Mar 12

Occupancy Sensors: Still Turn the Lights Out PLEASE!

By: Phil Nicholson

Since our childhood, most of us have had it engrained in our heads to "turn the lights off when you leave the room!" I find myself nearly daily instructing my four year old son to re-climb the stairs in order to turn off the lights in his bedroom. In commercial settings, it is probably safe to say the general population is at least used to seeing occupancy sensor installations. Automatic lighting shutoff has been a code requirement now for upwards of 15 years in some jurisdictions, including my home state of Michigan since March 2003 with the first adoption of the Michigan Uniform Energy Code. However, how much energy is actually saved when it comes to these now commonplace devices? Depending on the philosophy of the occupants, it may be far less than one might think.

The theory behind the savings garnered from an occupancy sensor is based primarily on the scenario in which the occupant has failed to the turn the lights off. A typical minimum occupancy time delay (duration the sensor keeps the lights on after the last occupancy detection) is 30 seconds and often as long as 30 minutes. Say an occupant has left their office to return home for the evening and failed to turn off their lights. This accident, assuming a reasonably typical 100 square foot office, would cost the facility owners $0.12 (assuming a W/SF of 1.0 and a utility rate of $0.10/kwh). If this happens six times over the course of one year, the occupant will have wasted $0.72 if an occupancy sensor was not installed.

Not an overly surprising scenario I'm sure, however, let's look at a contrasting scenario. Say the same office suite shares a private toilet room. For years, the occupants of this office suite have finished their visit to the toilet room by turning off the standard snap switch as they leave the room. Over the course of the day there are a total of 34 visits to the space, each time, as reminded repeatedly during childhood, the occupants darken the room upon exiting. Eventually a renovation project results in the installation of an occupancy sensor being installed within the space. The users find having the switch detect their presence and turn on the lights automatically to be so convenient. Similarly, upon exiting each time, the switch does its job and turns the lights out for them"

Tags: Education, Engineering, Government