Smart Power Strips

The modern office is full of energy-using equipment—computers, printers, copiers, task lights, fans, and other devices—much of which is left on throughout the workday and, in some cases, for 24 hours a day, 365 days a year. According to the American Council for an Energy-Efficient Economy, about 7 percent of all commercial electric energy is consumed by office equipment and computers. However, because more than two-thirds of a year consists of nonbusiness hours, and because employees often don’t stay at their desks throughout the entire workday, researchers at the National Renewable Energy Laboratory estimate that a number of common plug loads are used for as little as 10 percent of the year. This discrepancy can result in a substantial amount of wasted money for building owners and operators.

To mitigate that wasted energy, “smart” or “advanced” power strips use a variety of techniques, including timers, sensors, Internet connectivity, and automated scheduling functionality to turn equipment off when it’s not in use—saving enough electricity to enable simple payback periods of less than two years in many cases. Smart power strips can be used to control a variety of devices, including coffee machines, space heaters, printers, water coolers, and computer monitors.

A research effort in New York involving a small office building found that by implementing plug load controllers like smart power strips, overall (not just plug load) consumption could be reduced by as much as 10 percent through eliminating after-hours electricity usage. Table 1 summarizes which devices were most often left on after hours.

Table 1: Common devices left on after hours
In a study of a small office building in New York, water dispensers, coffee makers, and desktop computers were found to be the most common devices left on after hours.
Common devices left on after hours
What are the options?
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Here are some of the most common strategies you’ll encounter when shopping for smart power strips:

Occupancy sensors. Smart power strips equipped with occupancy sensors (either integrated directly into the power strip or connected to it via a cord) can sense the presence or absence of office workers and turn the attached equipment on and off accordingly. Passive infrared (PIR) sensors are the most common type of occupancy sensor and are able to “see” heat emitted by occupants. The sensor is triggered when a change in infrared levels is detected, such as when a warm object moves in or out of view of one of the sensors. PIR sensors are very resistant to false triggering. They are best used within a 15-foot range for two reasons: There are potential “dead” spots between their wedge-shaped sensory patterns that get wider with distance (see Figure 1) and, being passive, they do not send out any signal; instead, they depend on the intensity of the heat output of the moving part of the subject, which drops with the square of the distance.

Figure 1: Sensor coverage diagram
Ultrasonic sensors can detect motion at any point within the contour lines. Passive infrared sensors see only in the wedge-shaped zones, and they don’t generally see as far as ultrasonic units. The ranges are representative; actual sensors may be more or less sensitive.

Ultrasonic occupancy sensors, on the other hand, emit a high-frequency signal (over 20,000 cycles per second), outside human and animal audibility ranges, and listen for a change in frequency of the reflected signal. By emitting a signal instead of only receiving it, ultrasonic sensors are able to cover larger areas than PIR sensors and are noticeably more sensitive. However, they are also more prone to false triggering. For example, air motion created by a person running past a doorway or the on/off cycling of an HVAC system can cause false triggers. To try to overcome the unique drawbacks of each sensor type, smart power strips are also available that combine ultrasonic and PIR technologies in a single system.

Timers. Some smart power strips can turn office equipment on and off based on the calendar or clock time. This approach can be especially effective for equipment that’s used on a regular schedule or in cases where it doesn’t make sense to control equipment based on occupancy. For example, if an infrequently used laser printer or copier requires a long warm-up period and is kept in an area that lacks frequent foot traffic, the productivity lost while people waited for the printer or copier to warm up would quickly negate the savings from reduced energy use. Turning the device on and off based on a simple clock would ensure energy savings overnight, and a calendar function would enable weekend and holiday savings as well.

Activity or power monitors. Other smart power strips determine when equipment is in use or in an idle state by monitoring equipment activity—for example, the flow of data to a printer—or by monitoring the flow of current to the equipment. If the equipment is idle for a length of time specified by the user, it will be shut off and awakened when activity resumes. Some strips monitor the current from one device, such as a computer, to determine when to shut off or turn on all devices connected to the strip. This allows the strip to automatically shut off all peripheral equipment, such as monitors and printers, when the computer is shut off. These options can also be used together: A clock might turn a printer on at the same time each morning, and an activity monitor or occupancy sensor might turn it off when activity stops at the end of the day.

How to make the best choice
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Determine if smart power strips are cost-effective. Many buyers base their purchase decision on a simple visual observation that office equipment is left on longer than necessary. But to determine whether an investment in this type of equipment will really be cost-effective, you can monitor particular groups of equipment to see how much power they draw, how many hours they are left on, and how often they could be turned off. Several companies now offer standalone devices for measuring plug load power draw. Data from this type of device can be combined with a sensor to track energy usage and occupancy and estimate the savings that can be achieved through control of plug loads. Energy loggers can also be used to determine office equipment power draw and time of use. Once the equipment power draw, times of use, and occupancy schedules are known, a simple calculation (see sidebar) can be performed to determine the payback period of a smart power strip.

Alternatively, some newer Internet-connected smart power strips have built-in energy-tracking functionality. In certain applications, it may be economical to purchase a small number of this type of power strip, use them to identify the best applications, and see if it makes sense to purchase more to control the various plug loads in the facility.

Calculating payback on smart power strips

Although prices vary, the average smart power strip could add a premium of about $20 to the cost of a regular power strip with equivalent surge protection (or more for Internet-connected devices). If the strip controlled 50 watts worth of task lights and a monitor using 100 watts, and electricity cost an average of 10 cents per kilowatt-hour (kWh), the device would pay for itself after preventing about 1,333 hours of operating time—a matter of less than two months if equipment otherwise would be left on 24 hours per day. How did we get this number? By multiplying the power saved (in kilowatts [kW]) by the energy rate, then dividing the cost premium by that product: $20 ÷ (0.15 kW x $0.10/kWh) = 1,333 hours.

To determine how long it would take to eliminate 1,333 hours of operating time, first determine how many hours per day a smart power strip might prevent that equipment from operating. For example, a typical workday might be 9.5 hours long, during which monitors are inactive for 5.5 hours. Assume that the equipment is turned off at night and that the occupancy sensor time delay adds a half hour per day of operating time before the equipment is turned off. Then the smart power strip would save about five hours per day of operating time and pay for itself in 267 working days, or just over one year. If equipment is otherwise left on nights and weekends, the savings would be 143 hours per week for a payback of less than 10 weeks.

Pick the right control strategy. Smart power strips equipped with ultrasonic or PIR occupancy sensors will work in a typical office setting if they are installed correctly. However, each type has limitations. Ultrasonic sensors are more sensitive to movement than PIR devices, but they’re also more prone to false triggering; PIR sensors experience dead spots but are generally a better choice in a typical workstation where the line-of-sight requirement of a PIR sensor can be easily met. Strips that use equipment power monitors or timers may require less effort to install initially and might be a better solution in areas where a number of plug loads interact with each other in some way (such as computer peripherals), in spaces with low occupancy, where a usage schedule is well established (for example, a water cooler that can be turned off every night), or in areas where frequent foot traffic might lead to false triggers with occupancy sensing.

The National Renewable Energy Laboratory report, Plug-Load Control and Behavioral Change Research in GSA Office Buildings (PDF), shows actual measured savings from smart power strips. Researchers compared timer-based, load sensing, and combination approaches and their overall performance across several federal buildings (Table 2).

Table 2: Electricity use reduction by device type for three different control types
Energy reductions from smart power strips can be highly variable depending on building specifics and behavior. Total average savings across equipment type and with varied approaches were about 27 percent.
Electricity use reduction by device type for three different control types

Interestingly, the study found that schedule timer controls often outperformed load-sensing controls. The small negative savings could be attributed to normal usage variation coupled with disabled or broken load-sensing controls.

Consider Internet connectivity. A variety of products now feature the ability to connect to a Wi-Fi network. This allows users to adjust control settings and on/off schedules through a web portal while also monitoring real-time energy consumption. For facilities managers, this can offer a significant benefit because all the strips in a facility can be set up and monitored from a single location. These units also tend to have built-in power sensors, which can help identify plug loads that may yield the largest energy savings, provide a straightforward way to track actual energy savings, ensure persistence of savings (since a facilities manager can easily see if any smart strips have been unplugged or otherwise adjusted by employees), and open the door to employee engagement and awareness efforts. Although Internet-enabled smart strips can be around 2 to 5 times more expensive than nonconnected smart strips, the unique benefits they offer may nonetheless make them worth considering for many facilities.

Optimize occupancy sensors. Some smart power strips come with small-range occupancy sensors that only sense a presence in the workstation; others cover a wider area and detect the presence of people in the office around the workstation. Choose the latter if you want equipment to turn off less frequently and turn on sooner.

As is the case for wall- and ceiling-mounted occupancy sensors, the sensors that come with smart power strips are easily visible and can potentially be improperly adjusted, stolen, vandalized, or fooled into perceiving a human presence when a space is unoccupied. For continued energy savings, take the following precautions:

  • Involve building personnel in planning for the sensors.
  • Train maintenance personnel and office occupants to keep sensors operational rather than disconnecting them when problems occur.
  • Position sensors carefully so that they only see the area that you want them to see; the biggest cause of false triggering is incorrect sensor positioning.

Pick a product with an adjustable time delay. Smart power strip products may come with either adjustable or factory-set time delays that determine how long a period of nonoccupancy or inactivity must be before the equipment is turned off. A product with an adjustable time delay allows you to customize the device to suit individual work habits.

Target devices with large plug loads. Large plug load devices such as vending machines, mini refrigerators, and water dispensers will contribute the most energy-use reduction when plugged into a smart power strip.

What’s on the horizon?
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Going forward, smart power strips will likely offer increased Internet connectivity at a lower price point and begin to function as a part of more all-inclusive workspace controls. For example, a variety of companies are working to integrate plug load controls with office lighting or personal HVAC controls, and although no offerings have yet taken off, it seems only a matter of time before such functionality becomes more widely available.

Who are the manufacturers?
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These companies offer smart power strip products that have different key features:

Timer-based

Occupancy-sensing

Power-sensing

Internet-enabled

Content last reviewed: 
01/04/2018
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