility staff, you might try approaching them with the data. For smaller tenants you are limited to encouraging conservation by passing along green tips. And if you have a maximum power clause written into leases, this might offer some leverage on the issue or help identify rogue data centers driving up building expenses.

For tentants and owners, the actions from the PIER study are helpful:

  1. Get power management software: This was a big win at offices. 62% of the computers in the study were left in idle mode on nights and weekends, and simply enabling existing power management software cuts usage by 50%. Prep for pushback from your IT team, as they will want some time for updates for patching and remote access. With the shift to the cloud computing, try asking nicely; most patches can be installed during the day.
  2. Get a control strategy: Many inexpensive plug strips were tested in the study, including load sensing, remote control and timer based. The surprising winner was the remote control, which allowed users to easily power down everything without any unintended consequences. Large organizations might explore more sophisticated plug load monitoring and control systems. Some are even being built into office furniture!
  3. Remind your tenants: Gridium loves the power of simple emails. The study showed 6% savings from simple weekly calendar reminders to shut down everything before leaving for the weekend. That’s something even multi-tenant facility professionals can add to their toolkit.
About Tom Arnold

Tom Arnold is co-founder and CEO of Gridium. Prior to Gridium, Tom Arnold was the Vice President of Energy Efficiency at EnerNOC, and cofounder at TerraPass. Tom has an MBA from the Wharton School of Business at the University of Pennsylvania and a BA in Economics from Dartmouth College. When he isn't thinking about the future of buildings, he enjoys riding his bike and chasing after his two daughters.

One reply on “Pull the plug on excess energy use”

  1. Robert van de Walle says:

    Good looking graphs! One of my pet peeves is a poor graph.

Comments are closed.

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