The water detection unit is a sensor used with the water leak detection and reporting starter kit wlxs-007 (sold separately) amazon asin b01myujanr. Up to 49 sensors can be installed per system wlxs-007. Each sensor has a unique serial number that reports the exact location of the water leak and will send the owner of the home or facility a text message or e-mail. The system will also shut off your water supply to your house or facility. Each water detection unit requires two each AA batteries. Additional components are available for the waterleakxsystems starter kit including a signal repeater / temperature sensor srts-005, water detection unit skinny sensor for tight spaces wduss-004, solid state well relay wr-006 that shuts down the well pump, 1 in water shut off valve wsv-1", 1.5 in water shut off valve wsv-1-1/2" and water detection unit extension probes wduxp-303 inserted into floor carpets - all to detect and report water leaks.