The advantage of having a sensor per light is that you can realize maximum energy efficiency. This takes a bit of time to explain and I will try to be as brief as possible.
Consider a 30 Ft x 30 Ft room with 9 lights that consume 35W each. The room is only occupied 50% of a typical work year and when occupied only 2 lights see motion.
In Scenario 1 you have a sensor per room. All 9 lights will turn on 100% when motion is detected and off when the room is vacant. Applied to our example we would expect the following math:
8 hours per day, 5 days per week, 50 weeks per year = 2,000 hours/year. The space is only occupied 50% of the time so the sensors will only keep lights on for 1,000 hours/year.
9 x 35 watt x 1,000 hours/year = 315,000 watts/year
In Scenario 2 you have a sensor per light. The 9 lights with sensors can be linked together in a group such that when any sensor sees motion it tells the others to come up to a different brightness level. Note here that the human eye can only perceive light level differences that are 30% or more. This means that you can set lights to 70% and it would be difficult to discern 100% from 70%. Also note that brightness % correlates linearly to input power %, meaning that a 70% light level will roughly correspond to a 70% power level. Applied to our example we would expect the following math:
2 x 35 watt x 1,000 hours/year = 70,000 watts/year
7 x 35 watt x 70% x 1,000 hours/year = 171,500 watts/year
Total = 241,500 watts/year
Comparing the scenarios, you save nearly 25% annually on your power bills without affecting occupant experience. The added cost of having a sensor per light is quickly paid for.
Another consideration is daylight savings. With daylight sensors per light, each light adjusts to light levels independently such that lights closer to the windows dim more than lights deeper in the room. Using a single daylight sensor for the room located in the center of the room means that all lights must dim to the same levels. In my experience, the gross control systems have trouble as occupants do not appreciate having their lights noticeably change. They will complain and eventually the building operators will get tired of trying to adjust and end up disconnecting. The chances of success for both the occupant experience and savings are much better with discrete control per light.
Applications that benefit from discrete control are manyfold; offices, classrooms, retail, warehouses, etc… In summary, with motion/daylight sensoring per light there is an excellent opportunity to maximize energy savings without sacrificing the occupant’s visual experience.
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