As plug loads -- or energy used by equipment plugged into an outlet -- continue to race to the top of the list of energy issues confronting building managers, measuring and controlling them has become a more critical and challenging aspect of building energy management.

According to the U.S. Department of Energy, plug loads can account for 30 percent to 50 percent of U.S. commercial building electricity consumption. It’s estimated that plug loads in California office buildings alone consume more than 3,000 gigawatt-hours annually, costing more than $400 million each year, according to the U.S. General Services Administration.

The collective energy that computers, monitors, cellphone chargers, vending machines and other sources of plug loads consume has grown significantly in recent years, becoming a top concern for building managers who were once largely focused on driving down energy costs related to HVAC, lighting and other building systems.

In a sense, plug loads represent a new frontier for managing controllable energy costs, fueled by the ever-increasing use of portable electronics among building occupants, as well as the need for faster, more powerful office equipment. In a 50,000-square-foot building, plug-load management can save $40,000 to $60,000 over five years on average, according to the GSA. Needless to say, the opportunities for plug load-related energy reduction are significant.

Load management challenges

While effective management of plug load consumption can have a significant impact on the bottom line, it has traditionally been a difficult challenge for building managers. Unlike lighting, HVAC and other centrally controlled building systems, plug loads are decentralized and come from all corners of a building, from office cubicles to kitchens to server rooms.

Compounding this problem is the issue of ownership: as far as plug-in devices go, who owns what? In a typical building, the IT department owns computer and server equipment, facilities owns vending machines and kitchen appliances, and building occupants own cellphones, tablets, and other personal electronics that are typically plugged in during the workday.

Not only are plug loads decentralized, but they are largely deregulated as well. While certain standards like ASHRAE 90.1 are beginning to call for more aggressive plug-load control measures, plug loads have traditionally flown under the radar of building performance mandates.

“Miscellaneous electrical loads (MELs) are growing at a pretty big clip, and as the industry moves toward making things more energy efficient and designing net-zero buildings, these loads really need to be addressed,” said Jessica Rivas, an associate director with Navigant Consulting in Boulder, Colorado. “But it’s been a challenge, and in general, I think one of the issues is that plug loads are not yet well understood in the industry.”

Hard to measure, hard to manage

One of the biggest issues that has stood in the way of effective plug load management is that load consumption has been traditionally difficult to measure. While power monitoring equipment can be used to measure and record plug load consumption at the sub-panel or individual circuit level, the technology can be expensive and complex, typically requiring the assistance of an electrician to glove-up to install the gear inside the panel.

On the other side of the spectrum are lower-cost, simpler plug load meter devices that “plug in” between the load and the wall outlet. While many of these products offer both a real-time “meter” view of usage and logging capabilities, the logging in most cases is limited to providing a totalized view of consumption vs. time-stamped data over time.

“A lot of the available technologies give you accumulated energy, but nothing really time-based,” said Ryan Stroupe, measurement tools program coordinator for Pacific Gas & Electric at the Pacific Energy Center in San Francisco. “If you’re going to do any financial analysis on energy consumption, you have to know the specific time the consumption is happening. Plug-load studies would be much more accurate with a device that has the ability to provide time-stamped data recording versus just a total.”

Rivas agrees that time-of-use data is important, and feels that the accuracy of traditional plug load meters has fallen short of what’s needed in today’s commercial energy audits.

“In order to conduct our evaluations, we need something that provides high accuracy down at really low wattage,” she said. “The simple meters typically don’t provide this level of accuracy. You can grab small wattages with more sophisticated power meters, but these systems aren’t really designed for plug-load metering, and if there’s disruption to the power, you won’t see it in the data.”

A new class of monitoring devices

Bridging the gap between complex power monitoring systems and simple meters is a new class of devices, plug load data loggers. These devices combine the convenience and affordability of simple plug-load meters with time-based logging capabilities and high accuracy at low wattages.

These devices typically work by plugging the load to be monitored directly into the logger, and then plugging the logger into a standard wall outlet. They are able to operate both in a meter mode, where the device functions as a power meter and provides a real-time display of energy consumed, and in a logger mode, which measures and records data at a user-defined interval.

With the ability to view, measure and log a range of parameters including true RMS voltage (v), true RMS current (A), active power (W), active energy (Wh), apparent power (VA) and power factor (PF), plug-load data loggers are able to provide comprehensive trend logs of plug-load energy consumption to help building managers determine more effective load management and control strategies including permanently installed controls. And they can do it with high levels of accuracy and granularity. Some models provide up to 0.5 percent measurement accuracy with a measurement resolution of 1 watt, making them suitable for building audits where highly detailed energy use data is required.

“Plug-load loggers have a lot of versatility that allows you to do things more simply and quickly than traditional power meters or energy accumulation meters have been able to do,” said PG&E's Stroupe. “There are a variety of studies going on where these devices work well. For example, in hotels, you have guests plugging things in, and consumption is a big unknown, and in dorm rooms you have students bringing in things like refrigerators. It’s important to understand the variability of these loads, and with accurate time-series data, it becomes possible.”

According to Rivas, plug-load loggers make the most sense in building areas where equipment is consolidated.

“Printing stations where you have a printer, fax and other components following the same usage pattern are good targets for commercial buildings,” she said. “The same thing goes for residential, where you might have a master appliance like a TV and then a number of other components working in tandem with it.”

Using plug-load loggers

Setting up and configuring a logger typically involves connecting the device to a laptop or desktop computer via USB cable, and using accompanying logger software to make point-and-click selections that include how often the logger records a measurement and the time and date the logger will start recording. Some logger software packages allow for batch configuration to set up dozens of loggers at one time. Additionally, some packages enable users to configure the logger to calculate minimum, maximum and average statistics during the logging period at a fixed sampling rate.

The process of data retrieval involves offloading the collected plug load data onto a desktop computer, laptop, or a handheld data shuttle device, which can be used to store and transport data back to a computer.

Data analysis is typically performed using accompanying data logger software, which allows the user to quickly and easily translate the load data into time/date-stamped graphs that clearly show variations over a given period of data collection. 

The resulting information can help determine the potential return on investment for implementing new plug-load controls, and can be used to verify the energy savings once conservation measures have been put in place. The data can also be used to help educate building occupants on the need for and impact of equipment shutdown best practices. 

Beyond monitoring

While plug-load data loggers can be a key tool for establishing plug-load consumption baselines and verifying savings, awareness and behavioral changes among building occupants can also go a long way toward helping an organization realize its plug-load efficiency goals.

“At PG&E, we have thousands of laptops and desktops that our employees use,” said Stroupe. “Our IT department can make sure they all have the energy savings capabilities activated, but it’s up to the people using them to keep the settings active and not override them.”

At CLEAResult, a Portland, Oregon-based firm that designs and manages energy-efficiency programs for utilities and government organizations, internal awareness around plug loads is high.

“We did a plug load competition two years in a row, and reduced plug load energy by 14 percent the first year alone,” said Amber Buhl, an energy engineer with the firm. 

Buhl, who blogs about plug-load management and other building efficiency topics for CLEAResult, believes that plug-load reduction programs within an organization can be successful, provided there’s a committed and educated cross-functional team in place with stakeholders from facilities, IT and the general employee population.

“In most organizations, there will be people who quickly sign on for a plug-load reduction project, and others who are highly resistant” to the idea, she said. “Often, to reduce plug loads, we are asking people to adopt a new habit or change an existing practice. Taking time at the beginning of the process to understand their concerns and potential barriers to make changes will pay off in the long run.”

While contests can help generate more awareness, the need to monitor plug loads can vary case by case, according to Navigant's Rivas.
“In some offices, people are mainly using laptops without a lot of other things plugged in, so the power draw is relatively low,” she said. “But in areas like the creative department, they tend to have larger desktop machines, so evaluating those types of loads is useful.”

She adds, “There’s no question that there is growing interest in plug loads in the industry. [W]e recognize that having the right measurement tools is a key factor in getting programs designed better. This is what ultimately creates movement in the market.”


Evan Lubofsky is a communications director and writer for Onset