After 15 years outdoors in hot humid summers and cold dry winters of Canada, I had to replace the original MS-16A because the sensor window was cracked (from weather) causing the sensor to not work reliably. The new one made the system like new again. If you used this indoors, you might never have to replace it. I have this pointed down about 30 degrees so that people walking on the street about 30 feet away don't turn on the lights unnecessarily. I also recommend lithium batteries especially if you have winter temps below about 0 F or -18 C. Lithium work and last better than alkalines at those temps.