Abstract: A sensor apparatus and method of installation, and a system for controlling at least one electrical device and a method of use of such system. The apparatus comprises a mount adapted to be attached to a support and a sensor element adapted to be detachably connected to the mount. The method of installing a sensor apparatus comprises providing a sensor apparatus according to the first aspect of the present invention. The method further comprises the steps of retainably affixing the mount to, or within, the support and detachably connecting the sensor element to the mount. The system for controlling at least one electrical device comprises at least one sensor apparatus, at least one electrical device, and a controller, wherein the at least one sensor apparatus and the at least one electrical device are electrically connected to the controller.