Among the cardinal rules for designing (or re-designing) energy efficiency into buildings is to consider how the space is used by its occupants. If designed correctly, interior spaces induce people to make more efficient choices, such as using reduced light or water to attain the desired level of comfort.
The beauty of the built environment is that when people’s habits cannot be modified or when design is not conducive to efficient use of resources, the space can simply be re-designed. Sometimes very minor changes can make dramatic impacts.
My master bathroom is case-in-point. The toilet resides in a little closet separate from the rest of the bathroom. My routine for the past 6 months involves turning on that closet light first thing in the morning. As I complete my preparation and grooming for work, I sometimes become rushed to get out of the house, and occasionally that light is left on. Arriving home from work 10 hours later, I enter the bathroom only to discover the toilet closet light has been blazing away all day, wasting energy.
The Intermatic switch uses no electricity, and it functions via a mechanical timer. I selected this brand and configuration because it is designed to replace standard wall switches, and it fits into a conventional junction box. Replacing the ON-OFF switch with a timer was easy, and any do-it-yourselfer can make the swap with just a flat-head screw driver (provided the power is shut off at the breaker before beginning the work).
As I am trying to teach my energy engineering students at UNT, no energy retrofit should be undertaken unless it makes economic sense. So, how much energy did this little switch replacement save, and how long will it take to pay back?
I estimate that I leave the offending light on at least once per week for a duration of 10 hours per incident, and I am out of the house about 50 weeks per year. I recently swapped the toilet closet incandescent for a 23-watt compact fluorescent (see my post “Let the Residential Lighting Retrofit Commence!” for details). So, crunching the numbers yields 11.5 kW-hours per year of wasted energy. I pay roughly $0.131 per kilowatt-hour; thus, leaving the light burning costs me about $1.51 per year. The simple payback period for a $17.99 timing switch is just under 12 years.
As energy retrofits go, a 12-year payback is not stellar. Solar panels or new roof insulation should pay back about twice as fast. However, the project took me less than an hour to complete, and for about the cost of a pizza, I don’t have to come home once a week to realize I left the lights on anymore.
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